On Thu, 20 Feb 2003 12:01:01 -0500, "Timothy R. Cook"
<z993126bellsouth.net> said:

>Thomas Willhalm wrote:
>> I wouldn't car about the errors too much. It already looks good as it is.
>
>Well...it is I think better than 99.9% match...but a DEM from the USGS
>site and a topographic map made by the USGS ought to match up exactly.

I wouldn't expect them to match up.  I would, however, expect them to
be skewed along the north-south direction.  It's a orthorectification
and projection issue.  The DEM is probably projected in UTM and the
topographic quadrangle is normally projected in state plane coordinate
system.  The state plane coordinate system would be skewed the further
north the map is located.  Part of the orthorectification problem is
that they are made by two different independent departments within the
USGS organization that don't talk to each other (and in fact there is
a bit of a feud between them).

Another common pitfall is that you may be assuming the post distances
of the DEM are equidistant along both axes when in fact one axis may
have slightly different spacing than the other.

BTW, This could be easily fixed in about 5 minutes in a GIS software
package such as ERDAS/Imagine.

---
Bob Chmilnitzky (a.k.a. Jet Jaguar)
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