[Proj] projection of Madagascar - longitude

Noel Zinn ndzinn at comcast.net
Wed Apr 30 07:03:05 EDT 2008

Laborde gave the Laborde grid with respect to the center of the projection
near the Tananarive observatory with respect to Paris, not Greenwich.  For
those of us working with respect to Greenwich, we need to know the longitude
of Paris with respect to Greenwich.  Mr Mugnier offers the British RGS
(Royal Geo??? Society) opinion 2d 20m 13.95s East also adopted by the Army
Mapping Service and its successor, the Defense Mapping Agency (now
superseded).  EPSG offer the French IGN opinion of 2d 20m 14.025.  Who best
knows the longitude of Paris with respect to Greenwich, the French or the
British?  The EPSG choose diplomatically. 

With respect to authoritative Malagasy geodetic control, I don't know.  I've
seen derived oil-company coordinates that are old, but nothing except
numerical examples in the old manuscripts that are authoritative.  And those
are so near the central line that the 29702 approximation works just fine.
That's not true offshore in general, however.  

Noel Zinn

-----Original Message-----
From: proj-bounces at lists.maptools.org
[mailto:proj-bounces at lists.maptools.org] On Behalf Of OvV_HN
Sent: Wednesday, April 30, 2008 4:49 AM
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] projection of Madagascar

Gerald I. Evenden wrote:
> Just to add to the confusion, libproj has always contained the +proj=labrd

> projection based upon a development by John Snyder.  An example script
> be:
> PROJ="+proj=labrd +ellps=intl +azi=18d54 +lat_0=18d54S +lon_0=46d26'13.95E

> +k_0=0.9995 +x_0=400000 +y_0=800000"
> Are there any known values known to be acceptable by Madagascar 
> surveyors/government?

The longitude of origin of 46d 26'13.95E puzzles me.
This value is given in the Madagascar Grids & Datums article by mr.
Mugnier, but he gives a value of 21G with respect to Paris, which would
be the same as 46d 26m 13.95s East of Greenwich (see column 2 on page 
144 of said article).
Hmmm... that would be a new chapter in cartography!

Anyhow, the EPSG GN 7-2 value of 46d 26m 14.025s wrt Greenwich is 
equivalent with exactly 49 grad wrt Paris. Wouldn't that be a more 
logical value?

Oscar van Vlijmen

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