[Gdal-dev] Datum shift accuracy

Thu Dec 11 16:27:20 EST 2003

I'm assuming it's just a configuration problem as well.  However it is
related to the datum shifting.  Doing an Albers->WGS84 reprojection
produces identical results with ESRI.  For what it's worth, doing
NAD27->WGS84->NAD27 in OGR gives me back the same shape, but I'm not
surprised at the internal consistency from that case.  The shift from a
base WGS84 to the NAD27 based projection produces a similar though
backwards shift.

My data files are the exact set in the proj446_win32_bin.zip, with the
.lla files all having been run through that nad converter that was
suggested earlier to create binary equivalents.

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 3:19 PM
To: gdal-dev at remotesensing.org
Subject: RE: [Gdal-dev] Datum shift accuracy

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
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