Nepalese National Grid?

These are notes on a hunt for parameters for the Nepali national mapping grid, after a colleague noticed a mismatch between large scale GIS data, which used an imprecisely defined coordinate system, and handheld GPS positions. Even allowing for a GPS error of say +/- 10 metres, there was still a discrepancy of several hundred metres.

ArcGIS doesn’t have Nepal among its list of pre-defined coord systems (nor incidentally does GPS Utility), so a UN agency in Nepal had gone about defining its own (see ‘1’ below). In effect, this is “UTM Zone 45½”. However, using this, the GPS mismatch appeared. Note that this PRJ definition uses the Everest 1830 ellipsoid.

NGA lists details of the datum used in Nepal here – this refers to the Everest 1956 ellipsoid.

NGA further lists the various Everest ellipsoid parameters here.

Everest

Brunei & E. Malasia (Sabah & Sarawak)

EB

6377298.556

6356097.550

1/300.8017

India 1830

EA

6377276.345

6356075.413

1/300.8017

India 1956*

EC

6377301.243

6356100.228

1/300.8017

Pakistan*

EF

6377309.613

6356109.571

1/300.8017

W. Malasia and Singapore 1948

EE

6377304.063

6356103.039

1/300.8017

W. Malasia 1969*

ED

6377295.664

6356094.668

1/300.8017

Without some control data, I can’t readily check whether this ellipsoid difference accounts for the observed offset – that will be the next task.

Also, note that the scale factor in the modified PRJ file below is 0.09999, rather than UTM’s usual 0.9996; I’m not sure how much of an effect this would have “on the ground” (is there a graph somewhere of scale error against distance form central meridian?).

In the meantime, I came across a couple of other passing references to similar problems (Evaluating Land Use Dynamics and Forest Cover Change in Nepal’s Bara District (1973–2003) and Mapping cryptic invaders and invasibility of tropical forest ecosystems).

It’s also worth noting the Nepalese Journal on Geo-informatics

From a MapAction perspective, issues 5 and 7 may of of special interest, with various disaster management and hazard mapping articles.

(1)
PROJCS[“Nepal_MUTM_84”,
GEOGCS[“GCS_Everest_1830”,
DATUM[“D_Everest_1830”,
SPHEROID[“Everest_1830”,6377276.345,300.8017]],
PRIMEM[“Greenwich”,0.0],
UNIT[“Degree”,0.0174532925199433]],
PROJECTION[“Transverse_Mercator”],
PARAMETER[“False_Easting”,500000.0],
PARAMETER[“False_Northing”,0.0],
PARAMETER[“Central_Meridian”,84.0],
PARAMETER[“Scale_Factor”,0.9999],
PARAMETER[“Latitude_Of_Origin”,0.0],
UNIT[“Meter”,1.0]]

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