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Orthorectification method using regular grid
A. Sonyushkin
Background
The main features
Near real-time processing
Visual quality control
Speed up processing time
ScanEx Image Processor ®
How it works
Source image and its grid
( ) 00 10 01 11
( ) 00 10 01 11
Pn b b x b y b xy
Pn a a x a y a xy
00 ( 1)
10 ( 2) ( 3)
01 ( 3) ( 1)
11 ( 1) ( 2) ( 3) ( 4)
b P
b P P
b P P
b P P P P
00 ( 1)
10 ( 2) ( 3)
01 ( 3) ( 1)
11 ( 1) ( 2) ( 3) ( 4)
a P
a P P
a P P
a P P P P
Transformation on the screen Warped image
Nearest neighbour (BOX)Bilinear (Triangle)CubicHermitLanczosetc
( ') ( )I Pn I Pn f
The main source of the grid
TERRA / AQUA MODIS (MOD03)TERRA ASTER (L1A)RADARSAT 1/2
Pre-calculated (none ortho)
Calculated (ortho)
Rigorous model (by ray tracing)Inverse RPCForward RPC
Grid from forward RPC
Mathematical model
, ,L p h
, ,S r h ( , )h H
0 0 0 0, , ( , )GGTL p H
0 0 0 0, , ( , )GGTS r H
0GGT
dp dp dh dp dp dhL L
d dh d d dh d
0GGT
dr dr dh dr dr dhS S
d dh d d dh d
0
0
GGT
GGT
dp dp dh dp dp dhL L
d dh d d dh d
dr dr dh dr dr dhS S
d dh d d dh d
Grid from forward RPC
Problems and solutions
( , ) ( , ) ( , )GGT
newmbaS L G S L B S L
( , ) ( , ) ( , )GGT
newmbaS L F S L A S L
( , ) ( , ) ( , )GGT
newmbaH S L H S L C S L
Grid from forward RPC
Traditional method vs grid method
Source image Traditional method Grid method
Grid from forward RPC
Traditional method vs grid method
Source image Traditional method Grid method
Accuracy of the method
Test area – Maussane-les-Alpilles
Testing AOI GCP distributions
36% with slopes > 6°4% with slopes > 45°
33 GCP`s from GPS8 DEM`s with different density and accuracy
Accuracy of the method
Datasets for test
Image ID GSD (m) Off-nadir angle
04sep23103653-p2as-000000156169_01_p001 0.615 2.0
03jun18102321-p2as-000000055542_01_p001 0.625 8.4
04sep18103125-p2as-000000155025_01_p001 0.659 16.1
04jan28101403-p2as-000000102220_01_p001 0.729 25.5
04apr11104103-p2as-000000120208_01_p001 0.867 35.3
DEM name Density (m) H RMSE (m)
SGM-06 0.6 1.2
ADS-40 2.0 0.75
GM-5 5.0 1.36
GM-10 10.0 1.43
TOPO-25 25 1.38
SRTM X-SAR 30 1.83
ASTER GDEM 30 4.52
SRTM SIR-C 90 2.85
Accuracy of the method
How to measure accuracy of the model for each pixel
For each Image with each DEM, grids with different step (in pixels) were calculated
• 32 x 32 pixels• 16 x 16 pixels• 8 x 8 pixels• 4 x 4 pixels• 2 x 2 pixels
For each Image with each DEM, orthoimage by traditional method was calculated• Orthoimage• Image with sample numbers of source image as
intensity• Image with line numbers of source image as intensity
Coordinates for each pixel were extracted and compared
• RMSE• MAE• CE90etc
Accuracy of the method
Results: RMSE
Accuracy of the method
Results: Errors distribution for image with 35.3° off-nadir angle
Accuracy of the method
Optimal grid step provides sub-pixel accuracy of the model
off-nadir angle
DEM
SGM06 ADS-40 GM5 GM10 TOPO-25 SRTM X-SAR ASTER GDEM SRTM SIR-C
2° 32 32 32 32 32 32 32 32
8.4° 16 32 32 32 32 32 32 32
16.1° 4 8 16 32 32 32 32 32
25.5° 2 4 16 16 32 32 32 32
35.3° 2* 4 8 16 32 32 32 32
Image
DEMGGT
GSDStep
GSD
Performance of the method
Computing time for different grid steps
CPU – Intel Core™ i7 x980 3.33 GHzRAM – 24 GBOS – Windows 7 SP1 x64
Method Time (ms) SpeedUp (times)
Traditional 221161 -
Grid step 2 pixels 220621 1.002
Grid step 4 pixels 57643 3.837
Grid step 8 pixels 24167 9.151
Grid step 16 pixels 9828 22.503
Grid step 32 pixels 7942 27.847
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Photogrammetric processing and thematic mapping in ScanEx Image Processor software
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