[Proj] Portuguese Modified Grid - Datum 73
Roger Oberholtzer
roger at opq.se
Wed Jun 23 07:41:59 EST 2010
On Thu, 2010-02-18 at 13:24 +0000, Jose Gonçalves wrote:
> Hello
>
> Some history about that projection: that central point was chosen at a
> mid latitude of the country and 1d00'00"E of the Lisbon Meridian
> (9d07'54.862"W). The small false easting/northing of a few hundred
> meters, which is quite unusual, was chosen in order to create
> projected coordinates similar to projected coordinates in an older
> datum (datum Lisboa).
I have finally obtained test data for conversion from WGS84 lat/longs
into D73 northings/eastings. After earlier discussions on this list
(between Feb 16 and 18, 2010), it was suggested that the appropriate
specification for proj would be:
"+proj=tmerc " // Projection
"+ellps=intl " // Spheroid
"+k=1.0 " // Scale factor at central meridian
"+x_0=180.598 " // False easting
"+y_0=-86.98999999999999 " // False northing
"+lon_0=-8.131906111111112 " // Longitude of central meridian
"+lat_0=39.66666666666666 " // Latitude of origin
"+towgs84=0,0,0,0,0,0,0 "
"+no_defs"
With test locations, I have a rather large error.
Reference point (Santa Cruz):
Latitude: 41.6743830556 Longitude: -8.3312663889 (WGS84)
Easting: -16493.683 Northing: 222799.692
Using the above definition, I get this:
Easting: -16421.042 Northing: 222981.401
Which is 195.691 meters from the reference calculation. I have points
all around Portugal, and the all seem to be around 195 meters from the
expected location.
Either I am missing something, or the test data is in error. Or both.
Does anyone know what they would expect for the D73 northing/easting at
the WGS84 lat/long test point above?
--
Roger Oberholtzer
OPQ Systems / Ramböll RST
Ramböll Sverige AB
Krukmakargatan 21
P.O. Box 17009
SE-104 62 Stockholm, Sweden
Office: Int +46 10-615 60 20
Mobile: Int +46 70-815 1696
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