Elimination and simulation of Terrain Effects on Remote Sensing images using DTM
Method
We know that the pixel value N on the satellite image is quantified from the spectral energy E received by the sensor:
N = a * E + b [1]
a and b are constants.
The spectral energy E can be divided into three parts: 1) energy of the space light scattered directly into the sensor, EA. 2) energy of the space light reflected from the ground surface and coming into the sensor, ES 3) energy of the sunlight reflected from the round surface and coming into the sensor, ED. It can be expressed as:
E = EA+ES+ED [2]
Let us define:
r' : ground reflectance
LS: energy of space light received by pixels on horizontal plane
LD: energy of sunlight received by pixels on horizontal plane
T : atmosphere transmissivity
q : ground slope
n : ground aspect
G: ground viewfield factor
a: sun azimuth
b: sun elevation
j: incident angle of sunlight toward the ground pixel
Supposing that the atmosphere is homogenuous and the sunlight is parallel, the values of T, EA, a, b, LS, LD are contents for all pixels in the image extent. Also we have:
if the intensity of scattered space light is same at all directions and in any places, them:
Where D
S is a constant unknown, same for all pixels; N, G, cosq are known parameters, varying with the pixels; r is a variable unknown, varying with the pixel. As a result, Ds can be computed by the minimum square-root criterion using the pixels in the shadow.
From equation [6] for the pixels out of the shadow we have;
Where D
s, N, q, j and G are all known. S
D is a constant unknown,
r is a variable unknown, varying with the pixel. So S
D can be calculated by the minimum square-root criterion using the pixels out of the shadow.
Now that parameters D
s and S
D are known, the apparent reflectance map r can be computed:
In order to simulate the images at different sunlight directions, it is necessary to locate the shadow area at that direction at first, then the simulated image can be calculated using equation [10] for the pixels in the shadow and equation [6] for those out of the shadow.
If we let r = 1, q = 0, G = 1 for all pixels and set the incident angle as 900 if it is greater than 900, an image can be produced using equation [6], meaning that the single influence of incident angle is added onto the constant reflectance image in which the reflectance of all pixels are the same such as 1. In this way, the influence of other factors and combinations of factors can be added onto the constant reflectance image separately to produce the simulated images corresponding which are the comparative analysis basis of the influence of the single factor and factor combination.
Dicussion of test result
PHOTO 2 is the apparent reflectance map computed out using equations [12] and [13] from PHOTO 1, a LANDSAT MSS image (band 5). The terrain effects have been removed on PHOTO 2. Two reflecting features on PHOTO 1, the higher intensity value in upper part than in lower part and the high reflectance of the river, are represented on PHOTO 2.
PHOTO 3 is the artificially simulated image from PHOTO 2 using equations [6] and [10], the sunlight azimuth and elevation of which are 450 and 300. The southeast northwest structures on PHOTO 3 are obviously enhanced.
PHOTO 4 includes four image produced by adding different kinds of terrain effects onto the constant reflectance image. The upper left represent the influence of a single factor, the incident angle; and the upper right contains the terrain effect of two kinds, the incident angle and the ground slope. The lower left is the result influenced by the incident angle and the ground viewfield factor, while the lower right is produced by adding four factors, the shadow, the incident angle, the ground slope and the ground viewfield factor all together onto the constant reflectance image.
The similarity between the upper left and the lower right demonstrates that the in
fluence of the incident angle on the satellite image is the biggest. Because the upper right is more similar to the lower right than the lower left, it is considered that the influence of the ground slope is bigger then that of the ground view field factor. The in fluencies the shadow is virtually included in the incident angle affect, because most pixels in the shadow are shaded by the neighboring pixel, the incident angle of which are no les, then 900 . Only small number of pixels in the shadow are shaded by the distant pixel.
The above results can be explained theoretically. It is clear that the intensity of sunlight is much higher than that of the space light. The ground view field only influences the space light, so its effect is the smallest. Although both the incident angle and the ground slope influence the ground slope influence the sunlight, the sunlight, the dynamic range of cosq is from 0 to 1, much higher that of cosq, because the ground slope is limited. As a result, the effect of the incident angle is the biggest.
Conclusion
The element of the above approach to compute the apparent reflectance is to calculate the light parameters S
D and D
s by using the terrain parameters and intensity value of the satellite image to process statistically the reflectance variations, than to compute the apparent reflectance of each pixel. The whole process is completed automatically by the computer, without the human effect. The three hypotheses about homogeneous atmosphere, parallel sunlight and diffusing space light are rather reasonable. The terrain effects on the satellite image can be eliminated consequently.
Because the apparent reflectance map is the one under certain sunlight direction, all simulated images of different sunlight directions are produced on the hypothesis that the apparent reflectances of different sunlight directions are the same. These images can enhance the corresponding information.
Through the comparative analysis of the influences of individual factors, it is found that the incident angle contributes the most part of the entire effect, Using only the incident angle factor to simulate the whole terrain effect for the generation of artificial relief on some images can get reasonable results and be time- saving.
Due to the DTM accuracy, the test results are far from ideal. DTMs with the grid size similar to or smaller than the pixel size of the satellite image are highly recommended.
Reference
Kiyonari Fukue: 7th International Symposium áá Machine Processing of Remotely Sensed Dateññ , June 23-26, 1981
Li Xianhua: Correction of Terrain Effect on Remotely Sensed information, Journal of Survey and Mapping, Voll5-2, 1986.5
