To investigate the usability of 3D (Three- dimensional) photogrammetric models generated without GCPs (ground control points), two studies were conducted in two different provinces of Turkey with different characteristics. First one in Siva located in the south of Turkey with 1650 m. elevation (Altitude above sea level) and the second one in Artvin located in the northeast of Turkey with an elevation of 450 m. Since there is a big difference in height between two regions as well as another variety factors such as meteorological difference, slope, terrain topography and etc to investigate that, whether this principle is applicable in everywhere or not. The two areas were modelled once with GCPs and afterwards without GCPs. The data acquisition in the both areas are as follow.
5.1 Sivas
This study is presented in Fig. 1 and conducted in Shugul Canyon located in the Sivas province of Turkey. Oblique imagery of the work area is carried out and with the help of data obtained once the area is modelled with GCPs and once without GCPs and to compare the sensitivity of the two models, the length, area and volume of the same regions were measured in both models and the results were compared.
To model the area with oblique aerial photogrammetry method a DJI Phantom 4pro model UAV is used (Fig. 2). 10 well distributed GCPs (ground control points) were used to georeference the model. The measurement of GCPs was conducted with (CHC X91 GNSS) model RTK-GNSS receiver.
At the first step 9 well distributed ground control points were established in the work area, during establishing of these points, care was taken to ensure that they can be easily seen in the photos and were establish away from the natural and artificial objects such as trees and buildings that would prevent these points from appearing in the pictures (Fig. 3).
After establishing the GCPs the measurement of these points was done in Turkish National Reference System as TUREF / TM36 and (ITRF96 in universal system as EPSG: 5256) with a sensitivity of (2cm) and the obtained coordinates are given in Table 1.
Table 1
Coordinate list of ground control points
Point No
|
Y
|
X
|
Z
|
P.1
|
607223.539
|
4292394.086
|
1401.258
|
P.2
|
607217.702
|
4292407.654
|
1409.344
|
P.3
|
607141.923
|
4292390.866
|
1394.211
|
P.4
|
607094.008
|
4292376.559
|
1391.703
|
P.5
|
606994.953
|
4292671.055
|
1446.086
|
P.6
|
606986.964
|
4292597.873
|
1407.067
|
P.7
|
606845.780
|
4292703.114
|
1445.776
|
P.8
|
606793.686
|
4292683.833
|
1415.525
|
P.9
|
606664.304
|
4292784.330
|
1447.985
|
In the second step the flight plan and flight time was determined, since the work area was mountainous area so the appropriate time for flight was determined as 12:00–14:00. By selecting this time, the effect of shading that would cause errors in the photogrammetric evaluation was tried to be minimized. DJI Phantom 4Pro model UAV was used for the aerial photography, Phantom 4Pro is equipped with a 20MP camera which is able to take photos with 4K quality and also has a 1-inch sensor. DJI Phantom 4Pro model UAV is presented in Fig. 4.
The imagery was conducted with the above UAV and nadir and oblique images of the work area were obtained. The cross overlap in the imagery was 85% and forward overlap was %80, this overlap is sufficient to represent the topography and reconstruct the area virtually in three-dimension. Positions of GCPs, image overlap ratios and camera calibration are given graphically in Fig. 5. The errors of GCPs used during the photogrammetric evaluation are given in Table 2. The GCP errors calculated by the Agisoft/Metashape are given in Table 3.
Table 2
Correlation matrix calculated by the Agisoft/Metahshape
|
Value
|
Error
|
F
|
Cx
|
Cy
|
B1
|
B2
|
K1
|
K2
|
K3
|
P1
|
P2
|
F
|
3656.51
|
0.04
|
1.00
|
0.00
|
0.20
|
-0.28
|
0.04
|
-0.21
|
0.25
|
0.23
|
0.00
|
-0.19
|
Cx
|
-4.57
|
0.08
|
|
1.00
|
0.01
|
-0.02
|
0.04
|
0.01
|
0.00
|
0.00
|
0.94
|
-0.01
|
Cy
|
18.41
|
0.06
|
|
|
1.00
|
-0.21
|
0.00
|
-0.03
|
0.02
|
-0.02
|
0.00
|
0.85
|
B1
|
-7.25
|
0.02
|
|
|
|
1.00
|
0.02
|
-0.02
|
0.00
|
0.00
|
-0.01
|
0.06
|
B2
|
0.36
|
0.02
|
|
|
|
|
1.00
|
0.00
|
0.00
|
0.00
|
-0.12
|
0.01
|
K1
|
0.01
|
0.01
|
|
|
|
|
|
1.00
|
-0.97
|
0.91
|
0.00
|
-0.04
|
K2
|
-0.01
|
0.01
|
|
|
|
|
|
|
1.00
|
-0.97
|
0.91
|
0.02
|
K3
|
0.02
|
0.01
|
|
|
|
|
|
|
|
1.00
|
0.00
|
-0.02
|
P1
|
-0.01
|
0.01
|
|
|
|
|
|
|
|
|
1.00
|
-0.01
|
P2
|
-0.01
|
0.01
|
|
|
|
|
|
|
|
|
|
1.00
|
Table 3
GCP errors calculated by the Agisoft/Metashape
Label
|
X error (cm)
|
Y error (cm)
|
Z error (cm)
|
Total (cm)
|
Image (pix)
|
point 1
|
1.273
|
1.163
|
-2.676
|
3.184
|
0.320 (17)
|
point 2
|
-0.265
|
-1.345
|
1.950
|
2.384
|
0.299 (21)
|
point 3
|
-2.428
|
1.455
|
2.572
|
3.825
|
0.333 (15)
|
point 4
|
1.162
|
-1.346
|
-1.782
|
2.518
|
0.198 (22)
|
point 5
|
0.637
|
0.363
|
1.899
|
2.036
|
0.117 (5)
|
point 6
|
-0.113
|
-0.228
|
-1.186
|
1.213
|
0.201 (17)
|
point 7
|
-0.694
|
-0.402
|
0.301
|
0.858
|
0.087 (5)
|
point 8
|
0.479
|
-0.046
|
-0.851
|
0.977
|
0.067 (6)
|
point 9
|
0.017
|
0.190
|
0.491
|
0.527
|
0.063 (7)
|
Total
|
1.058
|
0.912
|
1.727
|
2.221
|
0.246
|
Afterwards, the obtained aerial photographs were subjected to photogrammetric evaluation and the measured coordinates of GCPs were also used during the evaluation. Finally, geometry of the subject area has been reconstructed formally and computationally in virtual form. As a result of the evaluation, a dense cloud including 8,193,681 points with global coordinates, 3D model, orthophoto and digital elevation model were obtained as shown in Fig. 6.
After the 3D model was generated with GCPs, once again 3D model of the work area was generated without GCPs.
5.2 Artvin
This study was conducted in the Artvin dam built on the Çoruh Riverwhich is located in the Artvin province of Turkey (Fig. 7). A 3D model of the area is generated by oblique photogrammetry, once with and once without GCP.
For modelling this work area via aerial photogrammetry method a DJI Phantom 4pro model UAV was used. 5 GCPs (ground control points) were used to georeference the model. The measurement of GCPs was conducted with TRIMBLE R6 GPS (CORS/GNSS) receiver (Fig. 8). The coordinates of GCPs were measured in Turkish National Reference System as TUREF / TM42 (3 Degree) and (ITRF96 in universal system as EPSG: 5258) with a sensitivity of (2cm) and the obtained coordinates are given in Table 4.
Table 4
Point NO
|
Y
|
X
|
Z
|
P1
|
480173.041
|
4534136.386
|
408.183
|
P2
|
480193.098
|
4534080.037
|
395.799
|
P3
|
480188.260
|
4533962.940
|
397.686
|
P4
|
480112.801
|
4534002.926
|
442.610
|
P5
|
480095.596
|
4533966.277
|
446.039
|
The imagery was conducted with the UAV in accordance with photogrammetric principles and as a result nadir and oblique images were obtained. The cross overlap in the imagery was 85% and forward overlap was %80, this overlap is sufficient to represent the topography and reconstruct the area virtually in three-dimension. GCP positions and image overlaps are given in the Fig. 9.
Positions and errors of GCPs used during the photogrammetric evaluation, image overlap, and camera calibration coefficients and correlation matrix are given graphically in Fig. 9. The correlation matrix is given in the Table 5. The GCPs position errors are also calculated and are given in Table 6.
Table 5
Correlation matrix calculated by the Agisoft/Metahshape
|
Value
|
Error
|
F
|
Cx
|
Cy
|
B1
|
B2
|
K1
|
K2
|
K3
|
P1
|
P2
|
F
|
3649.93
|
0.05
|
1.00
|
0.01
|
0.35
|
-0.09
|
0.05
|
-0.33
|
0.34
|
-0.31
|
-0.07
|
0.06
|
Cx
|
-3.28
|
0.06
|
|
1.00
|
0.12
|
0.03
|
0.04
|
0.01
|
-0.02
|
0.02
|
0.69
|
-0.23
|
Cy
|
16.63
|
0.06
|
|
|
1.00
|
-0.06
|
0.06
|
-0.03
|
0.01
|
0.00
|
0.32
|
-0.31
|
B1
|
-0.28
|
0.02
|
|
|
|
1.00
|
0.01
|
0.01
|
-0.02
|
0.02
|
0.01
|
-0.01
|
B2
|
0.21
|
0.02
|
|
|
|
|
1.00
|
0.00
|
0.00
|
0.00
|
-0.10
|
0.03
|
K1
|
0.01
|
0.01
|
|
|
|
|
|
1.00
|
-0.97
|
0.93
|
0.01
|
-0.01
|
K2
|
-0.04
|
0.01
|
|
|
|
|
|
|
1.00
|
-0.99
|
-0.02
|
0.01
|
K3
|
0.07
|
0.01
|
|
|
|
|
|
|
|
1.00
|
0.02
|
-0.01
|
P1
|
-0.01
|
0.01
|
|
|
|
|
|
|
|
|
1.00
|
0.79
|
P2
|
-0.01
|
0.01
|
|
|
|
|
|
|
|
|
|
1.00
|
Table 6
GCPs position errors given by the Agisoft/Metashape
Label
|
X error (an)
|
Y error (an)
|
Z error (an)
|
Total (cm)
|
Image (pix)
|
P.l
|
-0.93064
|
-1.11915
|
1.23013
|
1.90573
|
0.321 (18)
|
P.2
|
-0.68080
|
0.83722
|
0.64880
|
1.25912
|
0.206 (21)
|
P.3
|
1.10243
|
-0.46048
|
-0.47225
|
1.28468
|
0.425 (14)
|
P.4
|
-0.78656
|
1.07360
|
-0.71107
|
1.50895
|
0.543 (8)
|
P.5
|
-0.61781
|
1.47969
|
1.62688
|
2.28427
|
0.245 (32)
|
Total
|
0.84206
|
1.04953
|
1.03049
|
1.69484
|
0.320
|
Afterwards, the obtained aerial photographs were processed in accordance to photogrammetric principles, the measured coordinates of GCPs were also used during the photogrammetric process of images. Finally topography of the area has been reconstructed in three dimension with its global position. As a result of process, a dense cloud including 18,958,130 points with global coordinates, 3D model, orthophoto and digital elevation model were obtained as shown in Fig. 10.
After reconstructing the work area virtually in three dimensions using GCPs, once 3D model was generated without GCPs.