[Proj] WGS84 to ED50

[Hamish]

Roger Oberholtzer wrote:
> Is anyone using proj to convert WGS84 latitudes and longitudes into ED50
> (Spain and Portugal) northings and eastings?
> 
> My attempts get me to within a couple of meters of values calculated by
> Trimble and Applanix (which are within a mm of each other). I did ask a
> question about this a while back and got some answers that improved my
> values by some 100 meters. But I never did get the values I need. So I
> am trying again.
> 
> As an example, I have (via the C API):
...
> "+towgs84=-131.0320,-100.2510,-163.3540,-1.2438,-0.0195,-1.1436,9.3900"
> 			"+no_defs");
> 
> 
> When I transform:
> 
> lat=42.87645687 lng=-4.88298199 alt=0.00000000
> 
> I get Easting=346318.770 Northing=4749022.461
> 
> but I need Easting=346319.686 Northing=4749023.634
> 
> Granted the difference is 1.489 meters. But at other locations it can be
> a couple of meters away. The +towgs84 parameters cane from an e-mail
> from Karel Cech on this list. Using these did improve the my values. But
> I am not sure the values are correct.
> 
> As Applanix and Trimble provide the same values for Spain (the ones I am
> trying to get from proj), they must be using an existing and agreed upon
> method. I would be surprised if proj couldn't do the same, if I could
> just figure out how to tell it to do so!
> 
> The Applanix provides this information on their calculations:
...
>  Coordinate transformation from WGS84 to mapping frame datum
>     dX = 125.098545; dY = 76.000054; dZ = 156.198703; f =
> 0.999991695369;
>     R1 = 0.000000000000; R2 = 0.000000000000; R3 = -0.000005473550;


The above R3,f values makes me hesitate a little, but it may be that Applanix
and Trimble are both preforming a less precise 3-term transform, while PROJ is
doing a more precise 7-term transform. That would certainly explain a 1-4m
error.

Rather than trust either software or method implicitly, it would be better to
judge the results based on published test points from the Spainish and/or
Portuguese government mapping office, and double check the validity of the
7-terms params from an official document too. Depending on national convention,
the +- signs of the terms may or may not be switched.


Hamish



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