[Qgis-developer] Fwd: Re: Datum Transformation - parameters for mainland Portugal

[Pedro Venâncio]

Hi Andre!




> tfm 1571 is a bug in the EPSG database. It should be "Amersfoort to 
> ETRS89 (1)", but somehow the source CRS got wrong.
> 
> It was deprecated in 2001, and replaced by tfm 1751 (4289 to 4258) under 
> the same name.
> For some reasons, EPSG still keeps it in its database, marked 
> "deprecated". The online version of the database does not show it 
> anymore, so I think QGIS can drop transformations marked deprecated as well.


+1 to drop all deprecated transformations.



> Usually, ETRS89 and WGS84 are concidered identical. There is another tfm 
> 1149 from 4258 to 4326 with all parameters to zero.
> 
> The transformation table inside QGIS only has those tfm from the EPSG 
> database where the target CRS is 4326 (we want towgs84, right?)
> 
> So I suggest to set target CRS for all new portuguese CRS to 4326 as well.


Yes, I know Andre, in my proposal I put the transformations to 4326, except the transformations using NTv2 grids, since these have been prepared specifically for 4258, and do not use the "+towgs84" parameter, but "+nadgrids". What do you think?



> For the other bug about Lisbon datum with prime meridian Greenwich vs 
> Lisbon, I have to investigate a bit further. Maybe the low accuracy tfm 
> now used applies to both within the range of accurancy.


The difference between them seems to me only the Prime Meridian, one is referenced to Greenwich and the other to Lisbon: 

[EPSG:4803] 

Anchor Definition:
Fundamental point: Castelo Sao Jorge, Lisbon. Latitude: 38°42'43.631"N, longitude: 0°E (of Lisbon). 

Prime Meridian [Lisbon]
Code: EPSG::8902
Name: Lisbon
Greenwich Longitude: 9° 7' 54.862" W


[EPSG:4207] 

Anchor Definition:
Fundamental point: Castelo Sao Jorge, Lisbon. Latitude: 38°42'43.631"N, longitude: 9°07'54.862"W (of Greenwich). 

Prime Meridian [Greenwich]
Code: EPSG::8901
Name: Greenwich
Greenwich Longitude: 0°


I also do not know if this has any implications on the accuracy of the transformations, but which I find more strange is that all the transformations are set from 4207. Would, in the past, EPSG 20790/20791 were using Greenwich Prime Meridian, and this was updated recently?



> @pedro: I really would encourage you to get this into the EPSG database 
> as soon as possible.

I will do that Andre.


Thank you very much!

Best regards,
Pedro