[Proj] ED50 from C API

[Roger Oberholtzer]

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 "
>                                                 "+no_defs"):
> 
>         // To ED50
>         toProj   = pj_init_plus( "+proj=utm "
>                                                 "+ellps=intl "
>                                                 "+zone=30 "
>                                                 "+units=m "
>                                                 "+no_defs"):
> 
> 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?
> 
-- 
Roger Oberholtzer

OPQ Systems / Ramböll RST

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