<br><font size=2><tt>From my colleague Lynn Usery (USGS/ Geospatial Information
Office). Mike Finn</tt></font>
<br>
<br>
<br>
<br>
<br><font size=2><tt>Using 2 points is not sufficient. The user should
use a grid of at least <br>
20 x 20 points (400 total points) over the quad. Transform those between
<br>
the datums and resample the pixels based on this approach. This is a <br>
simple operation in Imagine which automatically locates the control <br>
points then applies the datum tranformation, interpolates and resamples
<br>
the data. Of course Jarrett said no commercial software, so he must find
<br>
a way to implement this process with open source code.<br>
<br>
To perform the datum transformation, two points is just not enough to <br>
handle the differences between the two datums. NGS used thousands of <br>
points in the transformation to determine the NADCON shifts between NAD
<br>
27 and NAD 83 for the US.<br>
<br>
Lynn</tt></font>
<br>
<br>
<br>
<br>
<br>
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<td width=40%><font size=1 face="sans-serif"><b>"Jarrett L. Redd"
<jarrett_l_redd@yahoo.com></b> </font>
<br><font size=1 face="sans-serif">Sent by: proj-bounces@lists.maptools.org</font>
<p><font size=1 face="sans-serif">01/27/2006 02:59 AM</font>
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<div align=center><font size=1 face="sans-serif">Please respond to<br>
"PROJ.4 and general Projections Discussions"
<proj@lists.maptools.org></font></div></table>
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<div align=right><font size=1 face="sans-serif">To</font></div>
<td><font size=1 face="sans-serif">gdal-dev@lists.maptools.org, proj@lists.maptools.org,
geotiff@lists.maptools.org</font>
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<div align=right><font size=1 face="sans-serif">cc</font></div>
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<div align=right><font size=1 face="sans-serif">Subject</font></div>
<td><font size=1 face="sans-serif">[Proj] Dataset mismatch?</font></table>
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<br><font size=2><tt>Howdy...<br>
<br>
Please forgive the long cross-posting. I'm new to this and don't
know exactly<br>
who will have a possible answer for this issue of mine.<br>
<br>
I'm a volunteer working on a avalanche terrain and runout mapping project
for<br>
potential use by our mountain search and rescue teams. I'm using
DRG 24k<br>
topographical geotiff slices in UTM NAD27 and then processing 1/3 arc-sec<br>
elevation .adf files in lat/lon NAD83 to match up and plot slopes and such.
<br>
All this is downloaded from the NED seamless website.<br>
<br>
Problem is, the two data sets don't match up precisely. That is,
the features<br>
on the topo seem to match up precisely with elevation data in some places
and<br>
not so precisely in others. At the worst, the error is around 500
feet. I'm<br>
using "libproj" to convert coordinates between the two datums.
I'm currently<br>
processing a section of Colorado, and I'm building "libproj"
to include the<br>
"conus" correction file. I've also verified my coordinate
conversions are<br>
correct by comparing against openEV and topoUSA. I'm also using "libgdal"
to<br>
pull out the elevation data and "libgeotiff" to grab the image
raster data and<br>
geo tags.<br>
<br>
However, like I said, I'm new to this. I'm converting the NW and
SE corners of<br>
the geotiff into NAD83 and then interpolating for each pixel to match up
with<br>
the elevation data using the origin and resolution of the various elevation<br>
pieces. I have a sneaky feeling that life isn't that simple. Am
I missing<br>
some fancy projection to correct for curvature of the earth or something?
Or<br>
is this just an inherent mismatch between the data sets? Or both?
Other<br>
suggestions to try?<br>
<br>
Please don't suggest using a commercially available mapping package since
we<br>
have no money and I need to do very extensive data processing once I have
the<br>
data sets matching up properly.<br>
<br>
Thanks.<br>
<br>
-Jarrett<br>
<br>
P.S. Here's an example of a nearly 500 foot mismatch error:<br>
<br>
A tiny rock spire on the topo map:<br>
13 0394704 4334970<br>
<br>
And the corresponding spike in elevation data:<br>
13 0394571 4335039<br>
<br>
[Coords provided by OpenEV cursor]<br>
<br>
I don't expect a perfect matchup, but the worst areas need to be corrected<br>
somehow to make the avalanche terrain maps useful.<br>
<br>
I can email an example image showing the error if someone is really interested.<br>
<br>
Thanks again.<br>
<br>
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Proj@lists.maptools.org<br>
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</tt></font>
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