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This is an Accepted Manuscript of an article published by Taylor & Francis Group in Geodesy and
Cartography on 1/04/2015, available online: http://www.tandfonline.com/10.3846/20296991.2015.1011863
COMPASS MEASUREMENT – STILL A SUITABLE SURVEYING METHOD IN
SPECIFIC CONDITIONS
Julián Tomaštik1, Daniel Tunák2
Department of forest management and geodesy, Technical University, Zvolen, Slovakia
T. G. Masaryka 24, 96053 Zvolen, Slovakia
tomastik@tuzvo.sk1, tunak@tuzvo.sk2
Abstract. The compass measurement is one of the customary surveying methods that is used almost from the
beginning of systematic geodesy and cartography. After the periods of the decreased use of this method, it was
partially renewed by the ascension of the Field-Map technology that connects the compass measurement with the
software tools for the environment mapping. It is currently mostly used for the forestry under-canopy mapping and
other special tasks, where the current progressive technologies, especially photogrammetry and GNSS, can be used
only with complication. The Field-Map set can include either the laser rangefinder Impulse LR 200 with MapStar
Compass Module II compass, or TruPulse laser rangefinder that allows also the azimuth measurement. The research
was aimed on the accuracy of mentioned devices. The measured lengths and azimuth were evaluated primarily. The
evaluation of the field condition results showed the mean length error 0.07-0.09 m for Impulse rangefinder, while
0.25-0.29 m for TruPulse rangefinder. The mean azimuth error was around 0.5 degree for MapStar compass and
above 3 degrees for TruPulse. After this basic evaluation the data were used for the computation of four compass
traverses using various measurement and computation methods. The results showed, that the compass measurement,
especially using the Impulse+MapStar set, can be still a suitable method for lower accuracy surveying, although
effective only in specific conditions.
Keywords: compass measurement, Field-Map, accuracy, forestry mapping, laser rangefinder, electronic compass
Introduction
The use of compass measurement as a surveying method
gradually decreased in the past. It was caused mainly by
the increased number of the objects that interfere with the
Earth’s natural magnetism in urbanized areas and by the
rise of new technologies in the last periods, especially
photogrammetry and GNSS. The ascension of the Field-
Map technology caused a renaissance of compass
measurement as a distinctive method used in forestry
under-canopy mapping and some other specific tasks. It is
currently increasingly used in non-geodetic forestry tasks,
such as national forest inventory (O'Donovan 2007;
Buksha et al. 2010), forests research (Cienciala et al.
2013), and the like. The technology of terrestrial and
airborne laser scanning could complement or even
substitute the use of the compass measurement for this
type of tasks in the future, but there is still a need to
resolve a number of technological and methodological
problems (see e.g. Smreček, Danihelová 2013;
Hackenberg et al., 2014). Another option could be the
indoor positioning systems (e.g. Mautz 2009, Curran et al.
2011), but the main disadvantages – low accuracy,
sophisticated infrastructures, limited coverage area and
inadequate acquisition costs – remain unsolved despite the
partial progress.
The main focus when determining the compass
measurement accuracy should be on primary measured
parameters, i.e. lengths and azimuths. They represent the
polar coordinates of each point and their errors cause the
positional shift. Višňovský, Čihal (1985) derived the
equation for the positional shift of the last point of the
compass traverse:
, (1)
where d is the total length of the compass traverse and s is
the average side length. The 0.004 coefficient takes the
standard errors of the azimuths and lengths into account
when using the devices available at that time. For the
length measurement it was the error ±25 cm at 100 m
using the stadia rangefinder, causing the longitudinal
displacement. For azimuth measurement the error was 10',
which caused lateral displacement 29 cm at 100 m.
The primary objective of this work was to assess the
accuracy and usefulness of the compass measurement
using the electronic devices, which are part of the Field-
Map sets. Sub-tasks were to determine the accuracy of the
measured azimuths and lengths, and then determine the
overall positional accuracy based on the values of the
positional shift of the last compass traverse break-point
and mean coordinate errors. Thereafter assess the usability
of examined measurement for various tasks by comparing
these values with the prescribed criteria.
Materials and Methods
The Field-Map set is modular and its components can be
changed according to user needs. In principle it consists of
surveying equipment to measure angle and length, support
(tripod) and field computer with Field-Map software.
Optionally it may include the GNSS receiver. The devices
for the compass measurements are: (i) TruPulse 360B
laser rangefinder that also allows the azimuth
measurement or (ii) the set of Impulse LR200 laser
rangefinder and MapStar Compass Module II compass.
These tools combined with the simple support (monopod)
were used in the present research. Field computer was not
used because the format of recorded data would not allow
their analysis.
When examining the used electronic devices it is clear
already by using the manufacturer provided data that some
changes in terms of accuracy have occurred. For the
Impulse LR200 rangefinder the manufacturer specifies the
length error 3 cm at 50 m and 5 cm at 150 m, which varies
according to the target quality. It is, therefore, a positive
change over the use of stadia rangefinder. For the MapStar
Compass Module II compass the manufacturer declares
the ±0.3 degree error that is about 20'. This value is two
times worse than the stated error of older devices and at
100 m it causes the lateral displacement of 0.52 m. After
substituting these values declared by the manufacturer, the
equation for calculating the positional shift changes to:
, (2)
For the TruPulse 360B device the declared error values
are even higher. For the length measurement manufacturer
declares the value ±30 cm for targets designated by the
manufacturer as "typical". These are probably targets with
good reflectivity, but it is not stated to which measured
length the error applies. For "very far" targets with less
reflectivity the manufacturer declares the value ±1 m. The
value ±30 cm at 100 m was used for the purpose of
adjusting the positional shift equation. For the azimuths
measurement, the manufacturer declares the mean error
±1 degree. This is a value where it is very difficult to talk
about the use in geodesy, as at 100 meters of measured
length it causes lateral displacement 1.75 m. However,
such a value could be suitable for other applications, in
particular in combination with shorter lengths of measured
sides. Using these values to calculate the mean error
coefficient in the positional shift equation it can be
modified as follows: , (3)
According to the above equations, it is clear that the
accuracy of compass measurement using new electronic
devices is lower to the to conventional compass
measurement devices already according to the values
declared by the manufacturer. This is mainly caused by
the lower azimuth measurement accuracy.
The above equations represent the theoretically achievable
accuracy in the use of these types of devices. It is
questionable in what conditions were the manufacturer
declared values achieved and therefore it is very important
to verify them through the practical measurement. Two
point fields were designed to meet this goal in the first
phase of experiment, separately for lengths measurement
and separately for azimuths measurement. The point field
on the straight part of the road in Technical University in
Zvolen areal was used for the lengths measurement. It
consisted of device occupation point and 11 points, which
were at distances 5, 10 meters and in 10 meter intervals up
to 100 meters. Maximum length was chosen because of
the small magnification of the optics used in specified
devices and because the use of monopod, where the lack
of precise stabilization complicates the measurement of
longer distances. Also the Technical guide of the forest
management (1984) declares the maximum side length of
the compass traverse to 70 m, but this was required when
using conventional devices for compass measurement.
Points were set out using the combination of measuring
tape and TOPCON GPT3002 total station. Despite the
multiple measurements, the lengths cannot be stated as
absolutely precise and it is needed to take the 1 – 2 cm
error into the account, especially for those more remote
points. Points have been set out in two lines in order to
allow the measurement of two alternative lengths. This
procedure was chosen in order to avoid any potential
memory effect of the rangefinder when measuring the
same length. The measurement device and also reflective
prisms were built on tripod.
The measurement of exact azimuth values is quite
difficult, because these vary with place and time. All
systematic errors in their measurements, however, can be
removed by taking the value of the orientation deviation
into account, which essentially represents the difference
between the azimuth and the grid bearing determined for
fixed geodetic points. The determination of the orientation
deviation, which incorporates magnetic declination and
meridian convergence, should be the part of every
evaluation of the compass measurement (Žihlavník 2009).
The simple point field, consisting of device occupation
point and 4 points, was also designed for the purpose of
determining the azimuth measurement accuracy. These
points were situated approximately in the 4 cardinal
directions. First point was stabilized approximately in the
North direction and the other 3 were set out with 100 g
offset using the total station. Points were stabilized at a
distance of about 10 m from the device occupation point
to avoid the ambiguity when targeting as much as
possible. The device has been built on tripod to limit the
impact of imperfect centration, leveling and other errors
which occur when measuring with the use of simple
support (monopod), respectively free-hand.
The basis of compass measurement is a compass traverse,
which presents the polygonal line, where the mutually
independent magnetic azimuths of the traverse sides are
measured.
There are basically two main methods of compass traverse
measurement:
- measurement on each traverse break-point
- measurement on even traverse break-points,
measurement "with skipping".
When measuring using the first method on every break-
point, both direct azimuth to the next point and inverted
azimuth to the previous point are determined. The length
of the side is also measured two times. Due to the higher
labor intensity and the need to comply with the difference
criteria between direct and inverse azimuth is this method
rarely applied in practice. Using the faster and more
economical method of measurement on even break-point
the compass measurement device is built only on each
other break-point. The inverse azimuth of the previous
side and direct azimuth of the next side is measured along
with the side length. The next point is skipped and the
device is built only at the other break-point. Reverse
azimuths are converted to direct azimuths by adding or
subtracting the value of 2R. The disadvantage of this
measurement method is that it is not possible to verify the
correctness of the azimuth and length measurement. The
principle of both methods is shown in Fig. 1.
Fig. 1. Two methods of compass traverse measurement
The point field that was used in the second experiment
phase was founded for the purpose of verifying the
accuracy of various geodetic surveying methods in the
forest environment. It consists of 73 points, whereby four
compass traverses with length of 772.0; 587.88; 426.33
and 166.66 m were used; the number of break-points is
26, 17, 20 and 8. The point field was founded near the
village Sielnica (SK), with the detailed points mainly on
the borders of forest stands belonging to the University
forest enterprise TU Zvolen. Individual points were
stabilized using wooden or iron stakes eventually using
colored cross and surveying nail. The coordinates of
points were determined using the total station TOPCON
GPT3002 and combination of various measurement
methods (polygonal traverse, method of polar
coordinates). Obtained data were used as the reference
etalon for the compass measurement, because according to
the reported standard measurement errors the total station
is significantly more precise than the devices used for the
compass measurement. Also, the experimental
measurements confirmed the possibility of achieving
centimeter accuracy using the indicated total station (e.g.
Žihlavník 2012; Žihlavník, Tunák 2010). The point field
was designed to suit the compass measurement, because in
opposite to for example GNSS there is a need for
intervisibility between the adjacent geodetic points.
Consequently, one of the main disadvantages of the
methods, which require visibility between adjacent points,
is the large number of "surplus" points needed to set-out a
straight line in the forest. The side lengths were in range
from 12.01 m to 84.79 m. Overall, the side length of 60 m
was exceeded only four times. The location of point field
and the course of individual compass traverses is shown in
Fig. 2
Fig. 2. The point field location and the course of individual
compass traverses
The measurement of individual compass traverses was
conducted using the method of measurement on even
traverse break-points, "with skipping". Using this
method, the side lengths and the azimuths were measured
only once, in contrast to the measurement on each break-
point. The tripod was not used unlike the previous phase
of the experiment. The set Impulse + MapStar was built
on a monopod and the measurement was realized with
enabled and disabled “Level Aid” function, which is used
to level the apparatus on the stand. The measurement with
TruPulse 360B took place using a monopod as well as
free-hand without support. The reflective prism was
placed on a pole with leveling according to the bubble
level. All readings were recorded by hand in notebooks,
because the Field-Map software currently does not have
an option to record readings for the purposes of geodetic
measurements. The orientation deviation 8,59o was
determined for the transformation to the used S-JTSK
coordinate system. All azimuths were modified using this
value before the computation of orthogonal coordinates.
Three methods were used to determine the orthogonal
coordinates of compass traverses break-points:
- the calculation of the point coordinates in
succession, without adjustment
- length adjustment of the compass traverse
- the method of polar coordinates, using the etalon
coordinates of device occupation point
The evaluation was conducted separately for each
compass traverse and after that the summary result were
calculated for the entire set of surveyed points. The mean
coordinate error mxy as well as mean errors of measured
lengths and azimuths were used as basic comparative
values for individual compass traverses, as well as the
methods of measurement and evaluation. Those were
calculated as the difference between the etalon values and
the values acquired using the Field-Map set.
Results
Each length of the first phase experimental point field was
measured 30 times. Subsequently, the average value, the
error of average value and the standard deviation were
calculated. Obtained values are shown in Tab. 1.
Table 1. Average values, average value errors and standard
deviation of obtained lengths according to used device [m]
Impulse LR200
Trupulse 360B
length
σ
σ
5
4.98
-0.02
0.06
5.08
0.08
0.2
10
9.95
-0.05
0.03
9.99
-0.01
0.16
20
19.94
-0.06
0.01
19.88
-0.12
0.11
30
29.97
-0.03
0.01
29.89
-0.11
0.07
40
39.98
-0.02
0.01
39.92
-0.08
0.05
50
49.97
-0.03
0.01
49.98
-0.02
0.06
60
60.01
0.01
0.01
60
0.00
0.05
70
70
0
0.01
70.03
0.03
0.05
80
80.01
0.01
0.01
80.03
0.03
0.05
90
90
0
0.01
90.02
0.02
0.05
100
99.98
-0.02
0.01
100.05
0.05
0.05
From the data shown, especially after taking the etalon
point field error into account, it is obvious that used
rangefinders achieved rather good results. It is important
to note that the Impulse LR 200 length resolution is 0.01
m, while for TruPulse 360B it is 0.1 m. The values of
lengths error were tested for the presence of the bias using
the Student t-test. The critical value t0,05(29) = 2,045 was
repeatedly exceeded during the individual lengths tests
using the values obtained by Impulse LR200, as well as
TruPulse 360B. It could be generalized that lengths up to
60 m are slightly underrated, while lengths over 70 m are
slightly overrated using the TruPulse 360B. This is also
evident from Table 1. In most cases, however, the values
are below 0.1 m, what is the TruPulse 360B length
resolution. The test criterion was also similarly exceeded
using the Impulse LR200. However, the average length
errors were relatively small. At such as low values it is
necessary to consider the errors of etalon lengths. Overall,
it can be concluded that the accuracy of length
measurement did not exceed the values stated by the
manufacturer. However, it must be remembered that these
are the values obtained using a tripod and reflective prism.
Both devices have the ability to measure lengths without
the reflective prism, but it is often very problematic,
especially in forest area.
The azimuth measurement in first phase of experiment
was conducted 30 times in all 4 directions. Computed
average values of azimuths, standard deviations and
reduced differences are noted in Table 2. The reduced
differences were calculated using the average value of
first azimuth and consequently the basic direction value.
Table 2. Average values, standard deviations and reduced
differences of obtained azimuths according to used device
Mapstar Compass
Module II
Trupulse 360B
average
value
standard
deviation
reduced
difference
average
value
standard
deviation
reduced
difference
Direction
0o
2,11o
0,32o
0o
2,47o
0,10o
0o
90o
92,29o
0,21o
0,18o
94,35o
0,14o
1,88o
180o
182,19o
0,42o
0,08o
184,45o
0,18o
1,98o
270o
271,97o
0,15o
-0,14o
273,10o
0,12o
0,63o
At a superficial analysis of these values, it would appear
that the MapStar Compass Module II is less precise, since
the dispersion of individual measurements around the
average value is higher. However, when comparing the
values of reduced azimuths, it is obvious that using the
MapStar compass the highest difference is 0.18 degree, in
opposite to 1.98 degree using TruPulse. The result is that
the Mapstar compass measurements were more or less
consistent in whole circle range, but TruPulse acquired
azimuths which were 2 degrees higher in 90o and 180o
directions in comparison with the other two directions.
Nor the recommended calibration did help. It shifted the
angles systematically, but the differences in each direction
remained unchanged. Also, the measurements at another
site confirmed this fact. As there was no opportunity to
compare this measurement with the other measurement
using device of the same type, it would be wrong to
generalize these results. However, if confirmed, this
would be a serious complication, because an error of this
nature is hardly removable by user. The result of the first
phase of the experiment confirmed that the precisions
indicated by the manufacturer are real, but the
measurement of shorter lengths for both types of devices
and inconsistent azimuth measurement using the TruPulse
360B is problematic.
Four compass traverses with different total length and
average length of the side were used in the second phase
of the experiment. Obtained results present the average
values of 4 measurements, conducted in 2010-2014. As
mentioned, when determining the accuracy of surveying
equipment it is necessary to determine the accuracy of the
primary obtained parameters first, which were the lengths
and angles in this case. The length and azimuth errors
were calculated for each side of used compass traverses.
The mean squared error for each variant of measurement
was computed using these errors. The results are listed in
Table 3.
Table 3. Mean squared errors of azimuths and lengths according
to measurement variant
IMPULSE+
MAPSTAR
TRUPULSE 360B
without
leveling
with
leveling
monopod
without
support
azimuth (o)
0,51
0,48
3,12
3,24
length (m)
0,09
0,07
0,25
0,29
Subsequently, the measured lengths and azimuth were
used in the calculation of orthogonal coordinates of all
break-points in the S-JTSK coordinate system. Three
methods of calculation were used. The first was the
calculation of orthogonal coordinates from polar
coordinates using the occupation point coordinates from
comparative etalon, which eliminates the transfer of
errors, because the coordinates of each point are
calculated separately based on the coordinates of the
occupation point and measured azimuth and length.
Furthermore, two methods of traverse evaluation were
used - without adjustment and with length adjustment,
where the break-point coordinates are calculated one after
another, what leads to the transfer of errors. After the
calculation of coordinates, these were compared to the
etalon coordinates. Coordinate errors for each break-point
and subsequently the mean coordinate errors were
computed according to different methods of measurement
and calculation, as is shown in Fig. 3.
Fig. 3. Mean coordinate error values according to measurement
and computation method
The above figure confirms the significant difference
between the accuracy of used devices. Differences for
each device with and without leveling (resp. using
monopod and without using it) are minimal. It should be
noted that the point field was located in a slightly warped
terrain. It can be assumed that the measurement in more
hilly terrain would be more affected by the correct
leveling.
The presented values of mean coordinate errors mxy
depend on the lengths of sides and the general compass
traverse configuration and therefore cannot be regarded as
universally valid. It is needed to judge the measurement
accuracy of used devices according to the positional shift
of the last point, as was marked above in equations (1)-
(3). Višňovský, Čihal (1985) stated that the maximum
deviation qmax=2,5q , (4)
will in case of connected or closed compass traverse
represent the allowable misclosure of numerically solved
compass traverse. Therefore, misclosures of every
compass traverse were determined according to the
measurement method. At the same time, the expected
values of the position shift q were calculated according to
the equations (2) and (3) for each compass traverse and
used device. These values are shown in Tab. 4.
Table 4. Basic characteristics of experimental compass
traverses, predicted positional shifts and obtained positional
misclosures according to used equipment and measurement
variant [m]
traverse
1
traverse
2
traverse
3
traverse
4
total
traverse
length
772.00
587.88
426.33
166.66
average side
length
32.17
36.74
22.44
20.83
qimpulse
0,95
0.88
0.59
0.35
misclosure
Impulse
without
leveling
0.65
1.16
0.82
0.77
Impulse
with
leveling
0.73
0.99
0.98
0.69
qtrupulse
2,84
2.65
1.76
1.06
misclosure
Trupulse
with
monopod
2.98
4.38
0.52
1.52
Trupulse
without
support
5.46
5.4
0.89
1.92
The results showed that the maximum deviation, equal to
2.5q was not exceeded in either case. Using the Impulse+
MapStar set misclosure was 0.7 to 2.2 times higher than
the predicted positional shift, using TruPulse 360B 0.3 to
2.0 times higher. The results also showed that equations
(2) and (3) are suitable for the prediction of compass
measurement accuracy with the use of examined
measurement devices. Of course, the reliability of this
conclusion should be confirmed by additional
measurements of compass traverses with different overall
lengths and average side lengths.
Discussion
The obtained values provide information on practically
achievable frameworks of errors that may be expected
with 68% probability when using examined devices. The
resulting mean error may still contain the random and
systematic part. Random component decreases with
increasing number of measurements, but the systematic
component (the so-called bias) remains constant.
Therefore, when using the surveying equipment, it is
necessary to determine the presence of systematic errors,
and if present eliminate it. The methodology of the study
was adapted to this requirement.
Just a few authors devote themselves to the evaluation of
compass measurement at the present. For example
Potočnik (2010) compares the compass measurement with
Suntoo to the measurement with total station and two
different GNSS receivers. He assesses the compass
measurement as quick and easy, but with low accuracy.
Šebeň et al. (2006) reported the maximum length error up
to 10 cm with the use of Field-Map set including Impulse
LR200 rangefinder. Azimuth measurement accuracy was
not engaged. They evaluate the consequences of errors of
measured lengths in forest inventory, where inaccurately
measured radius of circular inventory plots (even with 1
cm error) can significantly distort the results derived from
the inventory plot data. Ruan (1995) deals with the
transfer from graphic to analytic evaluation of compass
traverse, but works with an ordinary compass. The paper
published by Brach et al. (2013) contains the methodology
and uses the equipment that is closest to the one used in
the presented paper. Authors achieved almost the same
results in terms of mean errors of length and azimuth and
also the alike mean coordinate error.
In general, the examined devices achieved lower accuracy
in comparison to older device for compass measurement.
It is clear already from the accuracy values provided by
the manufacturer. When measuring lengths using the both
devices, paradoxically, the highest deviations and
variances were achieved at lower lengths. When using the
Impulse LR200 rangefinder, the reason could be in the
mismatch of telescope axis and measurement laser. For
lengths up to about 25 meters there is a need to place the
aiming crosshair more or less above the reflective prism,
thereby increasing the ambiguity of the aims and
apparently affecting the elevation angle, which is used in
the calculation of the horizontal distance. When using the
TruPulse 360B, this problem is not so significant, but
nevertheless errors are larger at lower lengths. This
finding is important, because the standard tasks where
Field-Map sets are used, often require the measurement of
lower lengths. Also the principle of the compass
measurement described in equations (1)-(3) says, that in
contrast to polygonal traverse, compass traverse is more
precise when the sides are shorter.
The evaluation of values achieved through the practical
measurement of compass traverses showed, that the values
for Impulse+Mapstar set are twice as high as the value
specified by the manufacturer. However, it should be
considered that the value of the resulting measurement
error includes not only the error of the device itself, but
also a variety of other errors, e.g. imperfect leveling error,
imprecise targeting, non-centric emplacement of the
device and the target, and so on. Similarly, the
comparative etalon values cannot be stated as absolutely
accurate, even when obtained using the devices with
rather high accuracy. Comparing to the conventional
devices for compass measurement, it can be stated that the
mentioned Impulse+Mapstar set achieves better results for
the lengths measurement, but only about 3 times worse
results for azimuth measurement. Using TruPulse 360B,
the length measurement error matches the value stated by
the manufacturer and is comparable to the values
achievable using the stadia rangefinders. In contrast, the
azimuth error is very high. The value above 3 degrees
would virtually rule the possibility of using the device for
surveying tasks out and also the use for other purposes
would be very questionable. Apparently the combination
of azimuths of the compass traverses sides in conjunction
with the inconsistent measurement caused such a high
error. Besides that, it is not a systematic error, since the
critical value for testing the bias was not exceeded. The
existence and the possible solution of such a high error
must be confirmed through the more future measurements.
Comparing the evaluation methods, the polar method with
the use of etalon coordinates of occupation points was
confirmed as the most accurate. In practice, this method
could be used in a combination of the mentioned devices
with other surveying methods, where the position of the
occupation point would be determined by more accurate
method (e.g. GNSS) and the surrounding topography
would be measured using noted devices. It is important
that with this method, the measured points (resp. azimuths
and lengths) do not influence each other. When using this
method and the Impulse + MapStar set, the criterion for
the 4th accuracy class according to the Slovak technical
standard STN 01 3410 was fulfilled, which is required for
the cadastral mapping outside the urbanized areas (forests,
agricultural areas). The other two methods represent the
traverse solution; ergo the coordinates of the points are
calculated sequentially and are mutually connected. This
causes the significant increase of the mean coordinate
error. The calculation without adjustment is a standard
procedure of the Field-Map use, where the first occupation
point is a known geodetic point, but the traverse is not
finished on another known point. Therefore the
adjustment is not possible. The calculation with
adjustment is possible when the compass traverse is
inserted between two known geodetic points. In that case,
the length adjustment is possible. The considerable
information for the practice is, that the final accuracy
within 1 meter could be achieved using the
Impulse+Mapstar set, which corresponds to the sum of the
traverse calculation without adjustment (establishing
occupation points) and polar method (measurement of
surrounding topography). The values obtained using
TruPulse 360B were significantly higher. Another
criterion, used in practice of Slovak forestry mapping is,
that the maximum misclosure of graphically evaluated
connected or closed compass traverse should not exceed
the 2% of the total traverse length. However, the accuracy
of the graphical evaluation is reflected in this criterion in
addition to the measurement precision. For the
experimental traverses this value means 15.4 m for
traverse 1, 11.8 m for traverse 2, 8.6 m for traverse 3 and
3.4 m for traverse 4. The obtained real misclosures are
lower than noted maximum misclosures in every case.
Based on the obtained results it can be stated that compass
measurement, even when using electronic devices, may be
still an applicable method for surveying tasks with lower
accuracy demands. These results showed that the source
of the highest errors is the azimuth measurement. These
cause the lateral displacement depending on the length of
the side to which was subsequently added the longitudinal
displacement caused by the error of the length
measurement. Thus it is confirmed, that it is preferable to
choose a larger number of shorter sides when using
compass measurement. The remaining advantage of
compass measurement over the use of theodolites,
tachymeters, respectively total station remains is the lower
demand on the existing point field, because no orientation
points are necessary for determining the basic direction. It
is especially convenient in rural areas where the point
field is sparse. Similarly advantageous is the possibility to
use a monopod, which facilitates and accelerates the
device stationing in the field. Despite the above result the
efficient use of compass measurement is currently more or
less limited only to under-canopy measurements in
forests, underground areas and other areas especially with
the obscure reception of GNSS signals. The reason is the
high efficiency of the photogrammetric evaluation of
elements that are identifiable on photogrammetric images
as well as the possibility of using global navigation
satellite systems. The accuracy of GNSS methods in areas
with good signal reception is much higher, while the time
consumption can be lower when compared to the compass
measurement. The results of the evaluation method using
the etalon coordinates of occupation points may show the
potential for increased accuracy in combination with other
methods, where the more accurate method will be used to
achieve the coordinates of occupation point (e.g. GNSS)
and the surrounding topography would be measured using
the method of polar coordinates for compass
measurement. The practical problem for using such a
combination, especially in forestry mapping, is, that the
linear objects, which are most widespread in forestry
mapping, cannot be measured from one occupation point,
while the correct determination of position using GNSS in
forest is problematic.
When using the national grids, as was the S-JTSK system
in actual study, it is very important to include the value of
the orientation deviation. It cannot be substituted by
magnetic declination; the meridian convergence must be
also taken into account. Therefore it is necessary to
determine the orientation deviation for every compass
measurement either by direct measurement (comparison
of grid bearings and azimuths on the known points), or by
calculating the magnetic declination and meridian
convergence for the particular area.
Conclusion
The current period is characterized by an increased
demand for spatial information in virtually all fields of the
human activity. These are often made available to the
wide public. The quality of the data, which is conditioned
by their origin, is often overlooked during the process of
creating and filling a variety of geographic information
systems. In the process of the introduction of new
technologies for obtaining these data it is therefore very
important to look not only on the efficiency of their use,
but also on the spatial accuracy of the data.
With regards to the achieved results the following
conclusions can be drawn:
- the mean coordinate error under 0.5 m can be achieved
by using the Impulse+MapStar set and compass
traverses with short sides and short total lengths. The
results when using the TruPulse device are many time
worse.
- the main source of error of examined devices is the
azimuth measurement.
- the fact, that compass traverses with shorter average
side lengths are more accurate, remains valid.
- it is necessary to keep clean from objects that affect
the Earth's natural magnetism.
- in terms of efficiency and relative small effect of
leveling choose as simple set as possible (preferably
with monopod).
- it is important to take the value of orientation deviation
into account when connecting the measurement into
the national grid.
The compass measurement is currently mostly used in
forestry research and forestry inventory. For this purpose,
it is sufficiently effective and fast. . The measurements in
forestry research are conducted mostly in local coordinate
system that is generally not necessary connected to the
geodetic points. In the field of research and inventory the
compass measurement successfully replaces customary
surveying methods, based on the measurement method in
the local orthogonal coordinate system using simple tools
such as stakes and steel tape. Compared to these methods,
the compass measurement with the use of laser
rangefinder and electronic compass is less laborious and
time-consuming. The important contribution to the
application of compass measurements as an appropriate
method for detecting the state of the environment is the
ability to use specialized software for automated
processing of measured data, which represents the
perspective for the preservation and further development
of tools for compass measurement. The main disadvantage
is the smaller effectiveness and accuracy in most areas,
where the GNSS or photogrammetry can be used. For that
reason the effective use of compass measurement is
currently limited to the forest under-canopy measurements
and some other specific measurements (for example
subterranean) where the actual most efficient methods
cannot be used without complications.
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Acknowledgement
This publication is the result of the implementation of the
project VEGA MŠ SR and SAV no.
1/0804/14:”Actualization of mapping, arrangement of
forest land ownership and determination of landscape
status by modern methods of geodesy and aerial survey”.
