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ifp

Universität Stuttgart

The Geometrical Accuracy of the VisionMap A³ Camera System

Final Report

Dieter Fritsch & Michael Cramer

Institute for Photogrammetry (ifp)

University of Stuttgart

Geschwister-Scholl-Str. 24 D

D - 70174 Stuttgart / Germany

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Final Report

The Geometrical Accuracy of the VisionMap A³ Camera System Empirical Testing in ifp Vaihingen / Enz Test Site

Dieter Fritsch & Michael Cramer Institute for Photogrammetry (ifp)

University of Stuttgart Geschwister-Scholl-Str. 24 D

D - 70174 Stuttgart / Germany

Contents

Project history ............................................................................................................................ 3

Empirical accuracy investigations .............................................................................................. 4

Comparison to DGPF-test results ............................................................................................. 11

A³ geometric accuracy in inner part of the block ..................................................................... 15

Summary .................................................................................................................................. 18

References ................................................................................................................................ 18

Appendix................................................................................................................................... 19

Stuttgart, June 14, 2012

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Project history The A³ camera test flight was flown over Vaihingen/Enz on Saturday, June 13, 2009, 08:07 – 08:48 UTC. The flying height / GSD has been proposed by VisionMap and agreed with ifp during the project preparations. Flights were proposed with GSD around 5cm. The flight lines of the later flight are plotted in Figure 1. This figure was made available by VisionMap. In total, 8 flight lines were necessary to cover the whole test site with almost 90% side lap. The corresponding GSD is 6cm, but this only is valid for the center part of the strips. GSD values will largely increase to the strip boarders because of the panoramic side viewing concept of the A³ system. All accuracy investigations are obtained from this flight only.

Figure 1: Flight pattern of empirical A³ flight test in Vaihingen/Enz @ June 13, 2009

(image provided by VisionMap)

The basic flight parameters are as follows (information provided by VisionMap):

• average altitude – 1972m; flight line direction – east-west, bi-directional;

• average ground speed – 113knot, difference in ground speed in two directions – 23 knot;

• GSD – 6cm;

• number of flight lines – 8;

• average forward overlap – p = 52%; average side overlap – q = 86% (when all 8 flight lines are considered);

The (pre-)processing of image data was exclusively done by VisionMap. A small sample image (about 2000 x 1000 pix size), which showed the Siemens star, was delivered to ifp on June 16, 2009, right after the test flight. Until now no other image data are available at ifp.

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The later image orientation / aerial triangulation was based upon up to 10 control points which ifp made available to VisionMap. This processing also was done at VisionMap only, without any additional interaction with ifp. Thus the following accuracy assessment presents a completely independent investigation of the full VisionMap processing pipeline including the personal experience!

Empirical accuracy investigations The external quality (accuracy) of 3D object point determination is analyzed from individual check point differences. About 162 signalized coordinated points, scattering the whole Vaihingen/Enz test field, have been used for this analysis. The point coordinates have been measured with static GPS-survey. The accuracy of these reference points is within the 1cm and 2cm range for horizontal and vertical component. Figure 2 shows the distribution of signalized points in the Vaihingen test field. The 10 control points are highlighted in white color; their coordinates were used by VisionMap to run different AT versions. The light-blue marked points served as check points only. Their coordinates are only available to VisionMap with a reduced accuracy of about 1m. Thus the analysis of check point differences is an independent estimation of the external accuracy obtained from this test data set.

Figure 2: The control and check point distribution of the Vaihingen/Enz test field

(Status: June 2009)

Overall four different versions of adjusted check point coordinates from AT have been delivered to ifp in a first step. As explained in the accompanying letter from April 4, 2012 four different versions have been computed using VisionMap LightSpeed software. Within the GPS shift enabled versions shift corrections in all three coordinate components have been considered.

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• Case 1 – 5GCP_Shift0: all 8 flight lines, adjustment based on 5 GCP with GPS shift disabled

• Case 2 – 5GCP_Shift1: all 8 flight lines, adjustment based on 5 GCP with GPS shift enabled.

• Case 3 – 10GCP_Shift0: all 8 flight lines, adjustment based on 10 GCP with GPS shift disabled

• Case 4 – 10GCP_Shift1: all 8 flight lines, adjustment based on 10 GCP with GPS shift enabled.

In a second step additional (up-dated) versions were provided by VisionMap to be analyzed against reference point coordinates. These versions have been made available to ifp after the results from cases 1-4 have been discussed. It is again four cases (cases 5-8), based on newly measured image point coordinates. Additionally, within these versions the number of flight lines used in bundle adjustment is reduced from 8 flight lines to 5 flight lines only. This is of direct influence on the effective side lap between the overlapping flight lines. As explained by VisionMap (accompanying letter from April 28, 2012), “…the average side overlap (in A3 flights) is 86%. With maximal FOV of 109°, this overlap is more or less a regular overlap for the A3 system for urban area mapping with low speed aircrafts, when the distance between flight lines are defined by relatively small (10°-20°) off-nadir angle to ensure a low building leaning on the orthophoto. For the rural area, where requirements for building leaning are lower, the off-nadir angle may reach 30°-32°. In this case the side overlap may be at 60% -70%. It is common practice and recommendation for the A3 users to fly with at least 60% side overlap. In order to evaluate the accuracy obtained from lower side overlap, every second strip (starting from the third one) from the original flight was removed in the AT. This reduced a number of strips from 8 to 5 and the side overlap - from 86% to 73% in average. In the north part of the block, two upper strips with 86% side overlap have been left, that enabled GCP measurements in this part. In the south part of the block, the GCP measurements were enabled with the new 73% side overlap.” The following Figure 3 illustrates the updated flight strip configuration.

Figure 3: Five strip configuration to reduced side overlap of flight lines

(A³ flight test in Vaihingen/Enz @ June 13, 2009, image provided by VisionMap)

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The different cases are explained in the following. Notice that the principle block configuration – compared to cases 1-4 (8 flight lines with high side overlap) – was unchanged for case 5 and 6, the only difference here was the re-measuring of image observations by VisionMap. Thus the accuracy difference between case 5 & case 2 and case 6 & case 4 only refers to the influence of re-measured image observations.

• Case 5 – all 8 strips, 5 GCP, GPS shift enabled, can directly be compared to version 2.

• Case 6 – all 8 strips, 10 GCP, GPS shift enabled, can directly be compared to version 4.

• Case 7 – only 5 strips, 5 GCP, GPS shift enabled.

• Case 8 – only 5 strips, 10 GCP, GPS shift enabled. The adjusted object point coordinates for all 8 versions, which are used to compute the absolute accuracy from check point differences can be found in the Appendix (see Figure 10 & Figure 11, starting page 19). The statistical analysis of all eight different versions is given here. The RMS values from check point differences are shown in Figure 4. The detailed statistics for cases 1-8 are shown in the following tables. These numbers are obtained from 162 check point differences; the tables are followed by two vector plots, which (exemplarily) depict the difference vectors for the case 1 (Figure 5) and case 2 (Figure 6) configuration.

Figure 4: Comparison of accuracy (RMS) [m] for those versions where GPS offset corrections are applied.

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0.07 DEast DNorth DVertical

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Case 1 – 5GCP_Shift0 (162 check points, all 8 flight lines, no GPS shift enabled)

DEast DNorth DVertical

Mean [m] -0.0032 -0.0154 -0.1079

Std.Dev. [m] 0.0159 0.0213 0.0383

RMS [m] 0.0163 0.0263 0.1145

Max.Diff. [m] 0.0470 @ PtNo 2253

0.0882 @ PtNo 2293

0.1835 @ PtNo 2253

Maximum difference (vector): 0.1916m @ PtNo 2253 Case 2 – 5GCP_Shift1 (162 check points, all 8 flight lines, GPS shift enabled)

DEast DNorth DVertical

Mean [m] -0.0110 0.0061 0.0135

Std.Dev. [m] 0.0177 0.0285 0.0540

RMS [m] 0.0209 0.0292 0.0556

Max.Diff. [m] 0.0588 @ PtNo 3051019

0.0863 @ PtNo 508019

0.1475 @ PtNo 108019

Maximum difference (vector): 0.1711m @ PtNo 108019 Case 3 – 10GCP_Shift0 (162 check points, all 8 flight lines, no GPS shift enabled)

DEast DNorth DVertical

Mean [m] -0.0058 -0.0131 -0.0801

Std.Dev. [m] 0.0169 0.0202 0.0346

RMS [m] 0.0179 0.0241 0.0873

Max.Diff. [m] 0.0488 @ PtNo 3051019

0.0782 @ PtNo 2293

0.1545 @ PtNo 508019

Maximum difference (vector): 0.1577m @ PtNo 508019 Case 4 – 10GCP_Shift1 (162 check points, all 8 flight lines, GPS shift enabled)

DEast DNorth DVertical

Mean [m] -0.0019 -0.0062 -0.0121

Std.Dev. [m] 0.0167 0.0210 0.0406

RMS [m] 0.0168 0.0218 0.0424

Max.Diff. [m] 0.0450 @ PtNo 105019

0.0882 @ PtNo 2293

0.1395 @ PtNo 106019

Maximum difference (vector): 0.1544m @ PtNo 2293

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Case 5 – 8strips_5GCP (162 check points, all 8 flight lines, GPS shift enabled, re-measured image observations)

DEast DNorth DVertical

Mean [m] 0.0096 -0.011 -0.0218

Std.Dev. [m] 0.0163 0.0264 0.0537

RMS [m] 0.0190 0.0286 0.0580

Max.Diff. [m] 0.0503 @ PtNo 207019

0.0893 @ PtNo 508019

0.1625 @ PtNo 2293

Maximum difference (vector): 0.1841m @ PtNo 2293 Case 6 – 8strips_10GCP (162 check points, all 8 flight lines, GPS shift enabled, re-measured image observations)

DEast DNorth DVertical

Mean [m] 0.0017 0.0043 0.0097

Std.Dev. [m] 0.0148 0.0206 0.0398

RMS [m] 0.0149 0.0210 0.0410

Max.Diff. [m] 0.0407 @ PtNo 2563

0.1082 @ PtNo 2293

0.1845 @ PtNo 2293

Maximum difference (vector): 0.2141m @ PtNo 2293 Case 7 – 5strips_5GCP (162 check points, 5 flight lines only, GPS shift enabled, re-measured image observations)

DEast DNorth DVertical

Mean [m] 0.0048 -0.0154 -0.0130

Std.Dev. [m] 0.0175 0.0218 0.0637

RMS [m] 0.0181 0.0267 0.0650

Max.Diff. [m] 0.0645 @ PtNo 2693

0.0772 @ PtNo 2293

0.2165 @ PtNo 504019

Maximum difference (vector): 0.2245m @ PtNo 504019 Case 8 – 5strips_10GCP (162 check points, 5 flight lines only, GPS shift enabled, re-measured image observations)

DEast DNorth DVertical

Mean [m] 0.0007 -0.0037 0.0113

Std.Dev. [m] 0.0170 0.0196 0.0514

RMS [m] 0.0170 0.0200 0.0526

Max.Diff. [m] 0.0535 @ PtNo 2693

0.0892 @ PtNo 2293

0.2025 @ PtNo 2293

Maximum difference (vector): 0.0221m @ PtNo 2293

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Figure 5: Check point difference vector plot (Case 1), scale vector (blue): 10cm.

Figure 6: Check point difference vector plot (Case 2), scale vector (blue): 10cm.

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The following comments can be drawn from the tables and figures:

• The influence of additional GPS drift correction parameters does improve the accuracy (case 1 & 2 and case 3 & 4). This can especially be seen from the vertical component. In cases 1 and 3 (5 GCP and 10 GCP) clear systematic offsets can be seen in the vertical component. This offset is around -10cm, which causes much larger RMS values. This can also be seen from Figure 5. After GPS offset correction the values of Std.Dev. and RMS are almost the same, which indicates, that the differences are free of any mean now. GPS offsets, or better to say – position offsets, which are constant for the whole block – are present in many data sets. They effectively are compensated by additional GPS offsets even though they also might be due to non GPS error sources, like not perfectly known camera calibration parameters.

• The versions based on the re-measured image coordinates have almost the same accuracy than the previous results from cases 1 – 4. In terms of absolute accuracy (RMS) there is almost no difference visible when comparing case 2 & case 5 and case 4 & case 6.

• The use of 10 GCP compared to the 5 GCP situation does improve the accuracy. This is valid for all configurations. In horizontal components the accuracy increase is almost in the sub-centimeter range, in the vertical component the accuracy (RMS) is increasing around 1cm. Still, not in all (operational) flight scenarios 10 GCP might be available. As the accuracy increase is relatively small when changing from the 5 to the 10 GCP distribution, the A³ block obviously is sufficiently controlled with the 5 GCP configuration – including the additional information from GPS (with GPS offset correction) and the high (cross-)overlap between imagery. This also is valid for the 5 strip configurations (case 7 & 8).

• The horizontal accuracy is very high. The RMS in east and north component is within the 2cm range for case 4 and case 6. Considering the reference GCP accuracy, which is assumed to be in the range of 1cm for horizontal components, the obtained value from A³ imagery is close to the reference accuracy. The statistics obtained from the check point differences might (partially) be influenced by the reference accuracy.

• The vertical accuracy is slightly worse and reaches about 4cm for the 10 GCP case with GPS offset correction (Case 4 & 6). This might be due to the base to height ratio and the intersection geometry of the specific A³ arrangement. As the detailed system parameters are currently not available to ifp the theoretical vertical accuracy from base-to-height ratio cannot be estimated. If one relates the empirical vertical accuracy of about 5cm to the flying height above ground (around 1700m) the obtained factor is around 0.03‰, which is well within the theoretical accuracy. But it should be mentioned, that the A³ is somewhat unusual as small GSD values can already be obtained from relatively large flying heights.

• The use of 5 flight lines instead of 8 flight lines has an influence on the absolute accuracy. This can be seen from comparison of case 5 & 7 (5 GCP configuration) and case 6 & 8 (10 GCP configuration). It can be seen, that accuracy (RMS) decreases mainly for the vertical component. This decrease reaches up to 1cm for the 10 GCP case. The accuracy in horizontal is almost the same – interesting to see, that the 5 flight line case is slightly better, but changes are really small, only in the mm-range. This also might be due to the quality of reference values, which is in the 1cm range also.

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Comparison to DGPF-test results In 2008 the German society of Photogrammetry, Remote Sensing and Geoinformation (DGPF) initiated a test on the evaluation of digital airborne camera systems. The focus of this test not only was on the analysis of the geometrical accuracy performance – which typically is / was of prime interest in classical photogrammetry – but also on the radiometric quality and the quality of derived products like 3D surface models. The whole test was coordinated at ifp under the umbrella of the DGPF. The ifp also played an important role in the geometrical accuracy and 3D surface model generation expert groups, which were established to structure the overall project evaluation. The project design and project progress was continuously documented on the DGPF web (but in German mostly, see DGPF 2012). A special issue of the PFG, the official regular journal of the DGPF, was dedicated to present the results of the test in a comprehensive way. This PFG 02/2010 contains the project reports of all expert groups in the test and is available online (PFG 2010). The empirical testing of the airborne systems was also done in the Vaihingen/Enz test field, using a quite similar check and control point configuration like for the VisionMap A³ test flight. The detailed flight configurations can be found in Cramer (2010). Most sensors were flown in two different flying heights, resulting in two blocks with the ground sampling distances of 20cm GSD and 8cm GSD (nominal values). The 20cm GSD blocks covered the whole test area; the GSD 8cm blocks were limited to the center part. The 20cm GSD blocks were flown with a forward overlap of p = 60%, whereas a higher forward overlap of p = 80% was aimed at the 8cm GSD blocks. Not for all systems this 80% forward overlap was finally realized, due to system limitations. The side overlap between image strips was consistently defined with q = 60%, but slight adaptations of the block geometry were necessary due to the given test site extensions and different sensor formats (mainly influencing the later side overlap). Results from the ifp evaluations of DGPF data sets are discussed in Cramer & Haala (2010). The results for the GSD 8cm flights of ZI-Imaging DMC camera (1st generation), Vexcel Ultracam-X and Leica Geosystems ADS 40 (2nd generation, sensor head SH 52) are cited below. These accuracy numbers will serve as comparison to judge the performance of the A³ system as their nominal GSD of 8cm is close to the GSD 6cm of the VisionMap test scenario. Still, significant differences in block geometry are present and should be considered when comparing the accuracy numbers. Especially different overlap conditions, influencing the individual block geometry, have to be mentioned here. Within the following configurations cross strips have been considered except for the ADS40 block. The nominal GSD also is different, thus the tables also show the RMS and Std.Dev. in relation the GSD, to make the numbers more comparable. The DGPF GSD 8cm flights were only done in the center part of the Vaihingen/Enz test field, thus covering a smaller area only. This also influences the number of available check points. And finally the results presented from the DPGF flights are only obtained from 4 control points – each point located in one corner of the block –, but additional observations from GPS/inertial data are also considered in AT. For reasons of comparability the A³ results of Case 2 are used only and added in the final part of the table. This 5 GCP case is chosen, because it is more similar to the 4 GCP configurations from the DGPF test scenario than the 10 GCP cases from A³ testing.

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DMC DEast DNorth DVertical

RMS [m]/[%GSD] 0.028 / 35% 0.044 / 55% 0.054 / 68%

Std.Dev. [m]/[%GSD] 0.012 / 15% 0.015 / 19% 0.028 / 35%

DGPF-test: DMC check point accuracy (110 images, GSD 8cm, hg=870m, 4 GCP, 113 ChP)

Ultracam-X DEast DNorth DVertical

RMS [m]/[%GSD] 0.060 / 75% 0.025 / 31% 0.044 / 55%

Std.Dev. [m]/[%GSD] 0.008 / 10% 0.011 / 14% 0.026 / 33%

DGPF-test: Ultracam-X check point accuracy (175 images, GSD 8cm, hg=1200m, 4 GCP, 111 ChP)

ADS 40 DEast DNorth DVertical

RMS [m]/[%GSD] 0.027 / 34% 0.031 / 39% 0.050 / 63%

Std.Dev. [m]/[%GSD] 0.016 / 20% 0.022 / 28% 0.040 / 50%

DGPF-test: ADS40 check point accuracy (6 image strips, GSD 8cm, hg=770m, 4 GCP, 121 ChP)

A³ DEast DNorth DVertical

RMS [m]/[%GSD] 0.021 / 35% 0.029 / 48% 0.056 / 93%

Std.Dev. [m]/[%GSD] 0.018 / 30% 0.029 / 48% 0.054 / 90%

VisionMap A³ check point accuracy (Case 2, 8 image strips, GSD 6cm, hg=1972m, 5 GCP, 162 ChP)

Looking to the tables above only small differences are visible for horizontal components. The RMS values are all in the range of 2-3cm, except the Ultracam-X East component and the DMC North component. The A³ system is fully comparable to the other systems. In terms of absolute numbers, the A³ accuracy in horizontal is best, close to the reference accuracy of check points. In vertical component the performance is slightly different, especially when the smaller GSD is considered. DMC, Ultracam-X and ADS40 are in the range of 55-68% of one GSD (RMS), the A³ performance is slightly worse and reaches 93% of one GSD (RMS). Notice that the camera-specific A³ geometry allows capturing of the same GSD values from higher altitudes above ground than the other systems. In addition, there still might be some potential improvement, for example when appropriate self-calibration is applied. As mentioned earlier, the DGPF results were all obtained from self-calibration bundle adjustment. Right now it is not known to ifp if and to which extend additional parameters also have been considered in the A³ processing so far. The important role of additional parameters might also be seen from the following paragraphs. Figure 7 compares the DGPF test results from ifp, University of Stuttgart (US) to the accuracy from other universities, using the same data, but different concepts for data evaluation (i.e. no use of any cross strips in the US configurations). These figures are cited from Jacobsen et al. (2010) and also should be given to reflect the variability of accuracy numbers. This only is due to the chosen approach / configuration and the individual experience of the person responsible for the data processing. As one can see it was ifp and two institutions (Institute of Photogrammetry and Geoinformation, University of Hannover (UH), Institute of

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Photogrammetry, Technical University of Vienna (TUV)) evaluating the DMC, Ultracam-X and ADS40 flights.

RMS from ChP analyses DMC

RMS from ChP analyses Ultracam-X

RMS from ChP analyses ADS40

Figure 7: Empirical accuracy of DGPF GSD 8cm flights from UH (University of Hannover), US (University of Stuttgart) and TUV (Technical University of Vienna) evaluations

(figures taken from Jacobsen et al. (2010)).

The DMC and Ultracam-X flights are considered first: The UH only uses traditional ground control based AT, with 9 GCPs. No direct sensor orientation, i.e. additional observations from GPS/inertial sensors, has been introduced. 12 additional parameters (BLUH model, which combines physical relevant and purely mathematical parameters) were used. The US results already have been shown before in more detail, it should be mentioned, that for DMC, Ultracam-X and ADS40 flights the cross strips have not been considered to generate a more realistic operational scenario. Results are based on integrated sensor orientation, i.e. the GPS/inertial data are used as weighted observables. The 44 additional parameter model (significant parameters only) of Grün was introduced. TUV did the processing based on 8 GCP, but with the use of GPS perspective center coordinates only. Their self-calibration was done using the 12 Ebner parameters completed by two radial and another two tangential correction terms.

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The ADS40 processing was only done by two of the institutions, because of the availability of the special line scanner data processing workflow. Line scanner processing requests for additional GPS/inertial data, thus both versions were processed in an integrated sensor orientation. UH used 9 GCP, US only 4 GCP again. Additional self-calibration was only done for the US version: six self-calibration parameters modeling radial lens distortions as implemented in the ADS Orima bundle adjustment software were applied. As one can see from all three cameras, the obtained RMS values are slightly different. As the input data is the same for all different institutions these variations nicely reflect the situation in operational use, where different quality might be obtained even the same sensor / data set was used. Still, the variations are small and in all cases the accuracy is well below the 1 GSD thresholds. The accuracy obtained from US evaluations is fully comparable to the numbers obtained from the two other institutes. This is to confirm the correctness of the reference values previously used for the comparison to A³ data.

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A³ geometric accuracy in inner part of the block It is VisionMap’s common practice and also recommendation for A³ users to choose the mapping area between the outer strips of the block only. Thus VisionMap recommended to estimate the absolute accuracy only from those check points, which are located in the inner part of the block between the A³ outer strips flown in the most north and south parts of the test site. In order to do so, the most north and most south located check points are eliminated from the check point differences. Thus the following statistics is obtained from remaining 136 check points only. The accuracy numbers from inner region check point analysis are given separately because all other digital cameras flown in the Vaihingen/Enz test field so far have been (geometrically) characterized using all available check points. This also was the case for the DGPF camera evaluations already mentioned before. Figure 8 gives the change in RMS values – the (b) cases are only based on the remaining 136 inner region check points. The more detailed statistics are given in the tables. Figure 9 shows the modified point distribution.

Figure 8: Comparison of accuracy (RMS) [m] using all ChPs (162 points: case 5,6,7,8) or ChP from

inner block area only (136 points: case 5b, 6b, 7b, 8b).

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0.04

0.05

0.06

0.07 dEast dNorth dVertical

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Case 5b – 8strips_5GCP (only 136 check point from inner region, all 8 flight lines, GPS shift enabled, re-measured image observations)

DEast DNorth DVertical

Mean [m] 0.0104 -0.0121 -0.0399

Std.Dev. [m] 0.0165 0.0196 0.0331

RMS [m] 0.0196 0.0231 0.0519

Max.Diff. [m] 0.0503 @ PtNo 207019

0.0649 @ PtNo 2513

0.1135 @ PtNo 3015

Maximum difference (vector): 0.1181m @ PtNo 3015 Case 6b – 8strips_10GCP (only 136 check point from inner region, all 8 flight lines, GPS shift enabled, re-measured image observations)

DEast DNorth DVertical

Mean [m] 0.0030 0.0049 0.0004

Std.Dev. [m] 0.0142 0.0170 0.0300

RMS [m] 0.0146 0.0177 0.0300

Max.Diff. [m] 0.0407 @ PtNo 2563

0.0494 @ PtNo 2263

0.1035 @ PtNo 2263

Maximum difference (vector): 0.1150m @ PtNo 2263 Case 7b – 5strips_5GCP (only 136 check point from inner region, 5 flight lines only, GPS shift enabled, re-measured image observations)

DEast DNorth DVertical

Mean [m] 0.0037 -0.01477 -0.0327

Std.Dev. [m] 0.0162 0.0179 0.0379

RMS [m] 0.0167 0.0232 0.0501

Max.Diff. [m] 0.0413 @ PtNo 207019

0.0590 @ PtNo 2503

0.1195 @ PtNo 210019

Maximum difference (vector): 0.1246m @ PtNo 506019 Case 8b – 5strips_10GCP (only 136 check point from inner region, 5 flight lines only, GPS shift enabled, re-measured image observations)

DEast DNorth DVertical

Mean [m] 0.0005 -0.0017 -0.0002

Std.Dev. [m] 0.0162 0.0164 0.0347

RMS [m] 0.0162 0.0165 0.0347

Max.Diff. [m] 0.0467 @ PtNo 2563

0.0380 @ PtNo 411039

0.1095 @ PtNo 210019

Maximum difference (vector): 0.1146m @ PtNo 210019

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Figure 9: Five strip configuration with modified check point distribution, 136 ChPs located in the

block center (A³ flight test in Vaihingen/Enz @ June 13, 2009, image provided by VisionMap)

As expected the use of check points in the inner region of the block positively influences the obtained accuracy measures. This is nothing unexpected because multiple overlaps in the inner block region and better geometry of the ray intersections will increase the number of image rays used for object point determination and the block stability itself. If one looks into the increase in the RMS values, the improvement is mainly in the vertical component – especially when the 5 GCP configurations are considered. This accuracy increase is about 1.5cm (case 8 to 8b). The improvement is less if one looks for the 10 GCP cases. This also illustrates, that the overall block geometry is better, when 10 GCPs are introduced in the bundle adjustment. Thus the differences in quality of object point determination are smaller, comparing the points in the block center to those located closer to the borders of the block. Notice, this increased accuracy numbers are not due to any changes in the processing of the A³ data, it is only because the check points used for the statistical analysis have been limited to those points from the inner part of the A³ block only.

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Summary The A³ sensor obtained very satisfactory empirical accuracy from this Vaihingen/Enz test. The analyses have shown that the overall accuracy is in the same range of other commercial systems. Still, with regard to the high flying altitudes along with the high resolution on ground (small GSD values) provided by the A3, some small improvements in vertical accuracy might be possible. Even though the concept of stepping frame cameras seems to be slightly more complex, it is completely controlled by the VisionMap process flow. It should be mentioned that same concerns also have been made in the early days of line scanner data processing, where people “complain” on the additional effort and problems due to the compulsory use of additional GPS/inertial sensor data. Later empirical analysis showed – and this DGPF test not only was the first test – the high geometrical accuracy potential of ADS line scanner. But such empirical testing was necessary to prove the ADS performance and also to convince potential customers. This now might be similar for the A³ system. New and maybe slightly unconventional sensor geometries can fully be controlled by according software and process flows, but it is up to the system providers to participate in independent and accepted performance tests to convince future users of the potential of their sensors.

References Cramer, M. (2010): The DGPF-Test on Digital Airborne Camera Evaluation – Overview and Test Design, Photogrammetrie - Fernerkundung – Geoinformation (PFG). Heft 2(2010), pp. 75-84. Cramer, M. & Haala, N. (2010): DGPF Project: Evaluation of digital Photogrammetric Aerial-based Imaging Systems – Overview and Results from the Pilot Center Photogrammetric Engineering & Remote Sensing Vol. 76, No. 9, September 2010, pp. 1019-1029. Jacobsen, K., Cramer, M., Ladstädter, R., Ressl, C. & Spreckels, V. (2010): DGPF-Project: Evaluation of Digital Photogrammetric Camera Systems – Geometric Performance, Photogrammetrie - Fernerkundung – Geoinformation (PFG). Heft 2(2010), pp. 85-98. DGPF (2012): Camera evaluation test, web reference: http://www.dgpf.de/neu/WWW-Projekt-Seite/DKEP-Allg.html , accessed April 2012. PFG (2010): Special issue of PFG Journal, The DGPF Camera Evaluation Test, http://www.dgpf.de/neu/pfg/2010/Heft_2.pdf , accessed April 2012.