HJ-1A/B CCD IMAGERY Geometric Distortions and
Precise Geometric Correction Accuracy Analysis
Changmiao Hu,
Email: akaishi@163.com,
Ping Tang
tangping@irsa.ac.cn
Institute of Remote Sensing Applications
Chinese Academy of Sciences
Chao Yang, Beijing 100101
Content
1. Brief Introduction of HJ-1 A /B satellites
2. Geometric distortion analysis of HJ-1 CCD
data
3. Comparison of different geometric
correction models for HJ-1 CCD data
4. Conclusion
1. Brief Introduction of HJ-1 A /B
satellites

HJ-1 A and HJ-1 B satellites
were launched on Sept. 5,
2008 by a Long March-2C in
Taiyuan Satellite Launch
Center, Shanxi Province,
China.

HJ-1 A and HJ-1 B satellites
together provide observation
revisit cycle in 48 hours. The
overall objective is to
establish an operational earth
observing system for
environmental protection and
disaster monitoring.
Note: HJ is the abbreviation for Chinese pinyin “Huan Jing” - means “environment”.
HJ-1 (Huan Jing-1: Environmental Protection & Disaster Monitoring Constellation)

HJ-1A is an optical satellite
with two CCD cameras and
an infrared camera; HJ-1B is
also an optical satellite with
two CCD cameras and a
hyperspectral camera. The
two pushbroom CCD
cameras form a WVC ( Wide
View CCD Cameras)
31°
31°
11°
30°
30°
CCD 1
360km
CCD 2
360km
HJ-1 A/B WVC
TM
185km
Landsat TM
Technical parameters for multispectral
CCD sensors of HJ and Landsat TM
HJ-1 A/B CCD
Landsat TM
Spatial resolution
30m (in nadir)
30m
Swath width
360km (CCD*2≥700km)
185km
Aspect angle
31°
5°
Revisit period
2 days
16 days
Spectral resolution
Band 1:(0.43-0.52µm)
Band 2:(0.52-0.60µm)
Band 3:(0.63-0.69µm)
Band 4:(0.76-0.90µm)
Band 1:(0.45-0.52µm)
Band 2:(0.52-0.60µm)
Band 3:(0.63-0.69µm)
Band 4:(0.76-0.90µm)
Band 5:(1.55-1.75µm)
Band 7:(2.08-2.35µm)
2. Geometric distortion analysis of HJ1 CCD data

Test data
Eight images from different satellites and CCD, which are after
systematic geometric correction processing and have map projection
information. The details of systematic geometric correction are unknown.
No.
Satellite
Sensor
Path
Row
Date
ID
No.1
HJ-1A
CCD1
3
72
2009-12-22
0000225232
No.2
HJ-1A
CCD1
4
72
2009-10-17
0000186716
No.3
HJ-1A
CCD2
1
72
2009-12-25
0000226660
No.4
HJ-1A
CCD2
1
72
2009-12-25
0000226699
No.5
HJ-1B
CCD1
4
69
2009-10-15
0000186267
No.6
HJ-1B
CCD1
3
72
2009-11-19
0000204443
No.7
HJ-1B
CCD2
1
72
2009-11-22
0000205299
No.8
HJ-1B
CCD2
2
72
2009-10-22
0000189452
These images are from Satellite Environment Center
(Ministry of Environmental Protection).
Eight images are displayed as a falsecolor composite (RGB-B432) after
applying an identical linear stretch.
No.1
No.2
No.3
No.4
No.5
No.6
No.7
No.8
Methods for distortion analysis
The automatic image matching method is adopted to obtain
image control points, where HJ data as the original images, the
Landsat TM GLCF images as reference images.
Drawing the displacement vectors of control points;
Calculate root mean squared error (RMSE) and analysis.

1 n
2
2
xi  x'i    yi  y'i 
RMSE 

n i 1

Image control points selection
Over 1000 points
200 points
50 points
About 1000 image control points evenly distributed are extracted by
image matching in each test image. All the image control points are
checked and the error matched points are deleted. Then 50, 200, and
1000 nearly even distributed control points are selected out, and some
points are used as check points.
Displacement vectors
for eight images, 50 points
There are both global system distortions of oriented shift and local
distortions exist within the eight images. These distortions are quite
different and not regular.
RMSE of eight images, geographical
coordinates (meters), 50 points
RMSE
No.1
No.2
No.3
No.4
No.5
No.6
No.7
No.8
x
257.0
379.7
356.1
365.7
348.7
643.1
448.8
800.6
y
1004.5
981.16
741.9
760.9
888.9
300.2
246.4
559.3
total
1037.1
1052.0
822.9
844.3
954.9
709.7
512.0
976.6
The geometric precision of eight images are low. The total RMSE is
from 500 to 1000 meters.
 After systematic geometric correction processing, the HJ-1 A/B CCD
images are still with low geometric precision and need to be geo-corrected
in high precision.
3. Comparison of different geometric
correction models for HJ-1 CCD data
 Three mathematical models are tested :
1) Polynomial model (Global method)
2) Thin plate splines (Global method with local characteristics)
3) Finite element method (Local method)
Polynomial model
n
i

i j j
u

a
x
y

k

i 0 j 0


n
i
i k
j
v 
b
x
y

k

i 0 j 0



u, v  :
the image coordinates.
x, y  : the ground coordinates.
ak bk : the coefficients, which always determined
by least squares regression analysis.
Polynomial model is a global method. It always be used in small
size image.
Polynomial model

RMSE in pixels. The control points are used for solving model. Both
control points and check points are used for accuracy analysis.
Degrees/points
No.1
No.2
No.3
No.4
No.5
No.6
No.7
No.8
3 Degrees/40 control
7.179 8.245 8.876 9.338 11.22 9.205 7.197 11.27
3 Degrees/10 check
12.03 6.137 5.890 15.95 9.275 8.721 10.26 12.22
3 Degrees/180 control
7.551 9.213 8.161 10.06 9.761 8.517 8.330 12.57
3 Degrees/20 check
6.413 7.731 6.450 9.388 11.43 7.232 10.79 12.85
5 Degrees/40 control
5.238 6.072 6.229 7.571 9.479 6.854 6.023 7.197
5 Degrees/10 check
13.55 5.392 6.544 15.51 9.163 12.12 10.77 14.67
5 Degrees/180 control
6.721 8.568 7.454 9.708 9.089 7.753 7.704 11.63
5 Degrees/20 check
6.519 6.978 6.291 9.668 10.36 8.171 9.845 12.36

The polynomial model is difficult to be used in correcting the eight
images. The errors always larger than five pixels.
Thin plate splines (TPS)
N

2
2
u

a

a
x

a
y

f
r
ln
r

0
1
2
i i
i


i 1

N
v  b  b x  b y  g r 2 ln r 2

0
1
2
i i
i

i 1
ri
N
2
2
2




 x  xi  y  yi
N
N
N
N
N
 f   f x  f y   g   g x   g y
i 1
i
i 1



i i
i 1
i
i
i 1
i
i 1
i i
i 1
i
i
0
u, v  :
the image coordinates.
x, y  : the ground coordinates.
ak bk fk gk : the coefficients.
TPS is a global method, and also with local characteristics. It
interpolates the control point rigorously, hence there are no
residuals for the control points.
Thin plate splines (TPS)

RMSE in pixels, for check points.
Points:
Check/Control


No.1
No.2
No.3
No.4
No.6
No.7
No.8
10/40
13.80
11.13
8.342 19.12 9.800
10.06
11.70
13.95
20/180
5.816
4.128 5.850
9.572
9.621
8.437 18.17
100/1000
2.465
12.15 94.54 5.241 7.668
7.334
118.8 6.994
120/1000
1.779
6.948 137.3 4.949 2.300
1.500
40.39 24.48
6.011
No.5
When the number of control points is 40 or 180, the errors in check
points are always larger than 5 pixels.
When the number of control points is over 1000, the calculation
results of TPS are not stable.
Finite element method


Firstly construct Delaunay tessellation using the control points;
Then calculate the transformation parameters. In a Delaunay
triangulation, use a 1st-order polynomial algorithm to do precise
geometric correction.
u  a1 x  b1 y  c1

v  a2 x  b2 y  c2

Finally, interpolates the intensity of each pixel in the transformed file.
Finite element method is a local method, local variations do not
directly affect the registration of the entire image.
References: Jonathan Richard Shewchuk. “Triangle” A Two-Dimensional Quality Mesh
Generator and Delaunay Triangulator. http://www.cs.cmu.edu/~quake/triangle.html
Finite element method

RMSE in pixels, for check points.
Points:
Check/Control
No.1
No.2
No.3
No.4
No.5
No.6
No.7
No.8
10/40
13.28 4.636 7.513 19.12 9.897 9.340 10.19 13.09
20/180
5.900 9.453 5.369 10.74 9.638 9.510 7.915 20.12
100/1000
1.565 1.852 2.410 2.069 1.226 1.236 1.772 1.587
120/1000
1.461 2.817 2.121 2.243 1.493 1.338 1.726 1.814
Finite element method can meet the requirement of precise geometric
correction for HJ-1A/B CCD imagery if the number of evenly distributed
control points are over 1000.
4.Conclusion

Both global system distortions and complex local distortions
exist within the HJ-1A/B CCD images;

Polynomial model gets the worst accuracy;

Thin plate splines significantly improve accuracy, but with the
increase in the number of control points, the calculation is not
stable.

Finite element method is recommended to be used of precise
geometric correction for HJ-1A/B CCD imagery if the control
points are enough and evenly distributed. Besides it is a local
method, and possesses the advantages of rapidity and stability.
Changmiao Hu,
Ping Tang
Email: akaishi@163.com, tangping@irsa.ac.cn