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

[Frank Warmerdam]

Grabowski, Hank wrote:
> 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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Hank,

 From the EPSG definitions I see the following:

7008,Clarke 1866,6378206.4,9001,,6356583.8,1,Original definition a=20926062 and b=20855121 (British) feet. Uses Clarke's 
1865 inch-metre ratio of 39.370432 to obtain metres. (Metric value then converted to US survey feet for use in the 
United States using 39.37 exactly giving a=20925832.16 ft US).,US Army Map Service Technical Manual No. 7; 
1943.,EPSG,1995-06-02 00:00:00,98.34,0

OGR computes the inverse flattening from the semimajor and semiminor
axis.  I would have to assume that ESRI uses a deliberately truncated
value for inverse flattening to discard "noise" digits beyond the real
precision of the value.

Best regards,

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