Clovis clovis.grinand at ird.fr
Tue Apr 29 12:27:48 EDT 2008

```Dear list,

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

Historically there is one dedicated projection called "Laborde", using
International 1925 ellipsoïd with others parameters.
From what i know it has been approximate with an oblique mercator
projection and another called hotine oblique mercator.

Then, I use ArcGIS usually for my application and want slowly to move
towards R and QGIS, espacially to transform point coordinates in R
(spTransform). The big problem is that none of three have the same
projection definition and do display large offset when projecting GPS in
WGS84 to "Laborde".

From the projection definition, you will see below that there is some
agreement (lat of origine, scale factor, false easting, false northing,
azymuth) but some are completly different : longitude of origine and
prime meridian etc.

As I consider ArcGIS definition and transformation to be the right one
(works well!), my question are:
- Is the EPSG parameters up to date?
- How can i tune the parameters to obtain the same coordinates that with
arcGIS.
- Why lonc and not lon_0 ? (lon_0 does not give anything good)

I hope I m not missing something obvious.

Clovis

# EPSG 29700 Via R
print(CRSargs(CRS("+init=epsg:29700")))
 " +init=epsg:29700 +proj=omerc +lat_0=-18.9 +lonc=44.10000000000001
+alpha=18.9 +k=0.9995000000000001 +x_0=400000 +y_0=800000 +ellps=intl
+towgs84=-189,-242,-91,0,0,0,0 +pm=paris +units=m +no_defs"

# EPSG 29700 Via GQIS
+proj=omerc +lat_0=-18.9 +lonc=41.76277083000001 +alpha=18.9
+k=0.9995000000000001 +x_0=400000 +y_0=800000 +ellps=intl
+towgs84=-189,-242,-91,0,0,0,0 +pm=paris +units=m +no_defs

# Laborde projection in ArcGIS
laborde
Projection: Hotine_Oblique_Mercator_Azimuth_Center
False_Easting: 400000,000000
False_Northing: 800000,000000
Scale_Factor: 0,999500
Azimuth: 18,900000
Longitude_Of_Center: 46,437229
Latitude_Of_Center: -18,900000

GCS_Tananarive_1925
Name: GCS_Tananarive_1925
Remarks:
Angular Unit: Degree (0,017453292519943299)
Prime Meridian: Greenwich (0,000000000000000000)
Datum: D_Tananarive_1925
Spheroid: International_1924
Semimajor Axis: 6378388,000000000000000000
Semiminor Axis: 6356911,946127946500000000
Inverse Flattening: 297,000000000000000000
```