Full text: Proceedings of the Symposium on Global and Environmental Monitoring (Part 1)

each element of the array the relative cover of 
ten cover types was measured. These were 
combined to form three main types: heather 
species, grass species and bare soil. The cover 
percentages of these types along the elements 
of the array form a pattern. In the image 
exists a similar pattern in pixel values, induced 
by the field pattern (figure 1). 
ground element 
Figure 1: Grass cover percentage along an array 
of ground elements, and pixel values on corres 
ponding positions in the image. 
Correspondence between field pattern and 
image pattern is supposed to reach a maxim 
um when those pixels are selected that co 
incide in space with the ground elements. The 
residual variance from the regression of 
ground cover percentages on pixel values can 
be used as a measure of this correspondence. 
Optimization of the image localization with 
respect to the field data was done by selecting 
the array of pixels that yielded the least pos 
sible residual variance. In figures 2 and 3, the 
hypothetical effect of shifting and rotating the 
array of pixels on the residual variance is 
depicted. When pixel and ground array do not 
overlap, the residual variance is supposed to 
fluctuate at random on a high level. 
Figure 4 shows actual contours of this vari 
ance when an array of pixels is shifted in the 
image. In this way, positioning the field data 
in the image can be done with subpixcl ac 
curacy. 
In most cases, ground elements will not 
align with pixels. Pixel arrays were therefore 
resampled from the image, allowing them to 
take every desirable position and orientation. 
Resampling was done by sampling a number 
of points (625 in this case) within the projec 
tion of the new pixel in the image, and by 
taking the average of the sampled values as 
the new pixel value. 
A line intercept method (Cialcs 1979) was 
used in the field to measure the cover of 
heather, grass and bare soil in a series of 
ground elements. Along three parallel lines, 
spaced 12.5 m apart, the lengths of the cover- 
types intersecting with (he line were recorded. 
These lengths were summed over a distance of 
25 m, the size of a ground clement. The sum- 
total of lengths for a cover type, relative to 
x-position 
Figure 2: Expected behaviour of residual varian 
ce as a function of the position of pixel and 
ground element arrays relative to each other. 
When they fully overlap, residual variance rea 
ches a minimum. 
Angle between pixel-array and ground elenent array 
Figiire 3: Expected behaviour of residual varian 
ce as a function of the angle between the array 
of pixels and the array of ground elements. 
Residual variance reaches a minimum when the 
arrays arc parallel.
	        
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