[Gdal-dev] USGS vs. ESRI vs. OGR

[Grabowski, Hank]

I followed your advise to compare to the USGS tool.  The conversion
results are listed below.  As we can see, the ESRI and USGS tools match
identically (to the precision of ESRI which was lower than the USGS
solution).  There is a convergence parameter from the USGS calculation
which is significantly larger than the error between OGR and ESRI/USGS.
Being a novice at this reprojection business, I don't know if that is
the bounds of the accuracy of the solution.  I would tend to believe not
however, since a 40 minute minimum accuracy would be a multi-kilometer
error in position.  On a hunch I looked at the two definitions used for
WGS84 by both OGR and ESRI, and their coefficients match to reported
accuracy.  I'll continue investigating this issue if no one has any
ideas what could be causing it.

UTM X	597196.62	Meters		
UTM Y	4116325.5	Meters		
				
Product	Lat			Lon			Convergence
Scale
		DD MM SS.sssss 	DDD MM SS.sssss 	DD MM SS.ss
Unitless
USGS		37 11 24.57282 	085 54 17.71610 	0 39 43.15
9.99716E-001
ESRI		37 11 24.57 	085 54 17.72		
OGR		37 11 24.76	      085 54 17.62		

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
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