GPS SUPPORTED AERIAL TRIANGULATION USING UNTARGETED GROUND CONTROL

GPS SUPPORTED AERIAL TRIANGULATION USING UNTARGETED GROUND CONTROL Mirjam Bilker, Eija Honkavaara, Juha Jaakkola Finnish Geodetic Institute Geodeetinrinne 2 FIN-02430 Masala Finland Mirjam.Bilker@fgi.fi, Eija.Honkavaara@fgi.fi and Juha.Jaakkola@fgi.fi Commission III, Working Group III/1 KEY WORDS: GPS, Aerial triangulation, Block adjustment, Ground control accuracy ABSTRACT Defining the orientations of images has always been a significant expense in mapping projects. Measuring the positions of the camera during the flight with GPS can reduce the costs, because less ground control is necessary to perform the aerial triangulation. The use of untargeted instead of targeted ground control could reduce the costs even more. Investigated is the accuracy of GPS supported aerial triangulation of conventional image blocks. By calculations with simulated image blocks and real test blocks the influence of the GPS accuracy and the accuracy and amount of ground control on the resulting block accuracy are investigated. The empirical tests show that the GPS solutions are not yet fully reliable in this case, but when drift parameters for the GPS are estimated per strip during the aerial triangulation, the results are good. Simulations show that the amount and accuracy of ground control can be reduced considerably in GPS supported aerial triangulation to get the same results as in aerial triangulation without GPS support. With little ground control the results are even improved when the accuracy of the GPS support is bad. Simulated and empirical cases show that with 4 accurate ground control points the results are better than in the traditional case with more ground control and no GPS. Untargeted ground control proved to give acceptable results. The results with 4 road crossings as control points were comparable with results calculated with 4 targeted control points. Ditch crossings gave in this case no acceptable results. 1. INTRODUCTION The wide spread use of geographical information systems has generated a need for the production of medium and small scale topographic data. Potential information sources are small scale aerial images (1:30 000 till 1:60 000) and high resolution satellite images (ground resolution 1-5m). Traditionally defining the orientations of images has been a significant expense in a mapping project. Recent technical developments like in aerial case the direct measurement of orientation parameters by the Global Positioning System (GPS) and inertial navigation systems (INS), have reduced the costs of the orientation process. Methods that totally rely on these techniques without the use of ground control are not yet fully reliable. To get reliable results an aerial triangulation still has to be carried out, using minimal accurate ground control. The use of untargeted ground control could help to reduce the costs even more in cases where the accuracy does not have the first priority. The fieldwork would be reduced in costs and would become independent of the flying date. GPS supported aerial triangulation of conventional blocks of 1:30 000 aerial imagery with the use of little and untargeted ground control is the subject of this study. Investigated is what the influence is of the GPS accuracy and the accuracy and amount of ground control on the resulting block accuracy. The principles of GPS supported aerial triangulation will shortly be discussed in chapter 2. Simulation techniques were used to find the influence of the GPS and ground control under ideal circumstances. The results are described in chapter 3. As last the effects were investigated in an empirical way with a real test field. The results can be found in chapter 4. This investigation is part of the ongoing research project ORIENTATION at the Finnish Geodetic Institute, which concerns orientation of aerial and satellite images for medium and small scale topographic mapping. 2. GPS SUPPORTED AERIAL TRIANGULATION Already in the early days of the Global Positioning System GPS people were thinking of its possible applications in photogrammetry. For example in (Schwarz et al., 1984) and (Ackermann, 1986) GPS is already mentioned in relation with the reduction or even removal of ground control in aerial triangulation. Now GPS has already been fully operational for several years and the use of it for determining the position of the perspective centre coordinates in aerial triangulation has become standard in many cases. The technical details of GPS supported aerial triangulation have been discussed in many publications, for example (Ackermann, 1992), (Blankenberg, 1992), (Burman, 1992), (Burman and Torlegård, 1993) and (Høgholen, 1992). The standard procedure is kinematic differential GPS, with one GPS receiver on the ground and one on the aeroplane. The carrier wave phase is observed and this means that ambiguities have to be solved. Solving the ambiguities is the main problem in the kinematic GPS positioning. An initialisation time is needed to find the ambiguities and every time after an interruption of the satellite signals new ambiguities have to be solved. In the last few years techniques have been developed which can restore the ambiguities after signal interruptions. However these so-called "on-the-fly" (OTF) techniques are not always reliable, especially when distances between ground receiver and moving receiver get large, see for example (Ackermann, 1996). Long

baselines are rather common in GPS flights in Finland and therefore they are also investigated in this study. The GPS processing program used in the study is the Ashtech PNAV program (version 2.1.03P), which calculates an OTF solution. The PNAV manual advises not to try to solve the ambiguities when the baseline distance is bigger than 10 kilometres. For the time to fix the ambiguities ( t) it gives the following approximation (Ashtech, 1994): t (min) = ½ [baseline (km) + recording interval (sec)] (1) In principle the use of ground control in aerial triangulation would not be necessary anymore when OTF solutions are good. However the reliability is questionable and systematic errors are still present. From the different models that exist for GPS supported aerial triangulation mostly a model is adopted with 4 (preferable double) control points in the corners of the block and at least two cross strips. In the block adjustment drift parameters are estimated for the GPS coordinates, either per strip or per block. See for example (Ackermann, 1992). Recent developments integrate an inertial navigation system (INS) with GPS. The two systems cover up for each others shortcomings and this way the position and the exterior orientation of the camera are measured. In principle ground control would not be necessary anymore and even the aerial triangulation could be omitted. More about theory and tests of GPS/INS integration can for example be found in (Cramer et al., 1997), (Schwarz, 1996) or (Škaloud et al., 1996). This study, however, is restricted to the use of GPS only. Many simulations have been carried out of GPS supported aerial triangulations to see the influence of different GPS block configurations on the block accuracy and reliability, see for example (Burman, 1992), (Bouloucos et al., 1992) and (Barrot et al., 1994). In (Ackermann, 1992) the block accuracy is investigated for different control cases when the GPS accuracy or the ground control accuracy is changing. For the different cases formulas are derived, where the block accuracy can be calculated from the standard error and the scale. For example in a case with 4 control points (CP), 4 vertical control points, 2 cross-strips and drift parameters per strip the block accuracy can be found in the following way (Ackermann, 1992): If, = s, = s, then: CP = 0 CP 0 0 GPS 0 0 X,Y 1.5 0 and r.m.s.z 2.0 r.m.s. 0 (2) In this study we investigate also cases were the condition CP 0 is not met. We focus on the influence of the ground control amount and ground control accuracy on the resulting block accuracy. The block accuracy is seen here as the accuracy of the adjusted ground coordinates. Two ways to present the accuracy of the block are distinguished: The theoretical block accuracy. These values are a result of the block adjustment. They are obtained from the covariance matrix of the adjusted ground coordinates in the bundle block adjustment. The empirical block accuracy. After the block adjustment check points are intersected. R.m.s. values are then calculated from the differences between the adjusted ground coordinates and their ground truth values. 3.1. Data sets 3. SIMULATIONS 1:30 000 Simulated data sets of scale 1:30 000 were made to investigate the influence of different parameters on the block accuracy under ideal circumstances. For all cases the following information was kept constant: 68 images, see Figure 1d flying height: 4600 m focal length: 153.28 mm overlap: p = 60%, q = 30% tie points: 5x3 per image s of image observations: s 0 = 5µm 15 cm in object space terrain height: 0 m no tilt or image deformations no GPS drift, but drift parameters per block are estimated in the block adjustment GPS antenna offset = (0,0,0) To see the influence of the ground control points and perspective centre coordinates measured with GPS, the following cases were considered: Influence of the amount of ground control points: Data sets with 4, 8 and 16 ground control points (GCP s) were made, see Figure 1. Influence of the ground control point accuracy: For each ground control point set random errors were generated from a standard normal distribution, N(0,), with = 1, 0.1, 0.3, 0.5, 1.0, 2.0 and 4.0 metres Influence of GPS coordinates for the perspective centres and their accuracy: Perspective centre coordinates were calculated with random errors from a standard normal distribution, N(0,), with = 0.1, 0.3, 0.5, 1.0 and 2.0 metres. Also the case without GPS was investigated. a) 16 Control b) 8 Control c) 4 Control d) Flight Strips Figure 1: Ground control configurations and flight strips in simulation Block adjustments of the possible cases were performed with the ESPA bundle block adjustment program (Sarjakoski, 1988). 3.2. Influence of ground control without GPS support To see what actually is the effect of GPS, first cases are calculated without GPS support. What happens when the ground accuracy is changing is shown in Figure 2. The r.m.s. values are almost the same for north and east. The accuracy in the north direction is about 1cm better than in the east direction. In the graphs this difference can not be seen and the r.m.s. values in east and north direction are therefore shown in one graph.

a. r.m.s. North, East 3.25 3.00 2.75 2.50 2.25 2.00 1.75 1.25 0.0 1.0 2.0 3.0 4.0 sigma GCP (m) b. r.m.s. Height 3.25 3.00 2.75 2.50 2.25 2.00 1.75 1.25 0.0 1.0 2.0 3.0 4.0 sigma GCP (m) 4 GCP s 8 GCP s 16 GCP s Figure 2: Theoretical block accuracy of a simulated block without GPS support and with varying ground control accuracy. As expected the accuracy of ground control is very important in the aerial triangulation without GPS support. In Figure 2 can be seen that the ground control points should have as high accuracy as possible to get high block accuracies. Especially for good results in height many ground control points with high accuracy are needed. 3.3. Influence of ground control with GPS support The influence of the ground control accuracy and amount of ground control on a block supported by GPS coordinates for the perspective centres is much less as can be seen in Figure 3. On the GPS coordinates errors with a standard deviation of 30cm were generated and the accuracy of the ground control points is varying. In Figure 3 can be seen that in case of GPS support the amount of ground control points does not have much influence when the accuracy of the ground control is high. When the accuracy of the ground control gets worse, the block accuracy decreases less fast than in the case of no GPS support (see Figure 2). This effect is biggest for the accuracy in height: the cases with GPS support (Figure 3b) are in height almost twice as good as the cases without GPS support (Figure 2b). When the ground control points accuracy is high, the use of double points will not improve the resulting block accuracy much, compared to the use of only 4 control points. This is because a double point can be seen as a single point with the accuracy of the control accuracy of the double points divided by 2 However the reliability will be better when double points are used. It can be concluded that in GPS supported aerial triangulation with the accuracy of GPS being 30cm or better, the amount of ground control can be reduced considerably and the accuracy of the ground control may be less than in the case without GPS a. r.m.s. North and East 3.25 3.00 2.75 2.50 2.25 2.00 1.75 1.25 0.0 1.0 2.0 3.0 4.0 sigma GCP (m) b. r.m.s. Height 3.25 3.00 2.75 2.50 2.25 2.00 1.75 1.25 0.0 1.0 2.0 3.0 4.0 sigma GCP (m) 4 GCP s 8 GCP s 16 GCP s Figure 3: Theoretical block accuracy of a simulated block with GPS support ( GPS = 0.3m) and varying ground control accuracy. support to get the same results as in the aerial triangulation without GPS support. 3.4. Influence of GPS support To see how important the accuracy of the GPS perspective centre coordinates is, a case was calculated where the ground control accuracy was kept constant at 5cm and the GPS accuracy changes. The resulting block accuracies are shown in Figure 4. a. r.m.s. North, East 0.15 0.05 0.0 0.5 1.0 1.5 2.0 sigma GPS (m) b. r.m.s. Height 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0.0 0.5 1.0 1.5 2.0 sigma GPS (m) 4 GCP s, GPS 4 GCP s, no GPS 8 GCP s, GPS 8 GCP s, no GPS 16 GCP s, GPS 16 GCP s, no GPS Figure 4: Theoretical block accuracy of a simulated block with GPS support. The GPS accuracy is varying and ground control accuracy is GCP = 0.05m.

In Figure 4 can be seen that the block accuracy decreases much slower than the GPS accuracy. When the GPS accuracy is 2 metres the result is in north and east direction practically the same as the case without GPS. In height there is however still a difference when minimal ground control is used. It can be concluded that in case of little ground control with high accuracy even GPS support with low accuracy can make the results better, although those results will not be as good as the cases without GPS and a lot of ground control. When the ground control accuracy decreases the amount of ground control will have to be increased to get still the same results as with little and accurate ground control. 4. TEST FIELD LAHTI 1:30 000 4.1. Description of the test field In the summer of 1997 a conventional 1:30 000 block of colour images was taken by FM Kartta Ltd in the neighbourhood of Lahti, about 90 km north east of Helsinki. During the flight GPS recordings were made on the aeroplane as well as on the ground. Technical details about the flight are the following: dates of flight : July 17th and July 20th, 1997 camera : Leica RC30 + PAV30 stabilised camera mount focal length : 153.28 mm average speed : 95 m/s block size : 21 x 35 km strips : 4 N-S + 2 E-W (cross strips) photo scale : 1:31 000 images : 4 x 13 + 2 x 8 = 68 overlap : p = 60%, q = 30% GPS receivers : Ashtech Z-XII recording interval : 1 sec GPS antenna offset : e x = -0.07m, e y = 0.09m, e z = 1.37m GPS ground station : Helsinki, ± 90 km from test field On the first day only the first cross strip was flown. Besides the station in Helsinki also the stations Vaasa, Kuusamo and Kevo of the Finnish Permanent GPS Network were recording during this flight at a 1 sec interval. These stations are situated at respectively 300, 580 and 990 kilometres from the Lahti area. 49 points were targeted to serve as ground control points or check points. The different ground control configurations can be seen in Figure 5. The images were scanned by FM Kartta Ltd with a pixel size of 25 µm. After the scanning one ditch crossings (ditch-ditch) and one road crossings (road-road or road-ditch) were chosen in the images in the neighbourhood of the 16 signalised points of Figure 1a. These 32 untargeted ground points were measured in the field with GPS. Geodata Ltd performed the image measurements using ESPA digital photogrammetric software, which measures in monoscopic mode. 4.2. GPS accuracy The GPS observations were processed with the Ashtech program PNAV in default settings for an aeroplane. It was tried to solve the ambiguities, although the PNAV manual advises not to try to solve the ambiguities for baselines longer than 10 kilometres (Ashtech, 1994). The coordinates of the perspective centres were obtained by linear interpolation. GPS solution The average r.m.s. values of the antenna coordinates calculated with different base stations are given in Table 1: Base Station Baseline length (m) Average r.m.s. (m) Helsinki day 1 90 0.07 Vaasa day 1 300 0.21 Kuusamo day 1 580 0.35 Kevo day 1 990 0.53 Helsinki day 2 90 0.08 Table 1: Baseline length and average r.m.s. values of the interpolated perspective centre coordinates. In case of the Helsinki base station the ambiguities were successfully solved for the data of the first day resulting in average r.m.s. values of 7cm. The r.m.s. values get bigger when the distance between the ground station and the test field gets longer. The program did regretfully not manage to solve all the ambiguities of the second day, strips 2-6. Only during the last one and a half strips the ambiguities were solved. The other coordinates are thus based on a floating point solution. The ambiguities of the second day were solved after about 90 minutes, while according to equation 1 this should in this case be about 50 minutes. An explanation for this big difference can be that the aeroplane started its mission at a much longer distance from Helsinki. For the first flight the ambiguities were solved already after about 20 minutes, which can be explained by the fact that the aeroplane started from a place closer to Helsinki. The coordinates were transformed to the Finnish national grid, KKJ. With this transformation a systematic error is introduced as the transformation between WGS and KKJ is not exact. The KKJ coordinates of the Vaasa, Kuusamo and Kevo solutions were then compared with the Helsinki solution. The statistics of the differences between the solutions can bee seen in Table 3. a) 16 Control b) 8 Control c) 4 Control d) 31 Check The differences between the Vaasa and Helsinki solutions are very constant. The differences between the Kuusamo and Helsinki solutions are a little less constant and the Kevo solution is the most unstable. Also the differences between the Kevo and Helsinki solution are very big. From this we may Figure 5: Control point configurations and check points in empirical test. conclude that the Kevo solution is not very reliable. The Vaasa solution seems to be good when compared with the Helsinki

dn de dh Vaasa - Helsinki av 0.074 0.120-0.047 0.074 0.120 0.049 6 8 0.012 Kuusamo - Helsinki av -0.012 0.241 0.010 0.016 0.241 0.022 0.011 0.011 0.020 Kevo - Helsinki av 8-3 9 9 4 0.312 0.015 0.027 0.040 Table 3: Statistics of the differences between the GPS solutions in metres. (av = average, = variation around zero and = variation around the average) solution. For the further investigations the Helsinki solution is used. Comparison with traditional block adjustment A traditional bundle block adjustment was carried out with all 49 targeted points as ground control to have an independent check on the Helsinki GPS solution. In this block adjustment Ebner parameters for image deformations were estimated. The perspective centre coordinates calculated in the block adjustment were corrected for the antenna offset using the orientations from the block adjustment. The coordinates were then compared with the GPS solution. The characteristics of the differences can be seen in Table 2. A part of the differences of Table 2 can be dedicated to the error in the perspective centre coordinates of the block adjustment. Their theoretical r.m.s. values are in north, east and height respectively 0.34, 0.35 and 0.15 metres. Other causes for the big variation around the average could be the interpolation method, the varying orientation of the camera with respect to the aeroplane and the GPS solution itself. Strip 1, measured on the first day, shows much bigger differences than the other strips. It might be caused by wrong ambiguity solution. The solving of the ambiguities of strips 5 and 6 does not seem to have any effect on the resulting differences. The differences obviously depend on the flying direction in all strips. A part of this systematic difference could be a wrong recording of the mid-exposure pulse, see for example (Høgholen, 1992). If the rising edge of the signal that comes out of the camera is recorded by the GPS receiver instead of the falling edge, this might lead to a timing error of 10 ms, which results in an offset in the flying direction. However, in this case the settings in the receiver were rightly put at the recording of the falling edge. Other causes for this flight direction dependent behaviour have not been found. strip to front left down strip to front left down 1 av 5.32 5.35 0.57 0.17 0.27 0.06 4 av 2.01 2.07 0.48 0.64 0.39 0.14 0.19 0.13 2 av 3 av 1.97 1.99 0.29 1.37 1.44 0.45 0.54 0.67 0.39 0.41 0.49 0.26 0.33 0.39 0.21 0.24 0.29 0.16 5 av 6 av 1.34 0.68 1.72 1.91 0.84 0.64 0.76 0.41 0.97 1.04 0.37 0.17 0.14 0.26 0.36 Table 2: Statistics of differences between GPS and block adjustment solutions of the antenna coordinates per strip. The values are given compared to the flying direction. (av = average, = variation around zero and = variation around average) Table 3 makes clear that in the GPS block adjustment drift parameters will have to be estimated per strip. 4.3. Block adjustment parameters The aerial triangulation of the Lahti test field was carried out with the ESPA bundle block adjustment program. Block adjustments were done with different amounts and types of ground control and changing amounts of strips. The standard deviations of the different input parameters were: a priori sigma: 0 = 5 µm image observations: tie point = 5 µm targeted = 5 µm untargeted = 7 µm ground observations: targeted = 0.05 m untargeted = m GPS: GPS = 0.15 m In cases with GPS support linear drift parameters were estimated for the GPS observations per strip. After each block adjustment 31 targeted check points (see Figure 5d) were intersected and these ground coordinates were compared with their ground truth values. The empirical r.m.s. values of the differences were calculated to give an indication of the block accuracy. 4.4. Influence of GPS support and amount of ground control To see the influence of the GPS support and the amount of ground control on the block accuracy, the following cases were considered: 6 strips (4+2); no GPS support 6 strips (4+2); GPS support 4 strips (no cross-strips); no GPS support 4 strips (no cross strips); GPS support 6 strips (4+2); GPS support; 12 Ebner parameters for image deformations All these cases were calculated for different targeted ground control configurations, see Figure 5. The empirical r.m.s. values are shown in Figure 6. It can be seen that the influence of the GPS is biggest in the height coordinates. The case with 4 strips, 16 GCP's and no GPS support can be considered as traditional. The r.m.s. values are for north, east and height respectively: 21, 18 and 52cm. The case with cross strips and GPS is in all three ground control cases better, for example the values are 16, 21 and 34cm in case of 4 GCP's. The case with 6 GPS strips and the estimation of Ebner parameters gives the best results with all r.m.s. values below 30cm. The case with 8 control points and cross-strips can be compared with the Ackermann's case as described in chapter 0, see also (Ackermann, 1992). In this empirical case = 15 0 cm and with equation 2 this would lead to the following r.m.s. accuracy of the block: r.m.s. X,Y 23cm, r.m.s. Z 30cm. The r.m.s. block accuracy values in Figure 6 are in this case in north, east and height respectively: 17, 20 and 36cm. The north and east values are indeed smaller than the values given by (Ackermann, 1992), but the height value is a bit worse. If the results are taken from the case where also Ebner parameters are estimated, then also the height value falls within the limits.

a. Empirical r.m.s. North number of GCP s b. Empirical r.m.s. East number of GCP s c. Empirical r.m.s. Height 1.20 1.10 0.90 0.80 0.70 0.60 number of GCP s 6 strips, no GPS 6 strips, GPS 4strips, no GPS 4 strips, GPS 6strips, GPS, Ebner Figure 6: Empirical block accuracy for cases of 4, 8 and 16 targeted ground control points, GPS and no GPS, 4 strips and 6 strips and Ebner image deformation parameters. It can be seen clearly that with the GPS solutions here used, cross strips are really necessary when only 4 or 8 ground control points are used. The cause for this is the necessity of GPS drift parameter estimation per strip. If the GPS solution would be better drift parameters could only be estimated for each flight and then the results without cross strips would become better. 4.5. Use of untargeted ground control points The ground accuracy of the untargeted points -ditch crossings and road crossings- was derived from intersection after a block adjustment with all 49 targeted points as ground control. The average r.m.s. values of the coordinate differences in these untargeted points were the following: road crossings: N = m, E = 0.21m, H = 0.37m ditch crossings: N = 0.58m, E = 0.35m, H = 0.80m The ground accuracy of the ditch crossings appears to be much worse than the accuracy of the road crossings. This is mainly caused by the monoscopic measurements in the images. Stereoscopic image measurements would probably improve the results considerably. Again block adjustments were carried out with 6 strips for the ground control cases of Figure 5, but now with road crossings as ground control. The empirical r.m.s. values of the targeted check points are shown in Figure 7. a. Empirical r.m.s. values North 0.35 0.15 0.05 number of untargeted GCP s b. Empirical r.m.s. East 0.35 0.15 0.05 number of untargeted GCP s c. Empirical r.m.s. Height 0.60 0.55 0.45 0.35 0.15 0.05 number of untargeted GCP s No GPS, road crossings No GPS, ditch crossings GPS, road crossings GPS, ditch crossings Figure 7: Empirical block accuracy for different cases of untargeted ground control Also the case with 16 ditch crossings as ground control is shown in Figure 7. As these results are already quite bad in height, no cases were calculated with 8 or 4 ditch crossings as control points. If we only look at the cases with road crossings as ground control we can say that the results are very satisfactory. The empirical r.m.s. values are comparable with the case of GPS triangulation with targeted ground control. For example with 4 targeted control points the empirical r.m.s. values are in north, east and height respectively: 16, 21 and 34 cm (Figure 6) and in the case with 4 untargeted control points the values are: 15, 20 and 33 cm (Figure 7). We can conclude that road crossings can very well be used as ground control. Ditch crossings in contrary do not give acceptable results in this case. Table 4 gives a summary of the r.m.s. values of the investigated cases with only four ground control points. Shown are also the values for very bad ground control. In the empirical tests the bad ground control accuracy was obtained by generating errors of 3.5m to the ground coordinates of the 4 targeted control points (Figure 5c).

simulations Lahti empirical tests GCP 0 theoretical (cm) 0 empirical (cm) GPS (m) (µm) N E H (µm) N E H no 0.05 5.1 19 20 83 6.4 28 21 45 yes 0.05 5.1 16 16 30 6.4 16 21 34 no 5.1 31 31 86 6.4 34 24 47 yes 5.1 23 23 34 6.4 15 20 33 yes 4.00 5.1 205 204 207 yes 3.50 6.4 160 194 245 Table 4: R.m.s. values of ground points for cases with 4 GCP's and 6 strips. GPS drift parameters were calculated in the simulations for the whole block with GPS = m and in the empirical tests per strip with GPS = 0.15m It can be seen that the values of the empirical tests come quite close to the simulation values. The results in height in cases without GPS are better in practice than in theory. The empirical case with very bad ground control gives high r.m.s. values, especially in height. The bad ground control has a lot of influence on the results, because strip wise GPS parameters are estimated in the calculations. 5. CONCLUSIONS Investigated are the influence of GPS support and the accuracy and amount of ground control on the resulting accuracy in aerial triangulation. Simulations showed that the ground control accuracy is very important in aerial triangulation without GPS support. It should be as high as possible. In GPS supported aerial triangulation the influence of the ground control is less. The amount and accuracy of ground control can be reduced to get the same results as in aerial triangulation without GPS support. With little ground control even GPS support with bad accuracy can improve the results. Empirical tests showed that with a baseline of 90 km the GPS solutions are still not reliable. However the GPS solutions are still useful in aerial triangulation when drift parameters per strip are introduced. Even with only 4 accurate ground control the results of the simulated and empirical GPS supported aerial triangulations proved to be better, especially in height, than the results of the traditional cases with much ground control and no GPS support. The use of untargeted ground control in GPS supported aerial triangulation gave acceptable results as far as it concerned the use of road crossings. The results with 4 road crossings as control points were comparable with results calculated with 4 targeted control points. Ditch crossings gave in this case no acceptable results. 6. AKNOWLEDGEMENTS This investigation has been financially supported by The Technology Development Centre of Finland and the Finnish mapping companies FM-Kartta Ltd and Geodata Ltd. The practical work for the project (production of imagery, field work and image measurements) has been carried out by FM-Kartta Ltd and Geodata Ltd. 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