[Gdal-dev] Datum shift accuracy

[Ed McNierney]

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
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