<br><font size=2 face="Courier New">Ed,</font>
<br>
<br><font size=2 face="Courier New">Thank you for the reply. I haven't
rolled up my sleeves on this problem but I follow you points clearly. Off
the top of my head then, if a transformation of this sort is only a meter
or two, and a pixel is 2.4m, then Jarrett's discrepancy of a couple hundred
meters, smells like a blunder somewhere to me. (Or, perhaps, he is at the
max accuracy he can expect considering the two sources he is using and
their pedigrees (the accuracies (errors) in their sources and methods of
collection).</font>
<br>
<br><font size=2 face="Courier New">Mike</font>
<br>
<br>
<br>
<br>
<br>
<br>
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<td width=40%><font size=1 face="sans-serif"><b>"Ed McNierney"
<ed@topozone.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 12:29 PM</font>
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<td><font size=1 face="sans-serif">"PROJ.4 and general Projections
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<div align=right><font size=1 face="sans-serif">Subject</font></div>
<td><font size=1 face="sans-serif">RE: [Proj] Dataset mismatch?</font></table>
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<br><font size=2 color=blue face="Arial">Mike -</font>
<br><font size=3> </font>
<br><font size=2 color=blue face="Arial">Yes, the USGS used thousands of
points, but they needed to cover the whole country! For CONUS that's
around 50,000 quads worth. We're only talking about one quad that's
only about 10 x 15 kilometers. If the NGS used 400 points per quad
they would have needed over 20 million points.</font>
<br><font size=3> </font>
<br><font size=2 color=blue face="Arial">I took a look at the Mount Sherman,
CO quad that is the subject of the original post. The northwest corner
of that quad is at 392131E 4345059N (NAD27) and 392084E 4345267N (NAD83).
The NAD27 -> NAD83 datum shift at that point is (-47E, +208N).
If I go to the southeast corner of that quad, the coordinates are
402746E 4331044N (NAD27) and 402699E 4331252N (NAD93). The NAD27
-> NAD83 datum shift at that opposite corner is (-47E, +208N) - identical
to the datum shift at the opposite corner! I cannot imagine one would
require 400 intermediate control points to accurately calculate a datum
shift across an area the size of a 7.5-minute quad - especially when the
shift at opposite corners is the same. You might, I suppose, manage
to get off by a meter or two, but since a single pixel on a 1:24K DRG is
2.4384 meters on a side, you're literally below the threshold of precision
on the data.</font>
<br><font size=3> </font>
<br><font size=2 color=blue face="Arial">Jarrett's seeing discrepancies
of hundreds of meters. Even if you measured only one point and presumed
the datum shift was identical across the entire quad, you couldn't produce
an error that large.</font>
<br><font size=3> </font>
<br><font size=2 color=blue face="Arial"> - Ed</font>
<br><font size=3> </font>
<p><font size=2>Ed McNierney<br>
President and Chief Mapmaker<br>
TopoZone.com / Maps a la carte, Inc.<br>
73 Princeton Street, Suite 305<br>
North Chelmsford, MA 01863<br>
ed@topozone.com<br>
(978) 251-4242 </font>
<p><font size=3> </font>
<br><font size=3> </font>
<br><font size=3> </font>
<br>
<br>
<hr><font size=2 face="Tahoma"><b>From:</b> proj-bounces@lists.maptools.org
[mailto:proj-bounces@lists.maptools.org] <b>On Behalf Of </b>Michael P
Finn<b><br>
Sent:</b> Friday, January 27, 2006 12:52 PM<b><br>
To:</b> PROJ.4 and general Projections Discussions<b><br>
Cc:</b> geotiff@lists.maptools.org; proj@lists.maptools.org; gdal-dev@lists.maptools.org;
proj-bounces@lists.maptools.org<b><br>
Subject:</b> Re: [Proj] Dataset mismatch?</font><font size=3><br>
</font>
<br><font size=2><tt><br>
>From my colleague Lynn Usery (USGS/ Geospatial Information Office).
Mike Finn</tt></font><font size=3> <br>
<br>
<br>
<br>
</font><font size=2><tt><br>
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><font size=3> <br>
<br>
<br>
<br>
</font>
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<td width=46%><font size=1 face="sans-serif"><b>"Jarrett L. Redd"
<jarrett_l_redd@yahoo.com></b> <br>
Sent by: proj-bounces@lists.maptools.org</font><font size=3> </font>
<p><font size=1 face="sans-serif">01/27/2006 02:59 AM</font><font size=3>
</font>
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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><font size=2><tt><br>
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>
_______________________________________________<br>
Proj mailing list<br>
Proj@lists.maptools.org<br>
http://lists.maptools.org/mailman/listinfo/proj</tt></font><font size=3><br>
</font><font size=2><tt>_______________________________________________<br>
Proj mailing list<br>
Proj@lists.maptools.org<br>
http://lists.maptools.org/mailman/listinfo/proj</tt></font>
<br>