EPSG/ESRI 37203 corrections (missing towgs84!)

[klokan]

This may be a problem of other from Proj4 ESRI derived systems in the EPSG.io database. May be solved properly by #24

Reported by:
[Dr.Sharad Lele, Senior Fellow, ATREE www.atree.org]
[email protected]

EPSG 37203 (or ESRI 37203) corresponds to GCS Everest India Nepal, which refers to Everest 1956 spheroid and India_nepal datum. This spheroid-datum combination is used in Survey of India toposheets of the 1960-2000 period.

The problem is that the proj.4 specification of 37203 is incomplete. It only specifies:

+proj=longlat +a=6377301.243 +b=6356100.230165384 +no_defs ...........................[1]

This implies there is no displacement of the datum with respect to the WGS84 centre of earth. But in fact, the India-Nepal datum is displaced significantly w.r.t WGS84. The displacement values are given in various documents (one such document is attached--search for Nepal). Accordingly, the correct proj.4 specification of this spheroid-datum combination would be:

+proj=longlat +a=6377301.243 +b=6356100.2284 +towgs84=295.00,736.00,257.00,0,0,0,0 +no_defs ...................... [2]

[PERHAPS a more accurate representation would be

+proj=longlat +a=6377301.243 +b=6356100.2284 +towgs84=295.00,736.00,257.00,12.00,10.00,15.00,7.00 +no_defs ............. .[3]

I am not sure about this].

As you know, QGIS software uses the proj.4 definitions directly. I have experimented in QGIS with using just 37203 as it is to geo-rectify scanned toposheets. But this results in the geo-rectified toposheet being displaced from its correct position by about 200meters. When i use my 'corrected proj.4 specification' [2], the geo-rectified toposheet sits perfectly in position. I have tested this with several toposheets and the same thing happens.

I hope I am making my point clear. I am not an expert on projections or GIS software as such, but I have over many years figured out how to correctly geo-rectify Indian toposheets. When I was using ERDAS Imagine to do this, I was explicitly defining the 'reference projection system' as "Geographic lat-long with Everest 1956 spheroid and India-Nepal datum", and things worked just fine. When I shifted to QGIS, I noticed this problem (because QGIS uses the proj.4 definitions), and so I thought of bringing it to your notice.

I would be happy to correspond further if any clarifications are required.