[Gdal-dev] OGR vs. ESRI Clarke 1866 Spheroid

[Grabowski, Hank]

Both OGR and ESRI are using the exact same semi-major axis parameter,
with an extra 4e-10 meters.  Both round to the same one-digit semi-minor
axis.  I didn't believe that these numbers were significant enough to
produce a multi-meter error, however I'm just reporting what I'm finding
along my investigative process.

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 4:22 PM
To: gdal-dev at remotesensing.org
Subject: RE: [Gdal-dev] OGR vs. ESRI Clarke 1866 Spheroid


Hank -

These appear to be in the range of binary floating-point storage
precision limits.  According to Snyder's Map Projections (USGS Bulletin
1395) the Clarke 1866 ellipsoid is:

Semimajor axis:	6378206.4
Semiminor axis:	6356583.8
Inverse flattening: 294.98

where the two axes are listed as "exact values" and the third is
computed (my desktop calculator gives 294.97869821390582...)

Since the axes are supposed to be exact at one decimal place, it appears
that the OGR values are more accurate than the ESRI ones.  In each case
the inverse flattening seems to be correctly computed as the inverse of
1 - (semiminor/semimajor).

	- 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 3:59 PM
To: gdal-dev at remotesensing.org
Subject: [Gdal-dev] OGR vs. ESRI Clarke 1866 Spheroid


There is a small discrepancy in the spheroid parameters for the Clark
1866 spheroid between ESRI/USGS and OGR.  Does anyone have a link to a
government standards site, like the USGS or USNO, that maintains these
constants?  Here are the sets of constants for the two programs from
their respective generated projection (.prj) files:

OGR:
  Spheroid: Clarke 1866
    Semimajor Axis: 6378206.400000000400000000
    Semiminor Axis: 6356583.799999999800000000
    Inverse Flattening: 294.978698213897990000


ESRI:
  Spheroid: Clarke_1866
    Semimajor Axis: 6378206.400000000400000000
    Semiminor Axis: 6356583.799998980900000000
    Inverse Flattening: 294.978698200000000000
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