[Proj] ED50 from C API

[Eric Miller]

I don't know about the "esri" file, but you have two different ellipsoids and no information to convert between the different models.  Don't you need a datum or towwg84 parameter for the ED50 definition?  I think the ED50 towgs84 parameters are missing because there is more than one candidate.  See, http://crs.bkg.bund.de/crs-eu/

There isn't really much difference between using cs2cs and pj_transform.  The init strings are the same.  Same for proj and pj_fwd/pj_inv.

>>> On 3/29/2007 at 3:40 AM, Roger Oberholtzer <roger at opq.se> wrote:
> 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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