[Proj] Portuguese Modified Grid - Datum 73

[Jose Gonçalves]

Hello

The proj string you refer is not correct. What is called in the EPSG
database "Portuguese Modified Grid - Datum 73" (epsg code 27492) is
the projection used in Portugal for the local datum called Datum 73.
Datum 73 was established in 1973 and is based on the Hayford
ellipsoid.
The correct definition of the projection can be found in the epsg file
distributed with proj:

# Datum 73 / Modified Portuguese Grid (deprecated)
<27492> +proj=tmerc +lat_0=39d00' +lon_0=-8.131906111111112 +k=1
+x_0=180.598 +y_0=-86.98999999999999 +ellps=intl +units=m +no_defs  <>

It is a Transverse Mercator projection with central point:
    lat_0=39d00'00.000"N
    lon_0= 8d07'54.862"W
No scale factor in central meridian is applied (+k_0=1)
It has a small false easting/northing
    x_0=180.598
    y_0=-86.990

This coordinate system was replaced by a new projection based on datum
ETRS89. The new projection, that the Portuguese Geographic Institute
(IGP) called PT-TM06, has a different central point. It's correct
definition is:

# ETRS89 / Portugal TM06
<3763> +proj=tmerc +lat_0=39.66825833333333 +lon_0=-8.133108333333334
+k=1 +x_0=0 +y_0=0 +ellps=GRS80 +units=m +no_defs  <>

In your proj string you had this central point mixed with the Datum 73
projection.

For a test point you can use the following (a geodetic point called Cabreira):

D73 Geographic - epsg:4274     (8d02'39.1243"W 41d38'17.2443"N)
D73 projected  - epsg:27492    (7488.375 218847.838)
ETRS89 Geographic - epsg:4258  (8d02'35.8302"W 41d38'20.2812"N)
ETRS89 Projected  - epsg:3763  (7483.752 218845.648)

To convert from Datum 73 to ETRS89 a Bursa-Wolf transformation can be
used. Using program cs2cs you can have, for example:

cs2cs +init=epsg:27492
+towgs84=-230.994,102.591,25.199,0.633,-0.239,0.900,1.950 +to
+init=epsg:4258 +towgs84=0,0,0

These parameters are given by IGP
(http://www.igeo.pt/produtos/Geodesia/Inf_tecnica/parametros_transformacao/parametros_Portugal.htm).

I have created NTv2 grid files to convert from datum 73 (and other
older national systems) to ETRS89 with better accuracy. They are
available in my page: http://www.fc.up.pt/pessoas/jagoncal/coordenadas
(sorry, only in Portuguese).

Regards

José A. Gonçalves
University of Porto, Portugal






2010/2/16 Roger Oberholtzer <roger at opq.se>:
> On Tue, 2010-02-16 at 14:53 +0100, OvV_HN wrote:
>> Dunno, but doesn't the utm projection uses a couple of preset values for
>> lat_0, k_0, x_0, y_0, and a calculated value of lon_0?
>> The tmerc projection seems more appropriate.
>
> You are surely correct. I will give this on a shot:
>
> +proj=tmerc +lat_0=39.66825833333333 +lon_0=-8.133108333333334 +k=1 +x_0=180.598 +y_0=86.990 +ellps=intl +units=m +no_defs
>
> Too bad it is always so hard to get appropriate test data for
> verification.
>
>
> --
> Roger Oberholtzer
>
> OPQ Systems / Ramböll RST
>
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