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International Journal of
Geo-Information
Article
A GeoSPARQL Compliance Benchmark
Milos Jovanovik 1,2,* , Timo Homburg 3and Mirko Spasi´c 2,4
Citation: Jovanovik, M.; Homburg,
T.; Spasi´c, M. A GeoSPARQL
Compliance Benchmark. ISPRS Int. J.
Geo-Inf. 2021,10, 487. https://
doi.org/10.3390/ijgi10070487
Academic Editors: Rob Brennan,
Brian Davis, Armin Haller, Beyza
Yaman and Wolfgang Kainz
Received: 22 May 2021
Accepted: 10 July 2021
Published: 16 July 2021
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
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iations.
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1Faculty of Computer Science and Engineering, Ss. Cyril and Methodius University in Skopje,
1000 Skopje, North Macedonia
2OpenLink Software Ltd., Croydon, Surrey CR0 0XZ, UK; mirko@matf.bg.ac.rs
3i3mainz—Institute for Spatial Information & Surveying Technology, Mainz University of Applied Sciences,
55128 Mainz, Germany; timo.homburg@hs-mainz.de
4Faculty of Mathematics, University of Belgrade, 11000 Belgrade, Serbia
*Correspondence: milos.jovanovik@finki.ukim.mk
Abstract:
GeoSPARQL is an important standard for the geospatial linked data community, given
that it defines a vocabulary for representing geospatial data in RDF, defines an extension to SPARQL
for processing geospatial data, and provides support for both qualitative and quantitative spatial
reasoning. However, what the community is missing is a comprehensive and objective way to
measure the extent of GeoSPARQL support in GeoSPARQL-enabled RDF triplestores. To fill this gap,
we developed the GeoSPARQL compliance benchmark. We propose a series of tests that check for
the compliance of RDF triplestores with the GeoSPARQL standard, in order to test how many of the
requirements outlined in the standard a tested system supports. This topic is of concern because the
support of GeoSPARQL varies greatly between different triplestore implementations, and the extent
of support is of great importance for different users. In order to showcase the benchmark and its
applicability, we present a comparison of the benchmark results of several triplestores, providing an
insight into their current GeoSPARQL support and the overall GeoSPARQL support in the geospatial
linked data domain.
Keywords: GeoSPARQL; geospatial data; benchmark; RDF; SPARQL
1. Introduction
The geospatial Semantic Web [
1
] as part of the Semantic Web [
2
] represents an ever-
growing semantically interpreted wealth of geospatial information. The initial research [
3
]
and the subsequent introduction of the OGC GeoSPARQL standard [
4
] formalized geospa-
tial vector data representations (WKT [
5
] and GML [
6
]) in ontologies, and extended the
SPARQL query language [7] with support for spatial relation operators.
Several RDF storage solutions have since adopted GeoSPARQL to various extents as
features of their triplestore implementations [
8
,
9
]. These varying levels of implementation
may lead to some false assumptions of users when choosing an appropriate triplestore
implementation for their project. For example, some implementations allow for defining
a coordinate reference system (CRS) [
10
] in a given WKT geometry literal as stated in
the GeoSPARQL standard (e.g., GraphDB). Other implementations do not allow a CRS
definition and instead only support the world geodetic system WGS84 (e.g., RDF4J) [
11
].
Such implementations, even though incomplete according to the GeoSPARQL standard,
still cover many geospatial use-cases and can be useful in many scenarios. However,
they are not useful, for example, for a geospatial authority that needs to work with many
different coordinate system definitions.
The requirements of GeoSPARQL compliant triplestores have been clearly spelled out
in the GeoSPARQL standard [
4
]. However, the Semantic Web and GIS community lack a
compliance test suite for GeoSPARQL, which we contribute in this publication. We hope
that our contribution may be added to the list of OGC conformance tests (OGC Test Suites:
ISPRS Int. J. Geo-Inf. 2021,10, 487. https://doi.org/10.3390/ijgi10070487 https://www.mdpi.com/journal/ijgi
ISPRS Int. J. Geo-Inf. 2021,10, 487 2 of 19
https://cite.opengeospatial.org/teamengine/ (accessed on 22 May 2021)), as they lack a
suitable test suite for GeoSPARQL.
Our paper is organized as follows. In Section 2, we discuss existing approaches
that worked towards evaluating geospatial triplestores, and Section 3introduces the test
framework of the benchmark and describes how the compliance tests were implemented.
Section 4
describes the application of the defined test framework against different triple-
store implementations, and we discuss the results in Section 5. In Section 6, we lay out the
limitations of our approach, before concluding the work in Section 7.
2. Related Work
Most standards define requirements which need to be fulfilled to satisfy the stan-
dard definition. However, not all standards expose explicit descriptions on how to test
compliance with their requirements or a test suite that tests the overall compliance to
the standard.
GeoSPARQL [
4
], as an extension of the SPARQL [
7
] query language, defines an
ontology model to represent vector geometries, their relations and serializations in WKT
and GML, a set of geometry filter functions, an RDFS entailment extension, a query rewrite
extension to simplify geospatial queries and further geometry manipulation functions.
First, it is important that we distinguish between performance benchmarks and com-
pliance benchmarks. Performance benchmarks try to evaluate the performance of system,
usually by employing a set of queries. Performance benchmarks may also consider seman-
tically equivalent implementations that are not following the syntax specified by a given
standard. On the other hand, compliance benchmarks are not concerned with the efficiency
or overall performance of a system, but rather with its ability to fulfill certain requirements.
Several benchmark implementations targeting geospatial triplestores, such as the
Geographica Series [
12
,
13
] or [
9
], try to evaluate the performance of geospatial function
implementations. Both approaches originate from the Linked Data community. Addi-
tionally, Ref. [
14
] shows that the geospatial community is interested in benchmarking
geospatial triplestores as well. Their benchmark includes a newly created dataset and
tests GeoSPARQL filter functions. While the aforementioned benchmarks might reveal if
functions are implemented, they do not necessarily reveal an incorrect implementation of a
given function.
The Tests for Triplestores (TFT) benchmark [
15
] includes a GeoSPARQL subtest. How-
ever, the subtest used here is based on the six example SPARQL queries and the example
dataset defined in Annex B of the GeoSPARQL standard [
4
]. Although these examples
are a good starting point, they are of informative nature and are intended as guidelines.
Therefore, any benchmark based solely on them does not even begin to cover all possible
requirements or the multiple ways in which they have to be tested, in order for a system to
be deemed as compliant with the standard.
Recently, the EuroSDR group reused the benchmark implementation of [
14
] to im-
plement a small GeoSPARQL compliance benchmark (EuroSDR GeoSPARQL Test: https:
//data.pldn.nl/eurosdr/geosparql-test (accessed on 22 May 2021)). This compliance bench-
mark consists of 27 queries testing a selection of GeoSPARQL functions on a test dataset. In
contrast to our benchmark, this implementation does not explicitly test all requirements de-
fined in the GeoSPARQL standard. In particular, GML support, RDFS entailment support
and the query rewrite extension, among others, have not been tested in this benchmark.
3. GeoSPARQL Compliance Benchmark
The GeoSPARQL compliance benchmark is based on the requirements defined in the
GeoSPARQL standard [
4
]. The 30 requirements defined in the standard are grouped into
six categories and refer to the core GeoSPARQL ontology model and a set of extensions
which systems need to implement, and which need to be tested in our benchmark:
1.
Core component (CORE): Defines the top-level spatial vocabulary components (Re-
quirements 1–3);
ISPRS Int. J. Geo-Inf. 2021,10, 487 3 of 19
2.
Topology vocabulary extension (TOP): Defines the topological relation vocabular
(Requirements 4–6);
3.
Geometry extension (GEOEXT): Defines the geometry vocabulary and non-topological
query functions (Requirements 7–20);
4.
Geometry topology extension (GTOP): Defines topological query functions for geom-
etry objects (Requirements 21–24);
5.
RDFS entailment extension (RDFSE): Defines a mechanism for matching implicit
(inferred) RDF triples that are derived based on RDF and RDFS semantics, i.e., derived
from RDFS reasoning (Requirements 25–27);
6.
Query rewrite extension (QRW): Defines query transformation rules for comput-
ing spatial relations between spatial objects based on their associated geometries
(Requirements 28–30).
Each of the specified requirements may be tested using a set of guidelines which
are loosely defined in the abstract test suite in Annex A of the GeoSPARQL standard [
4
].
While the abstract test suite defines the test purpose, method and type to verify if a specific
requirement has been fulfilled, it does not define a concrete set of SPARQL queries and a
test dataset which may be used for reference. We contribute the test dataset and the set of
SPARQL queries to verify each requirement in this publication.
In the GeoSPARQL compliance benchmark, each requirement is tested by one or more
SPARQL queries, where there is a single expected answer or a set of expected answers.
The number of queries used to test a requirement, as well as the number of expected
answers per query, depends on the nature of the requirement. For some of them, it is
sufficient to have a single query and a single expected answer to test whether the system
under testing complies with it. In contrast, other requirements have sub-requirements—
for example, requirements which refer to multiple properties or functions, requirements
referring to functions which can be used with geometries with different serializations, or
requirements which need a broader coverage of cases, to make sure they are fully met. In
these cases, multiple queries are used. Multiple logically equivalent expected answers are
used when the answer of a SPARQL query can be technically expressed in different formats
or literal serializations.
This approach of using queries and expected answers as tests allows us to mea-
sure the compliance of any RDF storage system by using the HOBBIT benchmarking
platform [16,17].
The output of the benchmark is a percentage which measures the overall compliance
of the tested system with the GeoSPARQL standard. It measures the number of supported
requirements of the system, out of the 30 specified requirements, as a percentage.
3.1. Benchmark Dataset
The GeoSPARQL standard defines an example dataset for testing in its Annex B [
4
],
which can be used with the set of six example test queries defined in the same section. This
example dataset contains six geometries. We wanted to use this dataset, but given that we
aimed to test all requirements of the standard, we had to substantially extend the dataset
both with new geometries and additional properties of the existing geometries. Figure 1
shows the geometries included in our extended dataset, while Listing 1contains an RDF
excerpt of the dataset, in Turtle syntax that represents the geometry A (Point).
The extended benchmark dataset contains 13 geometries of
Polygon
,
Point
and
LineString
types, all expressed as both WKT and GML literals. The total size of the RDF
dataset is over 300 triples. The dataset is available as part of the benchmark code [
18
,
19
], in
RDF/XML, GeoJSON [20] and GML representations.
ISPRS Int. J. Geo-Inf. 2021,10, 487 4 of 19
A (Polygon)
G (Polygon)
B (Polygon)
B (Point)
C (Polygon)
C (Point)
D (Polygon)
D (Point)
E (LineString)
F (Point)
A (Point)
G (Point)
J (Polygon)
K (Polygon)
L (Point)
M (Point)
Figure 1.
Abstract view of the geometries which are part of the benchmark dataset. Geometries
A, B, C, D, G, J and K represent
Polygon
geometries and (aside from J and K) all have a center
Point
geometry as well. Geometry E represents a
LineString
geometry, while geometries F, L and
M represent
Point
geometries. Geometries H and I are empty geometries and not visible in this
figure. All geometries are represented in the CRS84 geodetic system, except for geometry M which is
represented in EPSG:4326. Each geometry is represented both using WKT and GML literals.
3.2. Benchmark Queries
We provide here an overview of the approach we had in writing the queries used by
the benchmark to test the requirements of the GeoSPARQL standard. The requirements are
presented in order of the GeoSPARQL extension definitions presented in Section 3. The
benchmark queries are available as part of the benchmark code [
18
], along with a summary
table that maps the requirements to the relevant sets of queries, i.e., tests and sub-tests. The
details about how each test and sub-test is scored are presented in Section 3.3.
Req. 1
Implementations shall support the SPARQL Query Language for RDF [
7
], the
SPARQL Protocol for RDF [21] and the SPARQL Query Results XML Format [22].
We test requirement 1 with a single, basic SPARQL query which selects the first triple
where geometry A is the subject. To get consistent results across different systems, we have
to use a specific subject and have to order the results.
Req. 2
Implementations shall allow the RDFS [
23
] class
geo:SpatialObject
to be used in
SPARQL graph patterns.
Req. 3
Implementations shall allow the RDFS class
geo:Feature
to be used in SPARQL
graph patterns.
Requirements 2 and 3 are tested with single SPARQL queries, which select the first
entity of type
geo:SpatialObject
and
geo:Feature
, respectively. In order to get consistent
results for both queries across different systems, we order the results.
Req. 4
Implementations shall allow the properties
geo:sfEquals
,
geo:sfDisjoint
,
geo:sfIntersects,geo:sfTouches,geo:sfCrosses,geo:sfWithin,
geo:sfContains,geo:sfOverlaps to be used in SPARQL graph patterns.
ISPRS Int. J. Geo-Inf. 2021,10, 487 5 of 19
Listing 1: An RDF excerpt of the benchmark dataset, in Turtle syntax, which represents a
2D point geometry.
1@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
2@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
3@prefix ex: <http://example.org/ApplicationSchema#> .
4@prefix geo: <http://www.opengis.net/ont/geosparql#> .
5@prefix sf: <http://www.opengis.net/ont/sf#> .
6@prefix gml: <http://www.opengis.net/ont/gml#> .
7
8ex:APointGeom
9a geo:Geometry,
10 sf:Point,
11 gml:Point ;
12 geo:isEmpty false ;
13 geo:isSimple true ;
14 geo:dimension 2 ;
15 geo:spatialDimension 2 ;
16 geo:coordinateDimension 2 ;
17 geo:asWKT """
18 <http://www.opengis.net/def/crs/OGC/1.3/CRS84> Point(-83.4 34.3)
19 """^^geo:wktLiteral ;
20 geo:asGML """
21 <gml:Point xmlns:gml="http://www.opengis.net/ont/gml"
22 srsName="http://www.opengis.net/def/crs/OGC/1.3/CRS84">
23 <gml:pos>-83.4 34.3</gml:pos>
24 </gml:Point>
25 """^^geo:gmlLiteral .
Req. 5 Implementations shall allow the properties geo:ehEquals,geo:ehDisjoint,
geo:ehMeet,geo:ehOverlap,geo:ehCovers,geo:ehCoveredBy,geo:ehInside,
geo:ehContains to be used in SPARQL graph patterns.
Req. 6
Implementations shall allow the properties
geo:rcc8eq
,
geo:rcc8dc
,
geo:rcc8ec
,
geo:rcc8po
,
geo:rcc8tppi
,
geo:rcc8tpp
,
geo:rcc8ntpp
,
geo:rcc8ntppi
to be
used in SPARQL graph patterns.
We test each of the requirements 4, 5 and 6 with eight different queries (one per
property), to test the sub-requirements for each property specified. Since the queries for
requirements 28, 29 and 30 require the use of these same properties to test the system’s
compliance to the GeoSPARQL RIF [
24
] rules, we use an approach where the explicit RDF
triples needed to test requirements 4, 5 and 6 involve geometries which are the top result
when using the ordering of the query results. For this purpose, the queries for requirements
4, 5 and 6 order the results, and select the top result only, to ensure they test the existence
of the explicit and materialized RDF triple in the dataset.
Req. 7
Implementations shall allow the RDFS class
geo:Geometry
to be used in SPARQL
graph patterns.
Req. 8
Implementations shall allow the properties
geo:hasGeometry
and
geo:hasDefault-
Geometry to be used in SPARQL graph patterns.
Req. 9 Implementations shall allow the properties geo:dimension,
geo:coordinateDimension
,
geo:spatialDimension
,
geo:isEmpty
,
geo:isSimple
,
geo:hasSerialization to be used in SPARQL graph patterns.
The tests for requirements 7, 8 and 9 are done by selecting all entities of type
geo:Geometry
(Req. 7), or by selecting the object/value of geometry A denoted by the
property in question (Req. 8 and 9). Since requirement 8 specifies two distinct properties,
ISPRS Int. J. Geo-Inf. 2021,10, 487 6 of 19
and requirement 9 specifies six such properties, the tests for these requirements consist of
two and six queries, respectively.
Req. 10
All RDFS Literals of type
geo:wktLiteral
shall consist of an optional URI identi-
fying the coordinate reference system followed by Simple Features Well Known
Text (WKT) describing a geometric value. Valid
geo:wktLiteral
instances are
formed by concatenating a valid, absolute URI as defined in [
25
], one or more
spaces (Unicode U+0020 character) as a separator, and a WKT string as defined in
Simple Features [5].
We test requirement 10 by selecting and checking the datatype of a correctly defined
WKT literal from the dataset, to make sure the system under testing supports the specified
format of WKT literals and their datatype.
Req. 11
URI
<http://www.opengis.net/def/crs/OGC/1.3/CRS84>
shall be assumed as
the spatial reference system for
geo:wktLiterals
that do not specify an explicit
spatial reference system URI.
We test requirement 11 by first defining two geometries in the dataset: J and K, which
represent the same polygon, but geometry K has a WKT literal with an explicitly specified
reference system, while geometry J does not contain the URI and only contains the polygon
points in the literal value:
J: Polygon((-77.089005 38.913574,-77.029953 38.913574,
-77.029953 38.886321,-77.089005 38.886321,-77.089005 38.913574))
K: <http://www.opengis.net/def/crs/OGC/1.3/CRS84>
Polygon((-77.089005 38.913574,-77.029953 38.913574,
-77.029953 38.886321,-77.089005 38.886321,-77.089005 38.913574))
Then, we test whether these two geometries, i.e., their corresponding WKT literals,
are geometrically equal. This ensures that a correct answer to this test means that the
underlying system assumes CRS84 as the default spatial reference system for WKT literals
which do not specify one explicitly.
Req. 12
Coordinate tuples within
geo:wktLiterals
shall be interpreted using the axis
order defined in the spatial reference system used.
In order to test requirement 12, we define two new geometries in the dataset: L and M,
which represent the same point. Geometry L has a WKT literal which specifies the point
using the CRS84 coordinate system, while geometry M uses the EPSG:4326 coordinate
system [
26
]. Compared to one another, these coordinate systems use an inverted axis order:
L: <http://www.opengis.net/def/crs/OGC/1.3/CRS84> Point(-88.38 31.95)
M: <http://www.opengis.net/def/crs/EPSG/0/4326> Point(31.95 -88.38)
In order to test whether the system interprets the axis order correctly, i.e., according to
the spatial reference system, we test if the two geometries are equal based on the system
under testing.
Req. 13
An empty RDFS Literal of type
geo:wktLiteral
shall be interpreted as an empty
geometry.
We define two new geometries, H and I, for the purpose of testing requirement 13.
Geometry H represents a
LineString
geometry which has a WKT literal, which is an empty
string. Geometry I represents an explicitly defined empty LineString geometry:
H:
I: LineString EMPTY
Additionally, as most of the other geometries, these two geometries have a
Point
representation as well. In the case of geometry H, it is again represented by an empty value
of the WKT literal, while geometry I has an explicitly defined empty
Point
geometry in its
WKT literal:
ISPRS Int. J. Geo-Inf. 2021,10, 487 7 of 19
H:
I: Point EMPTY
The test then consists of two parts, where both check if the WKT literals of
H
and
I
are equal. The two parts refer to the separate testing of the equality of the
LineString
geometries and the
Point
geometries. Both parts should be correct in order for requirement
13 to be fulfilled and thus fully scored by the benchmark.
Req. 14
Implementations shall allow the RDF property
geo:asWKT
to be used in SPARQL
graph patterns.
We test requirement 14 by simply selecting the
geo:asWKT
value of geometry A and
checking it against the expected literal value.
Req. 15
All
geo:gmlLiterals
shall consist of a valid element from the GML schema that
implements a subtype of GM_Object as defined in [27].
For the purpose of testing requirement 15, we select all the values of the
geo:asGML
property, regardless of the RDF subject, and check whether all of them contain a valid
GM_Object
subtype in the value and whether its datatype is
geo:gmlLiteral
. The ordered
list of results is then checked against the expected answers, which include all valid GML
literals from the dataset.
Req. 16 An empty geo:gmlLiteral shall be interpreted as an empty geometry.
Similarly to requirement 13, we test compliance to requirement 16 by providing an
empty string as a GML literal value in one geometry—geometry H, and an explicitly
defined empty LineString in a GML literal—geometry I:
H:
I: <LineString><posList></posList></LineString>
Just like with requirement 13, here we use a
Point
representations as well. In the
case of geometry H, it is again represented by an empty value of the GML literal, while
geometry I has an explicitly defined empty Point geometry in its GML literal:
H:
I: <Point><pos></pos></Point>
The test for requirement 16 consists of two parts, as well, where both check if the GML
literals of H and I are equal. The two parts refer to the separate testing of the equality of the
LineString
geometries and the
Point
geometries. Both parts should be correct in order
for requirement 16 to be fulfilled.
Req. 17 Implementations shall document supported GML profiles.
Requirement 17 is the only non-technical requirement of the GeoSPARQL standard,
and therefore cannot be automatically checked and tested. This is the only requirement
omitted by the benchmark tests. To keep it simple, we assume that all GeoSPARQL
implementations fulfill this requirement and provide proper documentation for supported
GML profiles, which we believe to be a reasonable assumption.
Req. 18
Implementations shall allow the RDF property
geo:asGML
to be used in SPARQL
graph patterns.
Similarly to requirement 14, we test requirement 18 by simply selecting the
geo:asGML
value of geometry A and checking it against the expected literal value.
Req. 19
Implementations shall support
geof:distance
,
geof:buffer
,
geof:convexHull
,
geof:intersection,geof:union,geof:difference,geof:symDifference,
geof:envelope
and
geof:boundary
as SPARQL extension functions, consistent
with the definitions of the corresponding functions (distance, buffer, convexHull,
intersection, difference, symDifference, envelope and boundary respectively) in
Simple Features [5].
ISPRS Int. J. Geo-Inf. 2021,10, 487 8 of 19
In order to test requirement 19, we use separate tests for the nine functions in question,
i.e., we check each function separately. To test the full compliance of each function, we
run three sub-tests for them: (a) we test the function with geometry parameters which
are expressed as WKT literals, (b) we test it with geometry parameters expressed as GML
literals, and (c) we test it with a combination of WKT and GML literals. If the function
uses a single parameter, we only use the (a) and (b) sub-tests. If it uses two parameters,
we use the (a), (b) and (c) sub-tests, where (c) consists of two queries in which WKT is
the first and GML is the second parameter of the function (denoted as WKT-GML), and
vice versa (denoted as GML-WKT). With this, the test for each function consists of either
two sub-tests (WKT and GML), or of four sub-tests (WKT-WKT, GML-GML, WKT-GML
and GML-WKT). This ensures that the compliance score for each function is thoroughly
checked. The scoring details for these tests are presented in Section 3.3.
With this, the entire test for requirement 19 consists of tests for the nine functions,
each with two or four sub-tests, for a total of 28 SPARQL queries.
Req. 20 Implementations shall support geof:getSRID as a SPARQL extension function.
We test requirement 20 by using the
geof:getSRID
function in two queries: one with
the WKT literal of geometry A, and the other with the GML literal of geometry A. In both
cases, we check if the system correctly returns http://www.opengis.net/def/crs/OGC/1.
3/CRS84 as an answer.
Req. 21
Implementations shall support
geof:relate
as a SPARQL extension function,
consistent with the relate operator defined in Simple Features [5].
For testing requirement 21, we use a relate operator which denotes the
contains
relation (expressed as
T*****FF*
in DE-9IM [
28
]), and test it on geometries A and B, where
A contains B in the dataset. Given that the
geof:relate
function uses two parameters,
there are four queries for this test: WKT-WKT, GML-GML, WKT-GML and GML-WKT.
Req. 22 Implementations shall support geof:sfEquals,geof:sfDisjoint,
geof:sfIntersects,geof:sfTouches,geof:sfCrosses,geof:sfWithin,
geof:sfContains
,
geof:sfOverlaps
as SPARQL extension functions, consistent
with their corresponding DE-9IM intersection patterns [
28
], as defined by Simple
Features [5].
Req. 23
Implementations shall support
geof:ehEquals
,
geof:ehDisjoint
,
geof:ehMeet
,
geof:ehOverlap,geof:ehCovers,geof:ehCoveredBy,geof:ehInside,
geof:ehContains
as SPARQL extension functions, consistent with their corre-
sponding DE-9IM intersection patterns, as defined by Simple Features [5].
Req. 24 Implementations shall support geof:rcc8eq,geof:rcc8dc,geof:rcc8ec,
geof:rcc8po
,
geof:rcc8tppi
,
geof:rcc8tpp
,
geof:rcc8ntpp
,
geof:rcc8ntppi
as SPARQL extension functions, consistent with their corresponding DE-9IM
intersection patterns [28], as defined by Simple Features [5].
We test requirements 22, 23 and 24 by applying a separate set of tests for each of the
twenty-four functions specified. Each function is tested by employing four queries: one
with two WKT literals (WKT-WKT), one with two GML literals (GML-GML), and two with
a combination of WKT and GML literals (WKT-GML and GML-WKT). Each of the queries
tests if the relation implemented by the tested function is correct for the used geometries
from the dataset, and each of them returns a
xsd:boolean
answer. The geometries used
for the tests of each function are carefully selected in order to provide an unambiguous
assessment of whether the function is supported and correctly implemented in the system
under testing.
Req. 25 Basic graph pattern matching shall use the semantics defined by the RDFS Entail-
ment Regime [29].
For the purpose of testing requirements 25, 26 and 27, we use queries which require
the system to select both materialized RDF triples, as well as inferred RDF triples, based on
the specifics of each requirement.
ISPRS Int. J. Geo-Inf. 2021,10, 487 9 of 19
Therefore, we test requirement 25 using three separate queries: the first one selects
all instances of the
geo:Feature
class, where we expect the system to select instances of
the subclasses of the class, as well, e.g.,
my:PlaceOfInterest
; the second and the third one
select all instances with the
geo:hasGeometry
and
geo:hasDefaultGeometry
properties,
but expect the results to contain entities which use subproperties of these properties, as
well, e.g., my:hasExactGeometry.
Req. 26
Implementations shall support graph patterns involving terms from an RDF-
S/OWL [
30
] class hierarchy of geometry types consistent with the one in the
specified version of Simple Features [5].
For requirement 26, we use two separate queries: they select all instances of
sf:Surface
and
sf:Curve
, respectively, but expect the results to contain all instances of their subclasses
as well, such as sf:LineString and sf:Polygon.
Req. 27
Implementations shall support graph patterns involving terms from an RDF-
S/OWL class hierarchy of geometry types consistent with the GML schema that
implements GM_Object using the specified version of GML [27].
To test requirement 27, we use a single query which selects all instances of
gml:Surface
,
but the expected results include all instances of its subclass, gml:LineString.
Req. 28 Basic graph pattern matching shall use the semantics defined by the RIF Core En-
tailment Regime [W3C SPARQL Entailment] for the RIF rules [
31
]
geor:sfEquals
,
geor:sfDisjoint,geor:sfIntersects,geor:sfTouches,geor:sfCrosses,
geor:sfWithin,geor:sfContains,geor:sfOverlaps.
Req. 29 Basic graph pattern matching shall use the semantics defined by the RIF Core En-
tailment Regime [W3C SPARQL Entailment] for the RIF rules [
31
]
geor:ehEquals
,
geor:ehDisjoint,geor:ehMeet,geor:ehOverlap,
geor:ehCovers,geor:ehCoveredBy,geor:ehInside,geor:ehContains.
Req. 30
Basic graph pattern matching shall use the semantics defined by the RIF Core
Entailment Regime [W3C SPARQL Entailment] for the RIF rules [
31
]
geor:rcc8eq
,
geor:rcc8dc,geor:rcc8ec,geor:rcc8po,geor:rcc8tppi,
geor:rcc8tpp,geor:rcc8ntpp,geor:rcc8ntppi.
We test the requirements 28, 29 and 30 with eight different queries each, in order to
test the sub-requirements for each individual rule specified. The queries used here are
similar to the queries for requirements 4, 5 and 6, with the difference that the tests for
requirements 28, 29 and 30 require both materialized RDF triples and inferred RDF triples
to be selected for the query response. To ensure that the system selects all such entities
and therefore supports the semantics defined in the RIF core entailment regime for the RIF
rules, the tests require an ordered list of entities fulfilling the query request.
3.3. Benchmark Results
The benchmark can test whether the benchmarked system provides a correct or an
incorrect answer on each of the 206 benchmark queries. In order to transform these
individual results into an overall result, we calculate two benchmark results from a given
experiment:
•Correct answers
: The number of correct answers out of all GeoSPARQL queries,
i.e., tests.
•GeoSPARQL compliance percentage
: The percentage of compliance with the require-
ments of the GeoSPARQL standard.
The former is straightforward—it is the number of correct answers the system pro-
vided, out of the 206 test queries. The latter is calculated from the perspective of the 30
requirements and measures the overall compliance of the benchmarked system with the
GeoSPARQL standard. It measures the amount of supported requirements of the system,
out of the 30 specified requirements, where the weight of each requirement is uniformly
distributed, i.e., each requirement contributes 3.33% to the total result.
ISPRS Int. J. Geo-Inf. 2021,10, 487 10 of 19
If a requirement contains multiple sub-test queries, its 3.33% are uniformly distributed
among them. Therefore, for instance, each of the eight sub-requirements of requirement
4 contributes with 12.5% to the parent test score, i.e., with 0.4167% (3.33%
×
12.5%) to the
total benchmark compliance percentage score. This means that a single requirement from
the GeoSPARQL standard can be fully supported, partially supported or not supported
at all.
The only exceptions to this rule of uniform distribution of the weights between tests
on the same level are the sub-test queries which test GeoSPARQL functions with different
serializations of literals as parameters, i.e., requirements 19–24. When we test a function
for compliance to the standard while using (a) WKT-only literals, (b) GML-only literals and
(c) a combination of WKT and GML literals, the score is uniformly distributed between
these three logical groups, each contributing with 33.33% to the parent test score. However,
(c) is practically tested using two queries: one where WKT is the first and GML is the
second parameter of the function (denoted as WKT-GML), and vice versa (denoted as
GML-WKT). These two queries technically contribute with 16.67% to the parent test score
each, so that the total contribution from the logical group (c) remains 33.33%. With this,
the technical weight of the queries themselves is 33.33% for the WKT-only query, 33.33%
for the GML-only query, 16.67% for the WKT-GML query and 16.67% for the GML-WKT
query. Technically, on a query level, this is an exception of the uniform distribution rule we
practice, but, logically, on a group level, it still holds.
Given that requirement 17 is non-technical, and therefore not tested as part of the
benchmark, each system gets its 3.33% score points automatically, when it provides at least
one correct answer to the benchmark tests.
3.4. Benchmark Considerations
When creating the benchmark, we needed to take certain considerations and interpre-
tations which were implicitly given in the GeoSPARQL standard. We elaborate on these in
this subsection.
3.4.1. Geometry Literals
Many results of query functions defined in the GeoSPARQL standard return a
ogc:geomLiteral
as a result, following the GeoSPARQL standard definition. This means
that, according to the standard, a function such as:
geof:boundary(ogc:geomLiteral):ogc:geomLiteral
may take either a WKT, a GML 2.0, or a GML 3.2 literal as an argument, and may return
either a WKT, a GML 2.0, or a GML 3.2 literal as a result. The dataset we use for our
benchmark includes WKT and GML 3.2 formatted literals. However, we provide query
answers in WKT, GML 2.0 and GML 3.2 to support all possible outcomes from a system
tested by the benchmark.
The decision to include only GML 3.2 and not GML 2.0 literals in our dataset was
taken because GML 2.0 has been de-facto superseded by GML 3.2. GML 2.0 is not even
supported as an export option in current GIS software, such as QGIS, for instance. In
addition, in all systems, we benchmarked that the only GML variant that was supported
was GML 3.2.
3.4.2. Variations between Literal Serializations
Within the same literal type, different semantically equivalent representations of
geometries are possible. WKT serializations may include a CRS URI, but they may also
omit it (if it is missing, WGS84 CRS is assumed), and they may differ in the amount and
positioning of whitespaces. GML literals may differ in the order of attributes and definition
of namespaces. To be flexible about these variations, we apply a normalization process
before comparing the results from the tested system with the expected answer. WKT
literals are trimmed and their whitespaces are removed, and GML literals are converted to
canonicalized XML with normalized namespace definitions.
ISPRS Int. J. Geo-Inf. 2021,10, 487 11 of 19
3.4.3. Alternative Answers
The GeoSPARQL standard defines the results of GeoSPARQL functions as
ogc:geomLiteral
values but does not define which geometry types these literals should
serialize. Therefore, functions may not only return results in different literal types, but also
in different geometry representations even within the same literal serialization. One exam-
ple is the
geof:boundary
function which could return a
sf:LinearRing
or a
sf:Polygon
geometry as a result. Even supposedly simple return values such as an
xsd:boolean
may
be represented as either the xsd:boolean literals with value true and false or 1and 0.
In order to deal with these scenarios, we define alternative query answers for each of
the aforementioned possibilities. This means that each test consists of a single query which
is issued to the system under testing, and a set of several alternative correct answers, which
are logically equivalent, but may be technically represented in different serializations.
3.5. Implementation
We have implemented the benchmark as a benchmark for the HOBBIT platform
(Public instance of the HOBBIT Platform: http://master.project-hobbit.eu (accessed on
22 May 2021)), intended for holistic benchmarking of big Linked Data [
17
]. The HOBBIT
platform allows for users to define and execute benchmarks, on one hand, and provide
and add triplestore systems, on the other. A user can run an experiment on the platform
by selecting the desired benchmark and the target triplestore system to be tested. The
platform then loads the benchmark as a set of Docker containers (benchmark controller,
data generator, task generator and evaluation module), loads the system as a Docker
container (benchmarked system), and then runs the benchmark according to its logic,
programmed in the controller (Figure 2). The results of each experiment are stored in the
platform and are made publicly available on the Web.
In our case, the GeoSPARQL compliance benchmark first loads the dataset into the
benchmarked system, then reads all the test queries and sends them to the benchmarked
system for execution. The evaluation module reads the single expected answer or the set
of expected alternative answers for each query, and compares whether the benchmarked
system returns a correct or an incorrect answer, saving the result into the evaluation store.
After all tests are done, the evaluation module calculates two summarized results: (1) the
number of correct answers, out of all possible tests, and (2) the percentage of compliance to
the requirements of the GeoSPARQL standard, as described in Section 3.3.
We decided to use the HOBBIT platform for our benchmark due to its plug-in nature,
in which additional systems can be added by interested users, which will then be able
to run an experiment with the benchmark over their own system. A user can also run
our GeoSPARQL compliance benchmark over any triplestore system which is already
available on the platform. Additionally, the public nature of the platform allows for
greater transparency and reproducibility of the results of each benchmark, including our
GeoSPARQL compliance benchmark.
ISPRS Int. J. Geo-Inf. 2021,10, 487 12 of 19
Figure 2. The HOBBIT benchmarking platform.
4. Experimental Setup
In order to showcase the usability and usefulness of the GeoSPARQL compliance
benchmark, we set out to run a number of experiments over some of the most commonly
used triplestores. The set of chosen triplestores is shown in Table 1.
Table 1. Triplestores which have been tested using the GeoSPARQL compliance benchmark.
Triplestore Version Reference
Apache Marmotta 3.4.0 [32]
Blazegraph 3.1.5 [33]
Eclipse RDF4J 3.4.0 [34]
GeoSPARQL Fuseki 3.17.0 [9,35]
Jena Fuseki 3.14.0 [36]
Ontotext GraphDB 9.3.3 [37]
OpenLink Virtuoso 7.2 [38,39]
Stardog 7.4.0 [40]
TriplyDB 3.5 [41]
For each experiment, a system adapter has been created and published on a public
HOBBIT platform instance, as well as in the HOBBIT GitLab repository (HOBBIT Platform
Triplestores: https://git.project-hobbit.eu/triplestores (accessed on 22 May 2021)). This
allows for the reproduction of the experiments and the results. Each triplestore version
from Table 1was the most recent available stable version of the implementation at the
time of testing. If a triplestore requires a license file (e.g., Stardog, TriplyDB), its tests are
reproducible on the HOBBIT platform only until the embedded license of the integrated
system is valid. When the license expires, any interested party needs to submit their
own instance of the system to the platform in order to test it. For each of the triplestores
which have been tested, a system adapter implementation has been created which handles
the initial configuration of the triplestore, e.g., setting up a repository which contains
the data to be tested, enabling geospatial query support, etc. If possible, this adapter
implementation was added to the triplestore implementation in a joint Docker image or two
Docker images—the adapter implementation and the triplestore implementation—were
created for testing. It needs to be stated that not all of the aforementioned triplestores claim
to support GeoSPARQL. In fact, Blazegraph and Jena Fuseki do not support GeoSPARQL.
We included them in our experiments in order to show which GeoSPARQL requirements
are already supported by a non-GeoSPARQL implementation of an RDF triplestore which
at least supports the SPARQL query language.
ISPRS Int. J. Geo-Inf. 2021,10, 487 13 of 19
5. Results and Discussion
5.1. Overall Results
The results of the experiments with our benchmark and the systems listed in Table 1are
shown in Table 2and in Figure 3, and are available online on the HOBBIT platform (Results
on the HOBBIT platform: https://master.project-hobbit.eu/experiments/1612476122572,
1612477003063,1612476116049,1625421291667,1612477500164,1612661614510,161263753167
3,1612828110551,1612477849872 (accessed on 8 July 2021)). They show that none of
these widely used RDF storage solutions fully comply with the GeoSPARQL standard.
Aside from that, we can point out that one of them stands out with a significantly better
GeoSPARQL compliance score than the others, and, more generally, the top four stand out
from the rest. The triplestores in positions 5–8 share an almost identical result.
Table 2. Results from the GeoSPARQL compliance benchmark.
Triplestore Correct Answers GeoSPARQL Compliance
(out of 206)
GeoSPARQL Fuseki 3.17 177 82.75%
Ontotext GraphDB 9.3.3 80 69.75%
OpenLink Virtuoso 7.2 73 63.46%
TriplyDB 3.5 73 63.46%
Eclipse RDF4J 3.4.0 47 58.33%
Stardog 7.4.0 46 56.67%
Blazegraph 2.1.5 46 56.67%
Jena Fuseki 3.14 46 56.67%
Apache Marmotta 3.4.0 40 46.67%
Figure 3.
Results from the GeoSPARQL compliance benchmark, from the public instance of the
HOBBIT platform.
In order to see the reasons for these variations more closely, we made a breakdown of
the compliance results into the six extensions defined in the GeoSPARQL standard. These
results are shown in Table 3. As we can see from this table, the triplestores in positions
5–8 share the same result due to demonstrating full compliance with the CORE, TOP and
RDFSE extensions of the GeoSPARQL benchmark, but not with the other extensions. The
reason why almost all benchmarked triplestores comply with CORE, TOP and RDFSE is
simple: these requirements are designed in such a way that they are satisfied “out-of-the-
box” by most RDF- and SPARQL-compliant storage solutions. They refer to the use of
specific classes (CORE) and properties (TOP) in SPARQL query patterns, as well as RDFS
reasoning (RDFSE), which are features supported in most triplestores nowadays. Since
RDFS reasoning was not activated in the Marmotta version we benchmarked, it has no
ISPRS Int. J. Geo-Inf. 2021,10, 487 14 of 19
compliance for RDFSE so its score comes only from its compliance with CORE and TOP,
thus is lower than the scores of the other systems.
The bottom three systems are explicitly not GeoSPARQL-compliant, but we included
them in our experiments as baseline tests. As we can see, they all demonstrated compati-
bility with either two or with three extensions of the GeoSPARQL standard
(Table 3)
, and
scored 56.67% or 46.67% of the GeoSPARQL compliance score (Table 2). This, however, does
not mean that the benchmark score should start at 56.67% or 46.67%, since a benchmarked
RDF storage system may fail these tests too.
Table 3.
Support of the different GeoSPARQL extension by the tested triplestores. Full indicates full
support, comprised of correct query answers only, Full/E indicates that support is implemented but
erroneous, Partial [GML/WKT] indicates that support is partially implemented, None indicates that
support for this GeoSPARQL extension is not present.
Triplestore CORE TOP GEOEXT GTOP RDFSE QRW
GeoSPARQL Fuseki Full Full Full/E Full Full
Full/E
Ontotext GraphDB Full Full Partial [WKT] Partial [WKT] Full None
OpenLink Virtuoso Full Full Partial [WKT] Partial [WKT] Full None
TriplyDB Full Full Partial [WKT] Partial [WKT] Full None
Eclipse RDF4J Full Full
Partial [WKT CRS84] Partial [WKT CRS84]
Full None
Stardog Full Full None None Full None
Blazegraph Full Full None None Full None
Jena Fuseki Full Full None None Full None
Apache Marmotta Full Full None None None None
5.2. Discussion on the Results for Each Triplestore
First, we tested RDF triplestores which claim GeoSPARQL support. We wanted to
check how extensive their compliance with the GeoSPARQL standard is, and this list
included: GeoSPARQL Fuseki, GraphDB, Virtuoso, TriplyDB, RDF4J and Stardog.
GeoSPARQL Fuseki is the triplestore with the highest GeoSPARQL compliance score
in our experiments. It is the only system with full GML and WKT support and the only
system with a full implementation of all GeoSPARQL extensions (Table 3). However,
GeoSPARQL Fuseki produced incorrect results in many functions covered by the query
rewrite extension and in a few functions covered by the geometry extension. In addition,
just like all other triplestores we tested, GeoSPARQL Fuseki fails to handle empty WKT
and empty GML literals.
GraphDB provides a full implementation of all but the query rewrite extension. How-
ever, GraphDB can only handle WKT literals but not GML literals. This leads to a substan-
tially lower score in our benchmark, as many queries require either a GML literal as input,
or a combination of a GML and a WKT literal in order to be executed. Most functions with
WKT-only literals in the GEOEXT and GTOP extension tests produced correct results.
Virtuoso provides support for WKT literals, but not GML literals. Similarly to
GraphDB, it provides full implementation for all GeoSPARQL extensions, except for the
query rewrite extension. However, it has an additional issue: even though it returns logi-
cally correct results for the tests for the functions in requirement 19 (part of the GEOEXT
extension), the literals are transformed from WKT literals to an internal literal type which
is Virtuoso-specific. This renders a mismatch between the provided and expected answer,
and lowers the benchmark score for Virtuoso.
TriplyDB is a Linked Data solution which uses Virtuoso Open-Source and Jena Fuseki
for the storage of RDF data on the back-end. We used a Virtuoso-based version in our
experiments. Given that TriplyDB preprocesses the data during ingestion, preprocesses the
SPARQL queries before execution, and postprocesses the SPARQL results after execution,
it might provide different results and demonstrate different behavior compared to using
Virtuoso and Fuseki on their own. However, in the case of our benchmark, TriplyDB 3.5
scored exactly the same as Virtuoso Open-Source 7.2, meaning that the geospatial support
of TriplyDB is identical to Virtuoso.
ISPRS Int. J. Geo-Inf. 2021,10, 487 15 of 19
The RDF4J triplestore implements all the GeoSPARQL functions of the GEOEXT
extension and the GTOP extension for WKT literals. However, RDF4J fails almost all
of the GeoSPARQL tests from these extensions because it does not support CRS URIs
in WKT literals. While the GeoSPARQL standard acknowledges that the integration of
CRS URIs in WKT Literals is optional, they are used in various use-cases, especially at
geospatial authorities, and we expect them to be supported in every triplestore which
claims GeoSPARQL support. Thus, WKT literals with explicit CRS URIs are included in
most of the tests of the benchmark. In addition, RDF4J lacks support for GML literals and
the query rewrite extension.
The Stardog triplestore provides an implementation covering WKT literals and imple-
ments five geospatial functions which are similar to the GeoSPARQL functions, but not
fully compatible. More specifically, out of their five geospatial functions (
geof:within
,
geof:area
,
geof:nearby
,
geof:distance
and
geof:relate
), only the
geof:distance
function follows the signature of the GeoSPARQL function with the same URI. How-
ever, our tests for this function include WKT literals with explicit CRS URIs, which Stardog
does not support, so the test for this function fails. The tests for the other functions fail
either because functions with those URIs do not exist in the GeoSPARQL standard, or
because of a function signature mismatch. Thus, Stardog only scores in tests which cover
the CORE, TOP and RDFSE extensions.
Next, we tested triplestores which do not claim to support GeoSPARQL, but claim
support for other geospatial extensions. We expected that they will provide full support
for the GeoSPARQL CORE, TOP and RDFSE extensions which do not rely on the imple-
mentation of additional geospatial operators. They thereby constitute as baseline tests
for our approach, and this list included: Blazegraph, Jena Fuseki, Apache Marmotta and
Parliament.
Blazegraph supports some non-GeoSPARQL spatial functions in its GeoSpatial Search
Extension (https://github.com/blazegraph/database/wiki/GeoSpatial (accessed on 22 May
2021)). This extension allows the definition of
Points
via WKT literals but is otherwise not
GeoSPARQL-compliant. Blazegraph therefore fails the GEOEXT, GTOP and QRW tests,
as expected.
Jena Fuseki includes a customized spatial extension Jena Spatial (https://jena.apache.
org/documentation/query/spatial-query.html (accessed on 22 May 2021)) which is planned
to be replaced by the GeoSPARQL Fuseki implementation we tested. Jena Fuseki can cope
with WKT literals and defines a custom set of functions, none of which match the function
signatures defined in the GeoSPARQL standard. Hence, Jena Fuseki only gets awarded a
full score in the CORE, GTOP and RDFSE extensions.
Apache Marmotta has a GeoSPARQL implementation which was created in a Google
Summer of Code project (http://marmotta.apache.org/kiwi/geosparql.html (accessed on
22 May 2021)). At the time of testing, the extension was not included in the last stable
version of this triplestore; therefore, the version we tested was not GeoSPARQL-compliant.
Even though Marmotta supports RDFS reasoning, we were unsuccessful in our attempts to
activate it for the instance we worked with, so even though we expected it to achieve the
same score as the other triplestores which do not support GeoSPARQL, it only scored as
compliant with CORE and TOP.
Finally, we want to acknowledge that we also tested the Parliament 2.7.10 triplestore.
Parliament validates WKT and GML literals before they are added to the graph, and fails
to load a dataset if a validation error occurs. In our test, Parliament failed to parse GML
3.2 literals and the empty WKT literals. As a result, the benchmark dataset could not be
loaded, and we could not conduct the experiment with the Parliament triplestore.
6. Limitations of the Benchmark
The GeoSPARQL compliance benchmark does not test every GeoSPARQL function
with every available geometry type and their combinations. We do that with WKT and
GML serializations but not different geometry types. The reason for this is that the amount
ISPRS Int. J. Geo-Inf. 2021,10, 487 16 of 19
of possible combinations of geometries would be inconceivably too large and the benefit of
testing them far too low. WKT defines 27 geometry types, GML defines at least as many
which would need to be considered both in their GML 2.0 and in their GML 3.2 variants, to
be complete. Instead, our dataset consists of
Points
,
LineStrings
and
Polygons
, which
are the most widely used geometry types. With this, we believe we strike a good balance
between the benchmark being too extensive and being sufficiently precise in measuring a
system’s compliance with the GeoSPARQL standard.
Regarding the GeoSPARQL compliance percentage score: as we already stated, this
score measures the number of supported requirements of the system, out of the 30 specified
requirements, where the weight of each requirement is uniformly distributed, i.e., each
requirement contributes 3.33% to the total result. The reason we decided to use uniform
distribution instead of assigning requirement-specific weights is because adding weights
to different requirements would be somewhat arbitrary. Given that the authors of the
GeoSPARQL standard have not discussed or put any variable significance between the
different requirements, gives us a signal that, at least for the time being, we should treat
them as equally important. While that practically is not the case, and different stakeholders
may have different significance implicitly assigned to them, we do not think there is a
better universal way to address this.
7. Conclusions
This paper introduces a GeoSPARQL compliance benchmark which aims to measure
the extent to which an RDF triplestore complies with the requirements specified in the
GeoSPARQL standard. By doing a series of tests for each requirement, the benchmark
is able to assess whether the benchmarked system fully or partially supports a given
requirement, or not at all. The results from the 206 individual tests are transformed into
a GeoSPARQL compliance percentage which aims to provide a metric of the amount of
requirements covered by the benchmarked system.
In order to showcase the usefulness and usability of the benchmark, as part of the
HOBBIT platform, we ran a series of experiments with eight of the most commonly used
RDF triplestores. The overall results show that GeoSPARQL support varies greatly between
the tested triplestores. While the CORE, TOP and RDFSE extensions are supported in
almost every triplestore—as they only depend on SPARQL and RDFS functionalities and
are not GeoSPARQL-specific—the GEOEXT and GTOP extensions show varying levels of
implementation. Some triplestores, such as GraphDB or Virtuoso, chose to only implement
support for WKT literals, RDF4J supports only WKT literals without CRS URIs and only
GeoSPARQL-Jena provides a full GeoSPARQL-compliant implementations of all functions
with both GML and WKT compatibility. GeoSPARQL-Jena is also the only implementation
tested in our benchmark which implements the QRW extension of GeoSPARQL.
In conclusion, we can see that the GeoSPARQL standard, almost nine years after its
initial release, is often only partially supported by major triplestore vendors. We hope
that the contribution of our GeoSPARQL benchmark can help to motivate implementers to
improve their RDF storage solutions, give customers a guideline as to which implemen-
tation is most suitable for their given use-case, and provide a starting point for a further
standard-conform expansion of the geospatial Semantic Web.
Future Work
Recently, the OGC GeoSPARQL Working Group has been reactivated [
42
,
43
] to define
GeoSPARQL 2.0, a successor to the GeoSPARQL standard. It is a good practice of emerging
OGC standards to first be defined, then reviewed, and at the same time also implemented
as a proof-of-concept. During the course of this implementation, compliance testing
becomes increasingly common as can be seen by the establishment of the OGC Team Engine
(https://cite.opengeospatial.org/teamengine/
(accessed on 22 May 2021)), a compliance
test suite which enterprises may use to get official OGC compliance certifications for their
software implementations. Given that currently no OGC-endorsed OGC GeoSPARQL
ISPRS Int. J. Geo-Inf. 2021,10, 487 17 of 19
compliance test exists, we would welcome a collaboration with the OGC and would like to
extend our test suite to cover the changes which will be defined in GeoSPARQL 2.0.
Given that our benchmark is a compliance benchmark, we plan to develop a comple-
mentary performance benchmark, which would test the performance of the tested RDF
triplestore for each GeoSPARQL functions it supports. This would enable a more holistic
approach in the evaluation of geospatial RDF storage solutions. Despite the emergence
of many performance benchmarks for geospatial RDF triplestores (outlined in
Section 2
),
none of the existing benchmarks tests for every function defined in GeoSPARQL on a given
test dataset. Our performance benchmark would be able to utilize the results from the
compliance benchmark, and target the supported GeoSPARQL functions. The HOBBIT
benchmarking platform provides an excellent environment for performance benchmarks,
given that they all share the same infrastructure for the experiments and all results are
reproducible.
Author Contributions:
Conceptualization, Milos Jovanovik; methodology, Milos Jovanovik, Timo
Homburg and Mirko Spasi´c; software, Milos Jovanovik, Timo Homburg and Mirko Spasi´c; validation,
Milos Jovanovik, Timo Homburg and Mirko Spasi´c; formal analysis, Milos Jovanovik, Timo Hom-
burg and Mirko Spasi´c; investigation, Milos Jovanovik, Timo Homburg and Mirko Spasi´c; resources,
Milos Jovanovik, Timo Homburg and Mirko Spasi´c; data curation, Milos Jovanovik, Timo Hom-
burg and Mirko Spasi´c; writing—original draft preparation, Milos Jovanovik and Timo Homburg;
writing—review and editing, Milos Jovanovik, Timo Homburg and Mirko Spasi´c; visualization,
Milos Jovanovik and Timo Homburg; supervision, Milos Jovanovik; project administration, Milos
Jovanovik. All authors have read and agreed to the published version of the manuscript.
Funding: This work has been partially supported by Eurostars Project SAGE (GA no. E!10882).
Data Availability Statement:
The code of the benchmark is publicly available on GitHub, at https:
//github.com/OpenLinkSoftware/GeoSPARQLBenchmark (accessed on 8 July 2021). The results
from the executed experiments are available on the public instance of the HOBBIT platform, at https:
//master.project-hobbit.eu (accessed on 8 July 2021). More specifically, the results from Figure 3are
available at https://master.project-hobbit.eu/experiments/1612476122572,1612477003063, 16124761
16049,1625421291667, 1612477500164,1612661614510,1612637531673,1612828110551,1612477849872 (ac-
cessed on 8 July 2021), where each experiment is linked and can be viewed separately. The detailed
logs from each experiment are also publicly available for download from the same web location.
The HOBBIT platform provides reproducibility of our results, by allowing users to run their own
experiments with the GeoSPARQL compliance benchmark and any system(s) they are interested in
benchmarking.
Conflicts of Interest:
Milos Jovanovik and Mirko Spasi´c work for OpenLink Software, which is the
vendor of Virtuoso, one of the benchmarked triplestores. The funders had no role in the design of the
study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the
decision to publish the results.
Abbreviations
The following abbreviations are used in this manuscript:
CORE Core Component
CRS Coordinate Reference System
GEOEXT Geometry Extension
GML Geography Markup Language
GTOP Geometry Topology Extension
OGC Open Geospatial Consortium
QRW Query Rewrite Extension
RDF Resource Description Framework
RDFS RDF Schema
RDFSE RDFS Entailment Extension
SPARQL SPARQL Protocol and RDF Query Language
TOP Topology Vocabulary Extension
WKT Well-Known Text
ISPRS Int. J. Geo-Inf. 2021,10, 487 18 of 19
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