Parent Topic: <A href="http://www.pcigeomatics.com/cgi-bin/pcihlp/STG">STG</a><hr>
<H3>DETAILS</H3>
STG registers slant range or ground range radar imagery to a UTM
grid, given the elevation data for the area covered by the input
radar image and the flight path parameters previously calculated by
the FLIGHT program. If the input image is a slant range image, then
it is converted to a ground range image by the registration process.<p>
(Note: The SRTOGR program converts radar imagery from slant range to
ground range, without using elevation data or having to estimate
flight path parameters. If UTM registered elevation data is not
available, or if it is impossible to collect ground control points
on the input image, then SRTOGR can be used to convert slant range
imagery to ground range instead.)<p>
STG geometrically registers the input radar image data (DBIC) for
the input file (FILI), and transfers the registered radar image to
the output data channel (DBOC), such that it overlays the elevation
image (DBEC) on the output file (FILO). It is assumed that the
elevation image has been previously registered to a UTM grid using
the PACE Geometric Correction Package, and that the UTM coordinates
of the bounding rectangle for the output file have been defined with
the GEOSET program. If no elevation data is available for the area
(DBEC= ), the input radar image is registered to a UTM grid based on
a flat terrain model.<p>
STG determines the input radar pixel which corresponds to each
output radar pixel in the output file. In other words, the
registration process is output driven, not input driven. If there is
no input pixel corresponding to a particular output pixel, then the
output pixel value is not changed. This allows for mosaicking of the
output imagery. If desired, the user could run the CLR program to
pre-clear all pixels in the output channel(s), replacing the values
with a specified grey-level value, before running STG.<p>
The values of the input parameters FILI, FILO, DBEC, INPXSZ, ESCALE,
DELAY, RANGETYP, and HEIGHT are set by the user before running the
FLIGHT program, and normally do not have to be changed before
running STG. Likewise, the values of the parameters OALTI, OHEAD,
OPOINT, and OCOEFF, output by the FLIGHT program, should not be
changed before running the STG program.
<P>
<H4>RESAMPLING</H4>
Resampling is the process of sampling one or more input pixels to
create one output pixel. (This is different from the transfer phase,
which merely determines where the output pixel lies on the input
image). In general, resampling is accomplished by taking a weighted
average of a small window of input pixels around some point
determined by the transform phase. The quality of the corrected
output image and the time required in calculation are highly
dependent on the resampling method chosen. STG offers three of the
more popular resampling methods: Nearest Neighbour, Bilinear
Interpolation, and Cubic Convolution.<p>
NEAREST NEIGHBOUR: This method takes the single input pixel nearest
to the transformed point as the resampled output pixel. The
advantages of this method include a very low computational cost, and
the non-alteration of input pixel grey levels. This non-alteration
is important if you are registering theme or classified data. The
disadvantage is that the output image may be jagged and blocky in
appearance if there is much rotation or scaling.<p>
BILINEAR INTERPOLATION: This method takes a weighted average of the
four input pixels around the transformed point. The advantages of
this method include low computational cost and relatively smooth
output images. The disadvantage is that the output image may appear
slightly blurred.<p>
CUBIC CONVOLUTION: STG uses the 4-point classic method. This
variation of cubic convolution uses a 4-by-4 window of input pixels.
This method is closer to the perfect sin(x)/x resampler than the
Nearest Neighbour or the Bilinear Interpolation method. The
advantage is a smooth, sharp output image. The disadvantage is a
very high computational cost.
<P>
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