on the SAR images. For these reasons, the selection of suitable ground
control points was restricted to features such as centres of small lakes
and the well-defined corners of forests or woods. A further consequence
of this relates to the fact that,on all images of the test area, the
pattern of control points being tested is irregular in distribution as
will be seen on Figs. 1 to 4,
Both optically processed images were at a scale of 1:250 000. The resol-
ution of the survey optically processed image was in the order of 50-60m,
while that of the precision-processed one was about 25 m. The digitally
processed image by RAE was at a scale of 1:150 000 and has a resolution
of 20 m, while the digitally processed image by DFVLR was at a scale of
1:250 000 with a resolution of 25 m.
PROCEDURE OF THE GEOMETRIC TEST
The actual procedure followed for the metric tests of the four images of
the test area goes along the following lines: :
(i) Suitable ground control points were chosen on the four images
to serve as control or check points. The object positions of
those points were derived from the map with the help of a
coordinatograph. The accuracy of measuring the map positions
of the points was estimated to be in the order of * 0.2 mm,
equivalent to abeut * 13 m on the ground.
(if) The positions of the image points were measured using a mono-
comparator.
(iii) The set of measured points was divided into two groups:
(a) The control points which are used to determine the unknown |
transformation parameters by comparing their measured X,y image
coordinates and their X, Y map coordinates.
(b) The check points which are used to assess the metric accu-
racy of the images by transforming their image x,y coordinates
to the map system using the already determined transformation
parameters.
(iv) The residual errors at all measured points were plotted as vector
errors to give an immediate view of the error pattern to supplement
the numerical findings.
A number of coordinate transformations were used in order to transform
the radar images to the map system using the control points. These
coordinate transformations are:
(i) The Linear Conformal (or similarity) Transformation;
(ii) The Affine Transformation; and
(iii) A higher degree Polynomial Transformation of the form:
2 3 3 2,2 3,3
X = a +a xta yta xy*a x^«a x^ysa xy2?+a x +a x’y+a x?y?sa x?y
90-743! 22 3 4 5 6 7 8 9 10
Y
*b x?«b x?ysb xy?«b x?4b x?y«b x?y?4b x3y?
D Tb nsb yrb xy Ta S y | y ; ; y ; y Al
In practice,the terms of these polynomials are truncated one by one in
order to judge their significance on derived accuracy.
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