[Proj] ETRS1989 to Greek Grid (GGRS1987)

[support.mn at elisanet.fi]

I found the following data about Greek Grid and local datum:

"There is a new reference system used in Greece nowadays. It is
called the Greek Geodetic Reference System of 1987 (GGRS87)
where Fo = 38° 04' 33.8107" North, Lo = 23° 55' 51.0095" East of
Greenwich, No = 7.0 m, and the new Greek Grid is based on the
Transverse Mercator projection (presumably Gauss-Krüger) where
fo = 0°, lo = 24° E, the False Easting = 500 km, and the scale factor at
origin (mo) = 0.9996. Generally, I have serious doubts concerning any
“new” grid system that uses some non-standard variant of the UTM
Grid, but I understand that this particular one was devised by Professor
Veis of the Technical University of Athens. If Professor Veis approved
of this new grid, then there certainly must be a valid technical
reason for the curious parameters chosen. Thanks for the above
parameters go to Yannis Yanniris, a photogrammetrist in Athens.

The National Imagery and Mapping Agency (NIMA) has published
datum shift parameters from the European Datum of 1950 in Greece
to the WGS84 Datum where DX = –84 m, DY = –95 m, and DZ = –130
m; however, this solution is based on only two points and the accuracy
of the components is stated to be ±25 m. Users interested in
geodetic applications of GPS in Greece should read the NIMA
notice published next to my column in PE&RS October, 2002. The
European Petroleum Studies Group has published shift parameters
from GGRS87 to WGS84 as being DX = –199.87 m, DY = +74.79 m,
and DZ = +246.62 m. The EPSG published no accuracy estimates for
their parameters, so caveat emptor."

In the following link:

http://www.asprs.org/resources/grids/12-2002-hellenic.pdf

That might help you. You simply add the "towgs84" statement in the
local projection with those three parameters given in the above text
and you should get near the truth. Your local datum is shifted that
amount from the WGS84 datum. There might be additional shifting
required to the ETRS1989 from European 1950. The amount shift
needed is possible to figure out if you know some standard points
in Greekland in WGS84 and local datum. It is rather clear that the
Greek Grid is defined in the old local datum.

ETRS1989 (or EUREF89) is very close to WGS84 datum (difference is
less than 1 m) so that they can be considered the same. The following
datum shift for the Greek Grid was found in the esri data base. That
"towgs84" statement also fix your problem:

# esri:2100
# GGRS87 / Greek Grid
proj=tmerc
lat_0=0
lon_0=24
k=0.999600
x_0=500000
y_0=0
ellps=GRS80
towgs84=-199.87,74.79,246.62,0,0,0,0
units=m
no_defs
no_defs


Regards: Janne.

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Leonidas Liakos <leonidas_liakos at yahoo.gr> kirjoitti: 
> Dear all,
> recently I tried to convert some data from ETRS1989 (Lambert Azimuthal 
> EqualArea) to Greek Grid. Comparing the results of the conversion, it 
> seems that there is an important shift (error?) of those data.Is there 
> any special tranformation method and parameters that I should use? Yours 
> sincerely,
> 
> Leonidas Liakos
> Geographer
> 32nd Ephorate of Antiquities
> Hgoumenitsa,Greece
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