Do I need geocoded imagery?
  
   In a recent discussion of mountain areas, John Berry
   (ej10jlbs@shell.com) wrote:

        The problem that Frank has is that he is working in an area without
adequate maps:  therefore, he cannot geocode his Landsat using a DTM, because
the data available is neither detailed enough or accurate enough to use as an
input.

        He can georegister the imagery using using one or two accurately
located ground control points and the corner-point positions given in the
image header:  these are calculated from ephemeris data of, usually, unknown
accuracy (within +/- 1 km), but internal image geometry is good so an x,y
shift and a (usually) very small rotation can take care of everything to
better than the accuracy of his maps.  Positions used should be
topographically low, and at the same elevation.   GPS is the best solution, as
someone else pointed out, if Frank can get in the field.

        The next problem is the parallax error introduced by the high relief.
In his situation, the only answer* is to get SPOT stereopairs and make a DTM or
DEM from them.  Except in the case of very narrow gorges or slopes steeper
than 60 deg. there should be few problems with carefully chosen images (high
sun angles, etc).  ERDAS has an excellent module for doing this.  However, I
doubt that Frank has the budget.  I believe ERDAS`s Ortho module would then
allow Frank to make an Ortho image that would be a perfectly good map.

        *there may be some LFC or Russian stereo coverage in this area, which
would be a lot cheaper than SPOT but would require the use of analog stereo
comparators (probably).

        Even if there were good topographic contour maps for all of Frank's
area, the cost of digitising these and turning them into a usable DTM would
probably be prohibitive (though there are outfits in Russia who might be able
to quote a price affordable to a large western company).

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