[Proj] ED50 from C API
roger at opq.se
Thu Mar 29 05:40:04 EST 2007
Let me ask this another way:
proj has definitions for many conversions in the 'esri' file that that
it provides. These definitions appear to be used as changes from some
default initial settings. I say that as many say very little. The rest
must be specified elsewhere. When I want to add a new conversion via the
C API, I expect I could start by looking for an existing definition in
the 'esri' file. As the list can probably tell, each time I find I need
to add a new conversion, I never get it right and then ask the list.
Each time the corrections seem (to me) to be different. So, I am missing
some important part of the puzzle. Does no one else on the list use the
C API very much?
On Wed, 2007-03-28 at 16:04 +0200, Roger Oberholtzer wrote:
> I would like to convert WGS84 lat/longs from a GPS receiver into
> northings and eastings in ED50, fixed in UTM zone 30N. I have tried the
> following, but the results are always 100-150 meters off from what I
> expect. What have I missed? I based this in the ESRI file in proj 4.4.9
> Initialization (error checking removed here for clarity):
> // From GPS WGS84
> fromProj = pj_init_plus( "+proj=latlong "
> "+ellps=WGS84 "
> "+towgs84=0,0,0 "
> // To ED50
> toProj = pj_init_plus( "+proj=utm "
> "+ellps=intl "
> "+zone=30 "
> "+units=m "
> The call for each location:
> lng = LONG * DEGREE_TO_RADIAN;
> lat = LAT * DEGREE_TO_RADIAN;
> alt = 0; // We do not give this a height
> pj_transform(fromProj, toProj, 1, 0, &lng, &lat, &alt);
> For example:
> Input WGS84: lat=37.57508059 lng=-0.93032997 alt=0.00000000
> Obtains: Easting=682771.459 Northing=4160841.260
> But I am expecting: Easting=682876.090 Northing=4160892.020
> There is a difference in location of 116.293 meters.
> The error is in the initialization step. I suspect it is something that
> is not in the ESRI file that I need to add when doing this from the C
> API. But what?
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