WGS84 vs Indian datums: why your coordinates are 200 metres off

A land team sends over the corner points of a parcel from the revenue record. The GPS on site says the same corner is 200 m to the east. Both are "correct". The disagreement is not a measurement error but a datum mismatch, and it is one of the most common ways renewable-energy projects lose time.

What a datum is

Latitude and longitude only mean something relative to a model of the Earth. The model has two parts: an ellipsoid (the mathematical shape) and a set of rules for how that ellipsoid is anchored to the physical planet. Together they form a datum.

Change the datum and every coordinate shifts, even though the ground has not moved. The shift is not random: for a given pair of datums it is a smooth, predictable offset that varies slowly across a region.

The datums you meet in India

• WGS84. The global datum used by GPS, Google Earth, Google Maps and nearly all modern GIS data. Unless a source says otherwise, assume nothing about its datum until you have checked.
• Everest 1830 / Kalianpur. The historic Indian datum, defined by the Great Trigonometrical Survey and refined in 1937, 1962 and 1975. Survey of India toposheets, many state revenue maps and a lot of older engineering drawings are on a variant of this datum. Its ellipsoid is smaller than WGS84's and its origin is at Kalianpur in Madhya Pradesh.
• Indian 1954, Indian 1960, Indian 1975. Regional adjustments used in different periods and neighbouring countries, which appear in older aviation and defence data.

The offset between Everest and WGS84 across India is typically 100 to 300 metres, depending on the location and the exact Everest variant. That is the "200 m to the east" from the opening paragraph.

How to recognise a datum problem

• Coordinates from two sources disagree by a consistent amount and direction across the whole site.
• The disagreement is far larger than GPS error (a few metres) but far smaller than a typo (kilometres).
• Points from old survey sheets sit systematically off the roads and boundaries visible in satellite imagery.
• A CAD drawing declares a projection but not a datum, or names a datum in a title block that nobody has read.

How to fix it

1. Find out the datum of every source. Ask the surveyor. Read the toposheet legend. Check the CAD drawing's coordinate system definition. Assume nothing.
2. Transform, do not offset. Applying a single fixed shift is tempting and roughly works for a small site, but the correct fix is a datum transformation with published parameters. GIS software does this when you set the source and target coordinate reference systems correctly.
3. Use EPSG codes. WGS84 geographic is EPSG:4326. WGS84 UTM zone 43N is EPSG:32643. Kalianpur 1975 geographic is EPSG:4146, and its UTM variants have their own codes. Naming the code removes ambiguity.
4. Standardise on WGS84 for the project. Convert everything once, at the point of entry, and keep WGS84 as the working datum. Convert back only when an authority requires a specific datum in a submission.
5. Record what you did. A column noting the original datum and the transformation used saves the next person a week.

Why it matters more for wind than for most things

A solar boundary that is 200 m off is annoying. A wind turbine that is 200 m off can breach a spacing rule against a neighbour, land in a setback, or sit on the wrong parcel entirely. Because turbine spacing is checked in multiples of rotor diameter, a datum error of one or two rotor diameters can flip a layout from clear to clashing.

A caution about converters

Online converters, including the reneGIS Coordinate Converter, convert formats on a single datum, not datums. They will faithfully turn an Everest 1830 DMS value into an Everest 1830 UTM value, which is still 200 m from where GPS says it is. Fix the datum first, in a GIS with proper transformation parameters, then convert formats.