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Mapping without the sun
Zhang, Jixian

Figure 1-2 3D ortho Product generation algorithm flow
It’s difficulty to measure the Ground Control Point (GCP) by
GPS or other method in the West Region of China where the
1:50,000 scaled terrain maps are absent, because of the
mountainous terrain, rugged topography and abominable
climate of those area. So it’s an important method to make the
topographic map using the high-resolution remotely sensing
imagery with high precision orientation parameter to extract
DEM under the condition of lacking GCP, then the DEMs were
used to make the ortho image.
Up to till now, main emphasis has mainly given to the analysis
of geolocation error of remote sensing imagery. Zhang
Jixian(2000) researched the displacement of SPOT images,
M.S.Hashemiana(2004) evaluated the accuracy of DEM gener
ated form SPOT5 HRS imagies, Philip Cheng() described how
to correct QuickBird data using different geometric correction
model and the method of automatic DEM extraction using
QuickBird stereo data. Wang Yali(2005) analyzed the SPOT5
geometric error applying the computer simulation technique,
and A. M. Marangoz (2005) researched the geometric evalua
tion, automated DEM and orthoimage generation from along-
track stereo ASTER Images.
The aims of this paper is to assess the geolocation accuracy of
ASTER L3A image. If the accuracy satisfies the requirement
of China’s 1:50,000 map, then the ASTER image can be used
in the Mapping Project in the West Region of China. 2
2.1 the terrain of Beijing area
Beijing lies in the north of the North China Plain, at 39° 56'
N and 116° 20' E. The terrain of the Beijing area slopes from
the northwest to the southeast. Mountains snake round the
city's north, west and northeast, while the southeast part of the
city is a plain that slopes gently toward the coast of the Bohai
Sea. The Yongding, Chaobai and Juma rivers and the north
section of the Grand Cannel crisscross the area under Beijing's
jurisdiction. Most of the rivers originate from mountainous
areas in the northwest, cut through mountains and zigzag
through the plain in the southeast before emptying into Bohai
Sea. It covers 16,807.8 square kilometers. Mountainous areas
occupy 10,417.5 square kilometers, accounting for 62% of the
city's landmass. The rest, 6390.3 square kilometers or 38% of
the total, are flatland.
Figure 2-1 the terrain of testfield in Beijing area and the
distribution of check points
2.2 assessment approach
The geometric accuracy validation approach of ASTER Level-
3A image is that choosing character points form the image first,
then measuring the ground coordinates of the character points
by Global Positioning System (GPS), and the geolocation
accuracy was got by comparing the coordinates measured by
GPS and the image coordinates.
2.2.1 choose the character points in the image
The swath of ASTER image is 60 km, so we decided to choose
40 character points distributed well. Same with many satellite
datasets: road crossings and the end of bridges can easily be
selected as character points in ASTER scenes. In mountainous
area, it’s difficulty to choose the character points because there
are few roads and the roads always are not wide enough ( the
ground resolution of ASTER image is 15m) to recognize, so
other points such as turning points of a road was chosen as
character points. The distribution of character points is shown
in figure 2-1.
2.2.2 measure the coordinates of the character points
The ground coordinates of the 40 character points were meas
ured by GPS, and planimetric accuracy of these points can
reach to 1 m.
2.2.3 compare the coordinates difference