[gdal-dev] proj.4 +b parameter has different precision on i386 vs. amd64

[Etienne Tourigny]

Hi list,

It seems that the +b parameter of Proj.4 strings generated by gdal can
have different precisions, dependent on the architecture (i386 vs.
ams64) for various linux flavors and gdal versions (1.6 - 1.9).

The CRS from EPSG:4131 can have slightly different values: (
+b=6356075.413140239 vs. +b=6356075.41314024 )

i386:
+proj=utm +zone=48 +a=6377276.345 +b=6356075.413140239
+towgs84=198,881,317,0,0,0,0 +units=m +no_defs

amd64:
+proj=utm +zone=48 +a=6377276.345 +b=6356075.41314024
+towgs84=198,881,317,0,0,0,0 +units=m +no_defs


==> Can this happen with other values?

==> Is it an artifact of the calculation of the semi-minor axis from
semi-major and inv-flattening in ogr_srs_proj4.cpp ?

==> Is this a bug?


On my system (gdal 1.9, ubuntu 11.10, amd64) here is what the WKT and
PROJ.4 look like

$ gdalsrsinfo EPSG:4131

PROJ.4 : '+proj=longlat +a=6377276.345 +b=6356075.4131402401
+towgs84=198,881,317,0,0,0,0 +no_defs '

OGC WKT :
GEOGCS["Indian 1960",
    DATUM["Indian_1960",
        SPHEROID["Everest 1830 (1937 Adjustment)",6377276.345,300.8017,
            AUTHORITY["EPSG","7015"]],
        TOWGS84[198,881,317,0,0,0,0],
        AUTHORITY["EPSG","6131"]],
    PRIMEM["Greenwich",0,
        AUTHORITY["EPSG","8901"]],
    UNIT["degree",0.0174532925199433,
        AUTHORITY["EPSG","9122"]],
    AUTHORITY["EPSG","4131"]]



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http://lists.osgeo.org/pipermail/qgis-developer/2012-May/020357.html