[Proj] Convert coordinates from ED50 to Greek Grid
Mikael.Rittri at carmenta.com
Mon Jun 27 06:09:15 EST 2011
> I need to convert some coordinates from ED50 (EPSG:4230)
> to Greek Grid (EPSG:2100).I'm using the following parameters
> for cs2cs:
> cs2cs +init=epsg:4230 +to +init=epsg:2100
> but compared to already valid results, the output greek grid
> coordinates are wrong. Do I have to use any datum shift. And
> if so, what are the appropriate parameters for that?
I suggest using
cs2cs +init=epsg:4230 +towgs84=-86,-92.2,-127.5 +to +init=epsg:2100
but the datum shift may not be accurate enough (should be better than
The Greek Grid is based on GGRS87 (Greek Geodetic Reference System 1987),
and this datum has an appropriate datum shift in the file nad/epsg, at
least since Proj version 4.6.1.
However, ED50 has a datum shift in the trunk, but I think not in Proj
version 4.7. So you need to add one.
At http://www.epsg-registry.org , I have searched with "query by filter", using
Type: Coordinate Transformation
This gives three published datum shifts from ED50 to WGS 84:
Proj syntax Name Code Accuracy Area of Use Information Source
towgs84=-87,-98,-121 ED50 to WGS 84 (1) EPSG:1133 10 m West Europe (including Greece) U.S. DMA TR8350.2
towgs84=-84,-95,-130 ED50 to WGS 84 (8) EPSG:1140 44 m Greece - onshore and offshore U.S. DMA TR8350.2
towgs84=-86,-92.2,-127.5 ED50 to WGS 84 (19) EPSG:1440 999 m Greece - onshore and offshore Geodesy Department; Hellenic Petroleum s.a.
If you would use Proj from the trunk, you will automatically get the
first datum shift for ED50.
Otherwise, you must choose manually. In this case, the Accuracy field
isn't much help when choosing. The confidence of the accuracy is not
specified by EPSG, but varies with the information source. When it
happens to be U.S. DMA TR8350.2, I think it is one sigma, so the error
is worse about one third of the time. If so, then the accuraries of
the first two datum shifts cannot be usefully compared, even though
they come from the same information source, since they cover different
On the other hand, since DMA has bothered to give a specific Greek
datum shift, in addition to the general for West Europe, they must
think it is more accurate in Greece.
Personally, I would use the third datum shift, simply because it
comes from a Greek information source. The Accuracy 999 m is used by
EPSG to mean "don't know". (This datum shift doesn't differ from the
Greek DMA datum shift by more than about 4 meters horizontally.)
I don't know why the accuracy for ED50 in Greece should be as bad as
44 m. Maybe it is a pessimistic estimate, or maybe all those islands
made the ED50 realisation more difficult.
From: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] On Behalf Of leonidas
Sent: den 23 juni 2011 11:43
To: proj at lists.maptools.org
Subject: [Proj] Convert coordinates from ED50 to Greek Grid
I need to convert some coordinates from ED50 (EPSG:4230) to Greek Grid (EPSG:2100).I'm using the following parameters for cs2cs:
cs2cs +init=epsg:4230 +to +init=epsg:2100 but compared to already valid results, the output greek grid coordinates are wrong. Do I have to use any datum shift. And If so, what are the appropriate parameters for that?
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