[Gdal-dev] NAD83 vs. WGS84

[Grabowski, Hank]

I would find it to be an aggregious error that the ESRI code, which is
used in a signficant number of GIS installations, is not doing the datum
shift.  I'm going to call their tech support department in the next
hour, once they open, to determine if the software isn't performing the
task that they say it is performing.  If I look at the layer's active
coordinate conversions when I have a WGS84 based layer and I import the
NAD27 UTM data I see one coversion called NAD_1927_To_WGS_1984_1.  Doing
the same thing with a NAD83 layer produces a conversion called
NAD_1927_To_NAD_1983_NADCON.  The NAD27->NAD83 method is NADCON.  The
NAD27->WGS84 method is a Geocentric Translation method.  Having never
coded up a conversion method myself, I don't know if that is the
standard mechanism of conversion between those two datums.  

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: Friday, December 12, 2003 9:58 AM
To: gdal-dev at remotesensing.org
Subject: RE: [Gdal-dev] NAD83 vs. WGS84


Hank -

I'll look into this a bit, but for the location and precisions you're
talking about, the NAD27->WGS84 conversion IS the NAD27->NAD83
conversion.  The difference between the two should be invisible or at
best microscopic - nowhere near the delta you're seeing.  I'm inclined
to agree with Frank here, and you may just be grouping tools into those
that are doing the datum shift and those that are not.

	- 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: Friday, December 12, 2003 9:43 AM
To: gdal-dev at remotesensing.org
Subject: [Gdal-dev] NAD83 vs. WGS84


The problem as I've determined is that my "NAD27->WGS84" conversion is
really just doing a "NAD27->NAD83" conversion.  The NAD27->NAD83
conversion in ESRI identically matches the conversion I'm attempting in
OGR.  Is there something wrong with the syntax of my command for the
conversion to WGS84?  

ogr2ogr -t_srs WGS84 out.shp roads.shp

My program's internal conversion to WGS84 has the exact same difference,
so I figure once I determine how to get the conversion working in
ogr2ogr I should be able to map it over to my internal code as well.  Is
it such that NAD83 is the best I can do with OGR? If that's the case
that's fine.  I just need to make that fact known to our users.

Thanks for the help guys, you've all been very responsive.
Hank
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