[Proj] projection of Madagascar

OvV_HN ovv at hetnet.nl
Tue Apr 29 13:01:59 EDT 2008

Clovis wrote:

> I have been quickly through the archive but didn't find any topic on this.
> I have a projection issue concerning Madagascar.

There are no good nor usable alternatives for the Laborde projection.
Using the Oblique Mercator you'll get decimeters of error within the 
Madagascar territory.
One example from my software:

Laborde Madagascar:
lat = -16d 11m 23.280s; lon = 44d 27m 27.260s;
lat0 = -18.9; lon0 = 46d 26m 14.025s;
x0 = 4e5; y0 = 8e5; k0 = 0.9995; Az = 18.9;
international 1924 ellipsoid
-> x, y = 188333.848, 1098841.091

Oblique Mercator approximation with the following parameters:
lat0 = -18.9; lon0 = 46d 26m 14.025s; lonc = lon0;
x0 = 4e5; y0 = 8e5; k0 = 0.9995; alfac = 18.9; gamma = alfac;
international 1924 ellipsoid
-> x, y = 188333.847, 1098841.111

(angles in degrees wrt Greenwich, lengths in meters)

These differences may seem small, but wait until you get farther away 
from the central line.

See also section "Laborde projection for Madagascar", EPSG 
Guidance Note 7-2, version April 2008.
There you can also find omerc examples with an error of several decimeters.

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