[Geotiff] RE: [Gdal-dev] Dataset mismatch?

[Ed McNierney]

Jarrett -

I looked at both of your coordinates on TopoZone, and - assuming that
I'm identifying the "tiny rock spire" correctly, neither one of those
coordinates seems to be correct.

Assuming the coordinates you list below are NAD83, here is the "topo
map" location:

http://www.topozone.com/map.asp?z=13&e=394704&n=4334970&datum=NAD83&u=4

And here is the "elevation spike" location:

http://www.topozone.com/map.asp?z=13&e=394571&n=4335039&datum=NAD83&u=4 

Using the USGS NED data, I find a local elevation maximum of 11,999.8
feet at UTM 13 394644E 4335185N (NAD83), which corresponds quite well
with the location of the little "ring" on the topo map you can see in
both the links above.

Perhaps if you could give us the data you're using and walk through the
process showing intermediate results we can help diagnose the
problem(s).

	- Ed

Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
ed at topozone.com
(978) 251-4242 

-----Original Message-----
From: gdal-dev-bounces at lists.maptools.org
[mailto:gdal-dev-bounces at lists.maptools.org] On Behalf Of Jarrett L.
Redd
Sent: Friday, January 27, 2006 4:00 AM
To: gdal-dev at lists.maptools.org; proj at lists.maptools.org;
geotiff at lists.maptools.org
Subject: [Gdal-dev] Dataset mismatch?

Howdy...

Please forgive the long cross-posting.  I'm new to this and don't know
exactly who will have a possible answer for this issue of mine.

I'm a volunteer working on a avalanche terrain and runout mapping
project for potential use by our mountain search and rescue teams.  I'm
using DRG 24k topographical geotiff slices in UTM NAD27 and then
processing 1/3 arc-sec elevation .adf files in lat/lon NAD83 to match up
and plot slopes and such. 
All this is downloaded from the NED seamless website.

Problem is, the two data sets don't match up precisely.  That is, the
features on the topo seem to match up precisely with elevation data in
some places and not so precisely in others.  At the worst, the error is
around 500 feet.  I'm using "libproj" to convert coordinates between the
two datums.  I'm currently processing a section of Colorado, and I'm
building "libproj" to include the "conus" correction file.  I've also
verified my coordinate conversions are correct by comparing against
openEV and topoUSA.  I'm also using "libgdal" to pull out the elevation
data and "libgeotiff" to grab the image raster data and geo tags.

However, like I said, I'm new to this.  I'm converting the NW and SE
corners of the geotiff into NAD83 and then interpolating for each pixel
to match up with the elevation data using the origin and resolution of
the various elevation pieces.  I have a sneaky feeling that life isn't
that simple.  Am I missing some fancy projection to correct for
curvature of the earth or something?  Or is this just an inherent
mismatch between the data sets?  Or both?  Other suggestions to try?

Please don't suggest using a commercially available mapping package
since we have no money and I need to do very extensive data processing
once I have the data sets matching up properly.

Thanks.

-Jarrett

P.S.  Here's an example of a nearly 500 foot mismatch error:

A tiny rock spire on the topo map:
13 0394704 4334970

And the corresponding spike in elevation data:
13 0394571 4335039

[Coords provided by OpenEV cursor]

I don't expect a perfect matchup, but the worst areas need to be
corrected somehow to make the avalanche terrain maps useful.

I can email an example image showing the error if someone is really
interested.

Thanks again.

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