The rest of the exercise is devoted to geometric correction and
re-sampling. In order to illustrate the main principles, an
artificial image with 10 lines by 10 pixels is used initially. A
ground control point file is established from given pairs of
control points by GCP. (Keyboard mode is used for file- as
well as map-coordinates). The transformation equation is
determined by COORDN. LRECTIFY is used to produce one
image-file for each of the three resampling methods Nearest
Neighbour (N), Bilinear Interpolation (B) and Cubic
Convolution (C) as shown in Figure 4b. The students also
perform, based on the transformation equation, a "manual
resampling-calculation" for some selected pixels. In this
connection it can be mentioned that the way COORDN shows
the coeffisients of the transformation equation is somewhat
confusing.
Figure 4b. Transformation of a synthetic image with different
resampling algorithms.
An artificially distorted version of an uncorrected Landsat MSS
image (denoted "AR=6:1" in Figure 4c) is the basis for the part
of the exercise which deals with capture of ground control
points using the digitizing tablet. The result of this rectification
is denoted "GCP" in Figure 4c.
In the next part of the exercise, GCPs are measured on a map
and entered into ERDAS with the keyboard. The map is a large
scale topographic map (scale 1:5000) and the image is a SPOT
level 1B panchromatic subscene. The result of this rectification
process is denotetd "SPOT-MAP" in Figure 4c. When rectifying
the image, the students are asked to define corners
corresponding to one of the 1:5000 map sheets covering the
actual area.
In the last part of the exercise the goal is to perform a "system
correction" of the same distorted Landsat MSS image as is
mentioned above. Based on the "aspect ratio", the earth rotation
and the angle between the orbital- and the equatorial plane, the
transformation equation is calculated. The result of the
subsequent resampling is denoted "MOD" in Figure 4c.
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Figure 4c. Image rectification with various methods.
SATK2. Exercise 2. Determination of terrain elevation
from a single oblique SPOT scene.
The main emphasis in this exercise is rather on the under-
standing of the satellite image geometry than on the use of the
ERDAS system. By measuring distances (expressed in pixels)
by CURSES in a level 1A (not geometrically corrected) pan-
chromatic SPOT subscene, the students shall calculate the
terrain elevation for selected pixels in the image from known
true horisontal distances.
In addition to the subscene in which the measurements are
carried out, another obliqe SPOT subcene with a different view
angle covering the same area is also available. Together these
two subscenes form a stereo model By intelligent use of
READ, an image which can be stereoscopically viewed through
red/green glasses (the anaglyph method) is produced on the
screen. As this paper is in B/W, the effect cannot be
demonstrated here, but on the display screen of the ERDAS-
system, the effect is striking. Since the two images in the
stereo-model are taken from different orbits, the orientations of
the two images are slightly different when they are registered
to a common reference system, as shown in Figure 5.
Figure 5. Stereo model from two oblique panchromatic SPOT
subscenes.