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

Jarrett L. Redd jarrett_l_redd at yahoo.com
Fri Jan 27 13:53:05 EST 2006

Howdy, Ed...

Thanks for the response.  Sorry, I didn't specify that the coords I gave for
the rock spire are in NAD27.  I checked with topozone and it seems to match up
for that datum.

However, I think I may have found part of my problem.  I need to find the NAD83
coords for each pixel in the geotiff to match it up with the elevation data. 
To do this, I was linearly interpolating from the NW/SE corner coords.  This
seems to be the source of the error.  When I switch to using GTIFImageToPCS to
go from pixels to NAD27 UTM, then "libproj" to go from NAD27 to NAD83, I get a
pretty good match.  However, this is very slow, so I guess I'll still
interpolate, but over smaller regions.

However, once I have the pixel coords, I have to figure out the associated
elevation value.  I don't see any other way to do this but to interpolate from
the provided origin and lat/lon pixel resolutions in the ADF file.  Is this the
proper way?



--- Ed McNierney <ed at topozone.com> wrote:

> 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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