(162k)
The following image shows the result of the combined atmospheric and topographic
correction on SPOT-1 panchromatic imagery.
Landsat TM band 1 imagery. The scene was recorded on 19 April 1988 and contains
part of the Walchensee lake and surrounding mountains in the Bavarian Alps.
The solar geometry is: zenith angle 38 degree, azimuth angle 158.3 degree.
The top left presents the original scene, the bottom left the corrected scene. Top right
shows the aspect image (flat areas of the lake are coded bright), bottom
right the illumination image, i.e. the cosine of the local solar zenith angle,
so bright areas indicate a small local solar zenith angle (or high sun), dark
areas receive less or no direct solar radiation.
The ATCOR3 processing compensates the atmospheric/topographic influence
on the recorded signal and calculates a surface reflectance map for a flat
horizontal plane.
This example shows that atmospheric/topographic correction is even useful
for panchromatic imagery. Here, emphasis is on the elimination of topographic
effects, not on the surface reflectance retrieval.
(162k)
About the same area of the Walchesee lake is shown in the following figure. It presents part of a TM scene acquired 28 July 1988. The solar zenith angle is 34.6 degree, the solar azimuth angle 131.7 degree. The top left of the mosaic is the DEM (800 - 1800 m asl), the top right is the illumination image. The bottom left shows the original TM data (bands 3/2/1 coded RGB), the bottom right presents the surface reflectance image after combined atmospheric and topographic correction. Areas that appear dark in the original image due to a low solar illumination are raised to the proper reflectance level in the corrected image.
(137k)
The next image is a small Landsat-5 TM sub-scene of Makhtesh-Ramon (Israel) acquired 21
September 1995. The solar zenith angle is 44.5 degree, the solar azimuth angle 124.8
degree. The image was processed in the framework of a joint German-Israeli
Foundation project.
The top part presents the original data of TM bands 7/4/1 (coded RGB), the
bottom part is surface reflectance after combined atmospheric/topographic
correction. Most of the topographic features that can be seen in the original
image are eliminated in the surface reflectance image.
(90k) The topographic influence causes an enhancement of apparent surface reflectance values: surfaces oriented towards the sun appear to have higher reflectance values than the same surface cover oriented away from the sun if the topography is neglected. This effect is compensated for in the corrected image where the topographic information is employed to calculate a reflectance map for a flat terrain.