Full text: Mapping without the sun

Li Baipeng 3 ’* Yan Qin a Chen Chunquan 3 
3 China Academy of Surveying and Mapping, 16 Beitaiping Road, Beijing, China, 10039-baipeng001@sohu.com 
KEY WORDS: ASTER, Orthorectified image, accuracy assessment, geolocation, check point, Beijing area 
ASTER( The Advanced Spacebome Thermal Emission and Reflection Radiometer), a sensor aboard on NASA's Terra satellite, 
provides low-priced Visual and Near-Infrared (VNIR), Thermal Infrared (TIR), and Short Wave Infrared (SWIR) data. The VNIR 
subsystem of the data contains stereo bands ( VNIR3N and VNIR 3B) that can be used for the generation of DEMs. 
The approach of assessment is that choosing the characteristic points from ASTER L3A image first, then the coordinates of the 
specified points were measured by GPS, the geolocation accuracy was got by comparing the image coordinates and GPS coordinates. 
Accuracy assessment result proves that the precision of DEM and orthorectified ASTER image can satisfy the demand of 1: 50, 000 
map in Beijing area. 
1.1 ASTER and ASTER 3D Ortho product 
The Advanced Spacebome Thermal Emission and Reflection 
Radiometer (ASTER) is an advanced multispectral imager that 
was launched on board NASA’s Terra spacecraft in December, 
1999(ERSDAC, 2005). It was placed in a 705 km(at equator) 
sun synchronous orbit with descending node crossing at about 
10:30 am local solar time and the orbital inclination of 98.2 de 
The ASTER sensor is designed to provide image data in 14 
visible, near-infrared, short wavelength infrared and thermal 
infrared spectral bands with the spatial resolution of 15m, 30m, 
90m separately . Stereo image data are recorded only in Band 3, 
which is the near-infrared wavelength region from 0.78 to 0.86 
pm, using both nadir and aft-looking telescopes. 
The ASTER instrument has two types of Level-1 data: Level- 
1A and Level-IB data. Level-1A data are formally defined as 
reconstructed, unprocessed instrument data at full resolution. 
According to this definition, the ASTER Level-1A data consist 
of the image data, the radiometric coefficients, the geometric 
coefficients and other auxiliary data without applying the 
coefficients to the image data to maintain the original data 
values. The Level-IB data are generated applying these 
coefficients for radiometric calibration and geometric 
resampling. The ortho image is the image observed just above 
the target point. This means the ortho image includes no terrain 
error. The ortho image can be generated by correcting the 
terrain error using the elevation data for each pixel and the off- 
nadir observation angle. The 3D ortho product is the ortho 
product with the elevation data for each pixel, generated from 
the Level-1 A data. Its formal name is Level-3A01. Figurel-1 
shows the relationship between the 3D ortho data and the 
source data. 
The instrument geometric parameters such as the line of sight 
(LOS) vectors and the pointing axis vectors were precisely 
adjusted through a validation process using numerous GCPs. 
The DEM data, which is processed using only these system 
parameters, has been demonstrated to have extremely good 
*3D ortho Data are ortho images with elevation data for each 
Figure 1-1 Relationship between the 3D ortho data and the 
source data 
In 3D ortho data processing, the level-1A data is used as input 
image data. Moreover, the Level-4A01X(DEM XYZ) data is 
used as geolocation information for providing ortho graphic 
projection and map coordinates projection features to the 
Level-1A data. After performing collection to the Level-1A 
data and 
the DEM data, a geometric conversion is performed on the 
image data. At that time, the SWIR parallax erros in the along- 
track direction due to the detector alignment and in the cross 
track direction due to the Earth rotation are also corrected. 
The 3D ortho product generated is image data that has been 
subjected to ortho graphic projection processing and map 
coordinates projection processing. The DEM Z (elevation) data 
generated from the Level-4A01 X data for geolocation 
information on the image data, and DEM quality flag data are 
attached to the 3D ortho product after performing the same 
transformation of coordinates as for the image data. The DEM 
data used in the data processing is useful as quality information 
and, at the same time, may improve users’ convenience if the 
DEM geometrically matching image data is attached.

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