[Proj] TM06 for Portugal

[Jose Gonçalves]

Roger

The test data cannot be correct. ETRS89 coincides with WGS84 (or better
ITRF) in 1989 and the current discplacement is only a few decimeters. The
lat-long differences between your coordinates are 0.080" and 0.122", which
correspond to approximately 2.9 m in lat and 2.5 m in long, i. e the
differences you get in the projected coordinates.

Yout test point belongs to the portuguese geodetic network. The ETRS89
coordinates are correct, as you can check in:

http://www.igeo.pt/produtos/geodesia/vg/rgn/ficha.asp?num=873
(open with Internet Explorer)

Where did you get the WGS84 coordinates? They must be wrong.

Regards

José A. Gonçalves
Univ. of Porto, Portugal



2010/6/7 Roger Oberholtzer <roger at opq.se>

> 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
>
> Ramböll Sverige AB
> Krukmakargatan 21
> P.O. Box 17009
> SE-104 62 Stockholm, Sweden
>
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> Mobile: Int +46 70-815 1696
>
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