[Gdal-dev] Datum shift accuracy

[Ed McNierney]

Hank -

Given your setup problems, I'm slightly concerned that there may be some residual problem.  I think that's unlikely - I'd expect a complete failure, not a slightly inaccurate result - but this also may be a usage problem.

In my experience I have compared UTM PROJ results with the online NOAA tools and found them to agree to better than a millimeter.

	- 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: Grabowski, Hank [mailto:hgrabows at stk.com]
Sent: Thursday, December 11, 2003 2:43 PM
To: gdal-dev at remotesensing.org
Subject: RE: [Gdal-dev] Datum shift accuracy


This is true.  I was just hoping that one of the two watches had
documentation of validation against the Naval Observatory's Atomic
Clock, so to speak.  Being as I don't if either of these have been
rigorously validated against an industry baseline, if one exists, I'll
take your advice and throw a third watch into the mix.  Thanks!

Hank Grabowski
hgrabowski at stk.com
1-610-578-1000
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-----Original Message-----
From: Ed McNierney [mailto:ed at topozone.com] 
Sent: Thursday, December 11, 2003 1:55 PM
To: gdal-dev at remotesensing.org
Subject: RE: [Gdal-dev] Datum shift accuracy


Hank -

As the saying goes, the man with two watches never knows what time it
is!  It might be worth testing the conversion on a third source, such as
the "Geodetic Tool Kit" tools from NGS at http://www.ngs.noaa.gov/TOOLS
just to see whether they agree with your PROJ output or with your ESRI
output (or neither <g>).

	- 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: Grabowski, Hank [mailto:hgrabows at stk.com]
Sent: Thursday, December 11, 2003 1:45 PM
To: gdal-dev at remotesensing.org
Subject: [Gdal-dev] Datum shift accuracy


Hello again,

I want to thank everyone for their help getting the datum shifting
working again.  Converting the tables to binary form fixed the problem.
Now that I'm getting conversions however a second question has arisen.
What kind of precision should I expect in these conversions between
OGR/Proj.4 and ESRI ArcGIS?  

When converting Albers projections to WGS84 projections in both products
I get identical results.  However when I convert this NAD 1927 UTM Zone
16N data to WGS84 I get discrepancies.  There is a five to seven meter
difference, mostly in the North-South axis, between the OGR conversion
(using the ogr2ogr command) and the ESRI conversion.  Is this to be
expected?  My dataset is "roads.shp" from the the fourth exercise for
3DAnalyst in ArcTutor.  It is a city in the road data for a town in the
middle of Kentucky.  For this sort of conversion, is 6-7 meter precision
considered good or poor?  I would think it is poor, but I wanted to
defer to those with more experience with this sort of computation.

Hank Grabowski
hgrabowski at stk.com
1-610-578-1000
------------------------------------------------------------------------
----------
Experience the analysis and visualization power of STK 5.0 by upgrading
today! Order your free copy at http://www.stk.com to get started!
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