[Proj] TM06 for Portugal

[Roger Oberholtzer]

On Thu, 2010-02-18 at 16:40 +0100, Roger Oberholtzer wrote:

I now have some test data for the conversion from WGS84 lat/long to TM06
northing/eastings. The suggested proj conversion of:

      "+proj=tmerc "
      "+ellps=GRS80 "
      "+k=1.0 "
      "+x_0=0 "
      "+y_0=0 "
      "+lon_0=-8.133108333333334 "
      "+lat_0=39.66825833333333 "
      "+towgs84=0,0,0,0,0,0,0 "

seems in fact to work with ETRS89 lat/long, not WGS84 lat/long. When
provided with WGS84 lat/long, the resulting northings/eastings are a few
meters away. (Assuming my test data from the Portuguese Road Department
is correct. I have double checked, so it might very well be.)  In fact,
it is a small error, but I want to provide the best I can. I am guessing
that I need to change my +towgs84= to cause the WGS84 lat/longs to be
converted to ETRS89. Does anyone know the proper values?

This begs the question: what does
http://spatialreference.org/ref/epsg/3763/ assume is the datum of the
original lat/longs? Seems it assumes ETRS89, not WGS84.

Here is a sample location that I have been provided (all altitudes set
to 0.0):

 Ref (WGS84):    Latitude:   41.6743830556 Longitude:   -8.3312663889
Ref (ETRS98):    Latitude:   41.6744052222 Longitude:   -8.3312324722
  Ref (TM06):    Easting:    -16498.151 Northing:       222796.720

The proj conversion above properly converts the ETRS98 locations to
TM06. But I want to convert the WGS84 locations to TM06. When I try
that, I get:

  Easting: -16500.981 Northing: 222794.265

which is 3.747 meters away.

-- 
Roger Oberholtzer

OPQ Systems / Ramböll RST

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