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THE ACCURACY ASSESSMENT OF ORTHORECTIFIED ASTER IMAGE
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
ABSTRACT:
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. INTRODUCTION
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
grees.
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
accuracy.
*3D ortho Data are ortho images with elevation data for each
pixel.
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.
