ORTHORECTIFICATION OF STEREO SPOT PANCHROMATIC
AND RADARSAT FINE MODE DATA
Shahruddin Ahmad
Malaysian Centre For Remote Sensing (MACRES), No. 13, Jalan Tun Ismail
Abstract
Rectification of satellite data using ground control points (GCPs) in a polynomial fit is not practical
in areas with limited well-defined GCPs. This is the case in many forested areas or in coastal areas.
This study evaluated on the rectification accuracy of SPOT Panchromatic and Radarsat Fine Mode
data using satellite orbital parameters. The research revealed that the optimum number of GCPs
for rectification was 8 for SPOT Panchromatic and 10 for Radarsat Fine Mode giving respectively
RMSE accuracy of 0..34 and and 1.04. This research also studied on the accuracy of producing
orthoimages from Digital Elevation Models (DEMs), derived from stereo SPOT Panchromatic and
Radarsat Standard Mode datasets. The DEMs accuracies were validated using more than 100 spot
heights derived from digitized contour map. The DEM accuracies achieved for stereo SPOT data
was 26.3 m and stereo Radarsat data was 35.2 m. The resultant orthoimages of SPOT Panchromatic
and Radarsat Standard Mode data generated from the DEMs were evaluated for x-y accuracy by
merging with digital vector data of 1:25000 scale using 20 reference test points. The SPOT and
Radarsat gave respectively 4.76m and 6.82m mismatch.
1.0
INTRODUCTION
Rectification of satellite image by polynomial method is widely used in remote sensing applications.
However, this method is impractical not only in places where ground control points (GCPs) are
very limited - forested and coastal areas but also in landscape with a wide range of terrain. The use
of orbital parameters to alleviate this problem has been proven in many studies. These include
works carried out by Cheng & Toutin, 1997 and Westin, 1990 on rectification of SPOT Panchromatic
and Cheng and Toutin, 1997 and Keong, 1995 on ERS - SAR data using orbital parameters. Their
results showed an accuracy of 0.3 pixel for SPOT data and 1 - 2 pixels for ERS data.
A reliable Digital Elevation Model (DEM) is extractible from satellite stereo pair and it is useful
for producing satellite orthoimage. With stereo SPOT, a DEM produced could have an accuracy of
10 m (Al-Rousan et al., 1997 and Theodossiou & Dowman, 1990). Toutin (1998) showed that
stereo Radarsat data could produce DEM from various pair of modes and intersection angles with
an accuracy of 25 m.
45
The objectives of this study are (i) to validate the orbital parameter technique for geometrical
rectification of SPOT Panchromatic and Radarsat Fine Mode SAR data in a tropical environment
like Malaysia and (ii) to generate DEMs from stereo SPOT Panchromatic and stereo Radarsat
Standard Mode SAR data for orthoimage production.
2.0
STUDY AREA
The Klang Valley, Malaysia, which covers an area of 60 km by 60 km was selected as the study area
(Figure 1). This site is suitable for the study because it has terrain ranging from flat to hilly
Figure 1 : The Study Area
3.0
DATA REQUIREMENTS
Table 1 gives the satellite data and their specifications acquired for this study. The base height ratio
(B/H) for SPOT stereo pair was 0.8. Vertical parallax ratio (VPR) for the Radarsat stereo pair
acquired from the same side of Standard 2 and Standard 7 modes was 0.99.
46
Table 1 : Specifications of SPOT and Radarsat Data Acquired
Platform
SPOT
RADARSAT
Scene Number
270-344
269-343
14297
C0015070
C0015059
Sensor/ Mode
HRV-P
HRV-P
Fine Mode
(F5)
Standard
Mode (S2)
Standard
Mode (S7)
Level of
Processing
1A
1A
Path Image
Path Image
Path Image
Date
21.4.98
23.1.98
31.7.98
20.12.99
17.12.99
10x10
10x10
6.25x6.25
12.5x12.5
12.5x12.5
Pixel Size
(meter)
Viewing Angle
(degree)
-30.3
o
+15.8
o
o
45.5 - 47.7
o
o
24 -31
o
o
45 -49
Ground Control Points (GCPs) of high accuracy are necessary for rectification of SPOT Panchromatic and Radarsat Fine Mode data due to their high spatial resolution. For this reason, GCPs
collection was carried out using GPS real time differential correction technique. A 12-channel
Omni STAR receiver with sub-meter accuracy was used. A total of 35 GCPs, well distributed
over the image was obtained. In addition, contours digitized from a topographic map of scale
1:25000 at 20 m interval were used to validate the DEM to be generated from the satellite stereo
pair. Major roads were also digitized to evaluate the orthoimage
4.0
SATELLITE IMAGE RECTIFICATION USING ORBITAL PARAMETERS
The rectification process using PCI’s OrthoEngine Version 6.3 software was based on a co-linearity
transformation model between the image and ground space. This technique was developed by
Toutin (1995) from the Canada Centre for Remote Sensing. The inputs for the model were orbital
parameters gathered from header files and image coordinates (pixel, line) which corresponded to
the coordinates (X, Y, Z) of the Malaysian Rectified Skew Orthomorphic (RSO) projection.
The SPOT Panchromatic and Radarsat Standard Mode images were rectified. The rectified images
were transformed and re-sampled to create epipolar geometry to ensure both images were offset
only in the horizontal direction. Image matching was performed to match the corresponding pixel
with the reference image. This involved moving a template window in the search area of the epipolar
image until the best digital number match was obtained. The correlation coefficient between 0 and
1 was calculated for each pixel, where 0 represents a total mismatch and 1 a perfect match. The
difference between the centre location of template window and that of the pixel to be matched was
the parallax. This parallax value was used to compute the elevation at the centre of the template.
47
5.0
RECTIFICATION RESULTS
Rectification results of both SPOT and Radarsat Fine mode are discussed below.
5.1
SPOT Panchromatic Data
All 35 GCPs were used in the rectification either as controls or check points. The following number
of control GCPs were tested : 4, 6, 8, 10 and 12 to get the optimum number for rectification . The
corresponding balanced number of GCPs were then used as check points (CPs) to evaluate the
accuracy of rectification. Table 2 shows the Root Mean Square Error (RMSE) of both control
and check points GCPs. Figures 2 and 3 give the plots of the control GCPs and CPs with RMSE
respectively.
Table 2: RMSE of SPOT Rectification with Varying Number of GCPs
Ground Control Points
No. of GCPs
4
6
8
10
12
Check Points
RMSE (pixels)
X
0.28
0.33
0.27
0.25
0.24
Y
0.08
0.19
0.21
0.22
0.21
R
0.29
0.38
0.34
0.33
0.32
No. of CPs
31
29
27
25
23
Note: R = vector
48
RMSE (pixels)
X
1.32
1.08
0.34
0.32
0.32
Y
0.29
0.30
0.30
0.30
0.29
R
1.36
1.13
0.45
0.44
0.43
Figure 2: Plot of Control GCPs with RMSE for SPOT data
Figure 3: Plot of CPs with RMSE
From the table and plots, it was apparent that the optimum number of 8 control GCPs should suffice
for image rectification given that it has low RMSE accuracy of 0.45 pixels.
5.2
Radarsat Fine Mode Data
GCPs selection for Radarsat Fine Mode rectification was more difficult compared to SPOT due
to speckles present on the radar image. Speckle reduction was performed using Lee and Frost
filters. Like the SPOT data rectification , the following number of control GCPs were tested : 4,
6, 8 ,10 and 12. Table 3 shows the results and the plot of RMSE with GCPs appears in Figure 4.
A total of 10 GCPs was optimal for rectification. Given limited GCPs , evaluation of accuracy
of the rectified image using CPs was not performed.
49
Table 3 : RMSE of Radarsat Rectification
Using Varying Number of GCPs
Number of GCPs
RMSE (pixels)
X
Y
R
4
9.03
0.73
9.06
6
1.19
1.16
1.67
8
1.06
1.13
1.55
10
0.75
0.72
1.04
12
0.74
0.70
1.02
Figure 4: Plot of Control GCPs with RMSE
6.0
ACCURACY OF DEM
Different accuracy levels were expected from stereo SPOT Panchromatic and stereo Radarsat
Standard Mode DEM due to difference in data characteristics and spatial resolution. A study done
by Al-Rausan et al. (1997) showed that the accuracy of DEM from SPOT was <10 meter in a desert
area, while Toutin (1998) showed that the accuracy of Radarsat DEM using Standard Mode data is
20 meter in low relief and 50 meter in high relief area over the Sherbrooke region, Quebec, Canada.
Accuracy of both DEMs generated in this study from both SPOT and Radarsat Stereo-pairs was
validated on (i) flat to rolling (0-12o)and (ii) hilly terrains (12-20o)using more than 100 test reference
points derived from digitized contours (20m interval).
50
The flat to rolling areas cover mainly urban and vegetated areas, while most of the hilly areas are
covered by tropical rain forest. The results (Table 4) show that both DEMs gave better accuracy in
the hills compared to the flat to rolling terrain. From this study the overall accuracy of SPOT DEM
was 26.3 m and that for Radarsat DEM was 35.2 m. This was attributed to better spatial resolution
of the former datasets.
Table 4 : RMSE – Height for SPOT and Radarsat DEM
Types of terrain
RMSE ∆H (m)
from SPOT
RMSE ∆H (m)
from Radarsat
Flat to Rolling
32.1
41.0
Hilly
17.0
26.7
Overall Accuracy
26.3
35.2
Figure 5 shows the linear relationship between elevation from SPOT and Radarsat with elevation from digital contours (20m interval). Better correlation between SPOT and digital contour
elevation was observed. compared to Radarsat particularly in areas < 100m
Figure 6: Elevation Plot of SPOT, Radarsat and Digitized
51
7.0
ORTHOIMAGE PRODUCTION
Ortho-rectification is a process to remove the relief distortion using DEM to produce an orthoimage.
The DEM of SPOT was used complemented by Radarsat DEM. In isolated areas of cloud covers
and homogenous pixels the SPOT DEM did not provide elevation information. These areas were
masked and replace with corresponding elevation values from the Radarsat DEM. Figures 7 (a &
b) and 8 (a & b) are the orthoimages of Sepang (flat to rolling)and Langat (hilly) produced from
both SPOT DEM and Radarsat DEM. The accuracy of the orthoimages was evaluated with digitized
vector topographical layer. A total of 20 samples points over the study area were selected for the
accuracy assessment, which were mainly locations of roads intersections. The planimetric difference
between these points on the topography map and the corresponding ones on the images were measured
and the overall RMSE calculated for the 20 points. Accuracies of 3.15m (SPOT) and 6.36m
(Radarsat) were achieved for the Sepang image and 4.76 m (SPOT) and 6.82 m (Radarsat) for the
Langat image (Table 5).
Table 5 : Accuracy Assessment of SPOT and Radarsat Orthoimages
SPOT Panchromatic
Radarsat Fine Mode
Map Scale RMSE (E) RMSE (N) RMSE R RMSE (E) RMSE (N) RMSE R
meter
meter
meter
meter
meter
meter
Area
Number of
Points
Sepang
20
25000
2.38
2.06
3.15
3.8
5.1
6.36
Langat
20
25000
2.65
3.95
4.76
3.52
5.84
6.82
Figure 7(b): Radarsat Orthoimage of Sepang
Figure 7(a): SPOT Orthoimage of Sepang
52
Figure 7(b): Radarsat Orthoimage of Langat
Figure 7(b): SPOT Orthoimage of Langat
8.0
CONCLUSION
The study has proven that image rectification could be done reliably using orbital parameters and a
minimum number of GCPs. The rectification accuracy for SPOT with 8 GCP was 0.34 whereas that
for Radarsat fine mode was 1.04 using 10 GCPs. The study also indicated that accurate DEMs
could be generated using SPOT(P) and Radarsat standard Mode stereo pairs with Z- accuracies
respectively of 26.3m and 35.2m. The DEMs produced orthoimages with accuracy of about 0.5
pixel.
References
Al-Rousan, N., Cheng. P., Toutin. T. and Valadan Zoel, M.J., (1997). “Automated DEM Extraction from
SPOT Level
1B Imagery”. Photogrammetric Engineering & Remote Sensing, Vol. 63, No. 8,
August, pp 965-974.
Cheng, P. and Toutin.T., (1997). “On-Site Interactive GPS and Geometric Modelling: A Winning
Combination”. EOM, April 1997, pp. 35-37.
Keong, K.L., (1995). “Geocoding of Spaceborne SAR Imagery”. Proceedings of Seminar on the Integration
of Remote Sensing and GIS for Applications in South East Asia, 27-29 March 1995, Kuala Lumpur,
Malaysia.
Theodossiou, E.I. and Dowman, I.J, (1990). “Heighting Accuracy of SPOT”. Photogrammetric Engineering
& Remote Sensing, Vol. 56, No.12, December, pp. 1643-1649.
Toutin, T., (1995). “Multi-Source Data Fusion with an Integrated and Unified Geometric Modelling”.
Journal EARSel – Advances in Remote Sensing, Vol.4, No 2, pp. 118-129.
Toutin, T., (1998). “Stereo RADARSAT for Mapping Applications. Proceedings of ADRO Final Symposium,
Montreal, Canada, October 13-15, 1998.
Westin, T., (1990). “Precision Rectification of SPOT Imagery”. Photogrammetric Engineering and Remote
Sensing, Vol. 56, No. 2, February, pp
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