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Properties, configuration and applications: Oblique airborne photogrammetry

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Abstract

Oblique airborne photogrammetry is rapidly maturing and entering the workflow of service providers. Many applications embrace the advantages of airborne capturing combined with slanted viewing geometry which comes close to human perception of scenes while standing on the ground. Two types of occlusion may be distinguished: self-occlusion, for example when the rear side of a building becomes invisible, and occlusion by other objects. The west view enables the space to be observed between the two rear-side buildings while the south view allows the tower façade to be seen completely. Occlusion can thus be avoided or its effects diminished by increasing the number of images taken from different viewpoints and by increasing overlap. Providers and operators focus on two configurations: fan and Maltese cross. The first comprises two oblique cameras oriented either along or across track; the latter consists of five cameras, four oblique ones and one looking nadir.
| I N T E R N A T I O N A L | JANUARY 20141818
Oblique Airborne
Photogrammetry
PROPERTIES, CONFIGURATIONS AND APPLICATIONS
Oblique airborne photogrammetry is rapidly maturing and entering the workfl ow
of service providers. Many applications embrace the advantages of airborne
capturing combined with slanted viewing geometry which comes close to human
perception of scenes while standing on the ground. The authors provide an
overview of the properties of oblique airborne images, the most common
confi gurations and applications.
Today’s aerial surveys are often
carried out using multiple small or
medium-format cameras mounted
together and simultaneously
capturing nadir and oblique images.
façades, lamp posts, trees and other
vertical structures, but worsens
occlusions and introduces large-
scale gradients from foreground to
background. Two types of occlusion
may be distinguished: self-occlusion,
for example when the rear side of
a building becomes invisible, and
occlusion by other objects (Figure 1).
In the ‘north view’ of Figure 2, the
rear side of the tower is not visible,
and the tower also occludes parts
George Vosselman has
been full professor at
the Faculty ITC, University
of Twente, Enschede, The
Netherlands, since 2004.
He obtained his MSc from
the Delft University of
Technology, The
Netherlands, in 1986, and his PhD from the
University of Bonn, Germany, in 1991. From
1993 until 2004 he was professor at the Delft
University of Technology. His research interest
lies in information extraction from imagery
and point clouds acquired by airborne and
terrestrial sensors.
george.vosselman@utwente.nl
Yoeri Slagboom
has been owner
of Slagboom en
Peeters Aerial
Survey, Teuge,
The Netherlands,
since 1992. The
backbone is the
creation of orthoimagery using a wide
range of cameras owned by the
company. The present fl eet consists of
ve aircraft. In 2009, the company
started developing oblique products.
yoeri@slagboomenpeeters.com
Markus Gerke received an MSc
degree in geodetic sciences and
a PhD, both from the Leibniz
University of Hannover, Germany,
in 2000 and 2006 respectively. He
has been assistant professor at
the Faculty ITC, University of
Twente, Enschede, The
Netherlands, since 2007. He focuses on capturing
geometric and semantic information from images with
the emphasis on automatic processing and
interpretation of oblique airborne and UAS images. He
is co-chair of the ISPRS working group III/4 and
co-organiser of the ISPRS benchmark test on urban
object detection and reconstruction.
m.gerke@utwente.nl
Oblique images capture the scene
under a tilt angle, which is much
larger than in nadir views. e
tilt – the angle between nadir and
optical axis – improves visibility of
GIM0114_Feature_Gerke 18GIM0114_Feature_Gerke 18 07-01-2014 15:04:3907-01-2014 15:04:39
FEATURE
JANUARY 2014 | INTERNATIONAL |
19
BY MARKUS GERKE, YOERI SLAGBOOM AND GEORGE VOSSELMAN, THE NETHERLANDS
visible
self-occluded
occluded by
foreground
Nadir
direction
0
tilt
angle
optical
axis
of the building in the background,
which in turn occludes another
building. In the second north view
image, the façade of the rst building
facing is completely visible. e west
view in Figure 2 enables the space
to be observed between the two
rear-side buildings while the south
view allows the tower façade to be
seen completely. Occlusion can thus
be avoided or its e ects diminished
by increasing the number of images
taken from di erent view points and
by increasing overlap.
CONFIGURATIONS
In 2006, Track Air introduced
the MIDAS small-format system.
Today Leica/Hexagon o ers the
RCD30 Oblique system, where up
to ve digital mid-format cameras
enable the scene to be captured in
nadir and the four cardinal oblique
directions. Another example is
the new Osprey system o ered by
Microsoft. Providers and operators
focus on two con gurations: fan and
Maltese cross. e rst comprises two
oblique cameras oriented either along
or across track; the latter consists
of ve cameras, four oblique ones
and one looking nadir. Common tilt
angles of oblique cameras lie between
30° and 45°. Fan con gurations are
helpful for corridor mapping while
the Maltese cross, employed by
Series on Oblique Photogrammetry
This article is the fi rst in a series on oblique photogrammetry, a technology
introduced by service providers around 2005. The series is intended to cover concepts,
applications and camera systems currently available on the market. Readers are
welcome to contribute. To do so, please contact the editorial manager at
wim.van.wegen@geomares.nl or the senior editor at m.j.p.m.lemmens@tudelft.nl.
Figure 1, Sources of occlusion.
Figure 2, Examples of occlusion and how they can be avoided by multiple views
(Courtesy: Slagboom en Peeters).
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FEATURE
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04-11-2013 10:13:14
MIDAS, Pictometry or BlomOblique,
is used for capturing cities and other
large areas.
MONOPLOTTING
Unlike nadir images, oblique airborne
images depict scenes similar to the
way humans look at objects from
ground view, and therefore they
are often used for reconnaissance,
taxation, registration and inspection.
Tools are available to visualise the
images such as Pictometry Esri
interface or the Slagboom en Peeters
GIS viewer. ese tools are not meant
for measuring. Measuring can be
done by monoplotting, a technique
for collecting 3D coordinates from
a single (oblique) image combined
with a digital terrain model (DTM)
of su cient quality of the same
scene. Oblique images also enable
easy measurement of the height of
buildings and other objects (Figure
3).  e image coordinates of the
top and base point are measured
through heads-up digitisation,
from which dr can be computed.
e ray through the base point and
the projection centre (O) intersects
with a DTM, thus giving the planar
object space coordinates of the base
point. Assuming that the building is
vertical, the top point will have the
same planar coordinates as the base
point which allows to compute the
height di erence (dH) from dr. Such
measurements can be conducted with
the Pictometry Electronic Field Study
(EFS) system and BLOMDesktop
software.
3D CITY MODELLING
Oblique aerial views of urban areas
signi cantly ease the creation of
3D city models. As with mobile
mapping systems, street views of
buildings can be acquired although
the ground sample distance (GSD)
is usually much smaller. A distinct
advantage of airborne images is
that they provide views of the rear
sides of buildings and roofs. Services
enabling highly automatic creation
of 3D city models from oblique views
are BlomUrbex 3D and Slagboom en
Peeters 3D. Eagleview, a subsidiary
of Pictometry, creates detailed CAD
drawings from buildings (roof and
wall report) for easy estimation of
material needs for roof maintenance,
for instance. Visualisation and other
applications may su ce using closed
surface reconstruction alone. Since
2010, dense image matching has
entered photogrammetric production
work ows aiming at one height
value for each and every image pixel.
Meshing of the point clouds enables the
creation of a closed surface description,
followed by texturing. Figure 4 shows
a meshed point cloud. In addition
to semi-global matching (SGM) and
patch-based multi-view stereo software
(PMVS), both established techniques
published in some open source projects,
commercial software such as EADS
Street Factory is available.
CONCLUDING REMARKS
Automation is expected to increase
within scene interpretation and 3D
city modelling in the future.
FURTHER READING
- Fritsch, D., Kremer, J., Grimm, A. (2012), A Case Study of Dense Image Matching
using Oblique Imagery – Towards All-in-one Photogrammetry. GIM International, 26(4):
18-23.
- Furukawa, Y., Ponce, J. (2010) Accurate, Dense, and Robust Multi-View
Stereopsis. IEEE Transactions on Pattern Analysis and Machine Intelligence, 32(8):
1,362-1,376.
- Hirschmüller, H. (2008), Stereo Processing by Semi-Global Matching and Mutual
Information. IEEE Transactions on Pattern Analysis and Machine Intelligence, 30(2):
328-341.
- Höhle, J. ( 2008), Photogrammetric Measurements in Oblique Aerial Images.
Photogrammetrie Fernerkundung Geoinformation, 2008(1): 7-14.
- Petrie, G. (2009), Systematic Oblique Aerial Photography using Multi-frame
Cameras. Photogrammetric Engineering & Remote Sensing,75(2): 102-108.
Figure 3, Principle of monoplotting.
Figure 4, Dense matching result (PMVS2: patch-based multi-view stereo
software) and meshing (Meshlab, Poission reconstruction).
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Conference Paper
Full-text available
Aerial photography has a long history of being employed for mapping purposes due to some of its main advantages, including large area imaging from above and minimization of field work. Since few years multi-camera aerial systems are becoming a practical sensor technology across a growing geospatial market, as complementary to the traditional vertical views. Multi-camera aerial systems capture not only the conventional nadir views, but also tilted images at the same time. In this paper, a particular use of such imagery in the field of building inspection as well as disaster assessment is addressed. The main idea is to inspect a building from four cardinal directions by using monoplotting functionalities. The developed application allows to measure building height and distances and to digitize man-made structures, creating 3D surfaces and building models. The realized GUI is capable of identifying a building from several oblique points of views, as well as calculates the approximate height of buildings, ground distances and basic vectorization. The geometric accuracy of the results remains a function of several parameters, namely image resolution, quality of available parameters (DEM, calibration and orientation values), user expertise and measuring capability.
Article
At the present time, oblique aerial photography enjoys a much higher profile world-wide than it has done for many years. This is due partly to the activities of the Pictometry company in the United States and its numerous licencess and competitors who operate multiple oblique cameras world-wide. These companies are currently acquiring enormous numbers of oblique aerial images in a systematic manner using digital cameras, mainly over urban areas in more highly developed countries. However many other airborne digital frame camera systems based on oblique pointing cameras have also come into use for mapping, reconnaissance and surveillance applications. This article reviews and highlights the technologies that have been developed recently to acquire multiple digital aerial oblique photographs.