[Proj] [gdal-dev] gdalwarp EPSG:32662 problem
Antti Castrén
antti.castren at iki.fi
Mon Feb 10 03:41:59 EST 2014
2014-02-09 8:40 GMT+02:00 Andre Joost <andre+joost at nurfuerspam.de>:
> Am 09.02.2014 00:42, schrieb Even Rouault:
>>
>> But Antti guess seems right. Instead of +ellps=WGS84 (or +datum=WGS84), if
>> you
>> play with the a (semi-major axis) and b (semi-minor axis) parameters, you
>> can
>> see that only +a has an influence, so latest proj version seems to use a
>> spherical version of eqc.
>
>
> If you look at
> <http://trac.osgeo.org/proj/browser/trunk/proj/src/PJ_eqc.c>
>
> and the chapter 12 of Snyders manual, you will only find formulas for the
> sphere. So I guess there is no other way to calculate eqc.
>
> Maybe older versions calculated another radius for the sphere when an
> ellipsoid was given.
Stephen's "shift" was about 20km south, which correlates quite well if
you use semi-minor axis of WGS84 as radius of sphere while calculating
forward, and semi-major axis as radius of sphere while calculating the
inverse. At latitude of 55 degrees the difference is ca. 20 530 meters
(55 degrees -> 54.8156 degrees).
There are several different radii of the Earth, and some of them could
arguably be used in this context in place of semi-major axis.
All radii ar not created equal: http://en.wikipedia.org/wiki/Earth_radius
You have to bear in mind that this projection is intended for small
scale mapping, for example mapping the whole world. In that scale 20
km is nothing. If you need better representation of the Earth you have
to use a projection which takes ellipsoidal properties into account.
Of course the beef in this thread is not about choosing a projection,
but the change/difference in formulae used, which can create problems
as Stephen pointed out.
Cheers,
Antti
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