[GRASS5] PROJ_INFO and prime meridian

Michel Wurtz mjwurtz at wanadoo.fr
Wed Jun 4 17:08:54 EDT 2003

Markus Neteler wrote:
 > A projection related question:

 > - prime meridian Monte Mario (old!): 12d27'7.1E (today 12d27'8.4E)
 > - "spheroid":     Bessel
 > - Origin:         14E, 45:54N

Origin Meridian is not the prime Meridian... It's unusual

 > - False Easing:   800000 meters
 > - False Northing: 601000 meters

Strange False northing too (did the surveyors find a nice cave full
of good wine ? ;-))

 > lat_0: 45.9000000000   <- given 45:54N
 > lat_1: 33.0000000000   <- calculated by GRASS, given 'return'
 > lat_2: 45.0000000000   <- calculated by GRASS, given 'return'

Hmm... The problem may be here : I don't understand the values of lat_1
and lat_2 here : The conic projection may be tangent => Lat_0, LAt_1 and Lat_2
should be the same, or the conic projection is secant => Lat_0 is between
Lat_1 qnd Lat_2... BUT some peoples use a tangent projection, but with
a scale (at the tangent parallel) lesser than 1 (0.998...) which gives
results very closes of a secant projection (that's true for the french
LAmbert projection

 > Ideas (comments welcome):
 > - Lambert Conic Orthomorphic != LCC (I hope not)
I think it's synonyms

 > - the optional definition of the true prime meridian may solve the
 >   rotation problem (the map center is around 11 degree East from Monte
 >   Mario)

uses 12°27'7.1" in r.proj ? (but how use 14° for false easting ???...
In the same way look at the values for Lat_1 and Lat_2 (use 45.9 ?)

 > - the bessel definition of 1945 may be different from the current bessel
 >   in GRASS

seems unprobable, the name would be different

 > - the US Army used a different datum for LCC (where to find out?)

But I doubt they do else than put new détails on an old map, thus
respecting the original projection...

 > - the Gauss-boaga location with recent GIS data is wrong (I hope not)

Maybe, but difficult to verify, execpt if you have the opportunity to
go with a gps receiver on a easely identifiable point on both old
and new data and look what the GPS will give (a simple Garmin or Magellan
for outdoor will be enough to give you the name of the faulty set, since
the precision of this kind of GPS is about 10 m, comparing to your 400/800m)

Hoping this will help you

Good luck,

Michel Wurtz - Auzeville-Tolosane

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