[Proj] Map projection of Portugal
jagoncal at gmail.com
Mon Dec 30 19:39:03 EST 2013
Dear members of the list
I would like to put some questions related to map projections being used in
Europe. Most of the European countries are now using map coordinate systems
based on the ETRS89 datum. The Portuguese Geographical Institute has
adopted in 2006, for the continental Portugal, a projection, called
“PT-TM06” (EPSG code 3763), similar to the ones used before with older
geodetic datums (Datum Lisboa and Datum 73). It has, in my opinion, some
annoying characteristics that make users loose time, and even make errors
with costly consequences.
The projection is a transverse Mercator, with a central point in the centre
of the country, no scale change, i.e. k_0=1, and no false easting/northing,
so coordinates can be positive or negative. Some old coordinate systems in
other countries also had the origin inside the country or region, but I
think that now, most of them do not have this situation. So, my first
question is if anyone knows of map coordinate systems in use, for
topographic surveying and cadastre, which have negative coordinates.
The second bad aspect of our new coordinate system is that, being the
projection similar to the older ones, coordinates are also very similar.
Differences are in general smaller than 6 meters, so when one looks at a
coordinate pair it is not obvious if it is EPSG:3763 or older systems
(datum 73, EPSG:27493 or datum Lisboa, EPSG:20791). For many years people
will have to deal with data in the older systems, so making mistakes
becomes very easy. The reasonable approach would have been to add a big
shift so that coordinates are not mistaken, as I think most countries do.
This leads to my second question: does anyone know of similar situations of
very close coordinate systems?
I know that in Belgium a projection was established for ETRS89 (Belge 2005,
EPSG:3447) which had coordinates similar to an older system (EPSG:31300).
Meanwhile a new system was defined with new false easting/northing (Belge
2008, EPSG:3812). Was the change motivated by users?
Regards and a happy new year
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