Coordinate Reference System
The mathematical system that defines how coordinates relate to real-world geographic locations — the standard for ensuring spatial data from different sources aligns correctly.
A Coordinate Reference System (CRS) is a framework — combining a geographic datum, a map projection and a coordinate system — that defines how coordinates (x, y, z) relate to real positions on the Earth's surface. Different datasets use different CRS, and if they are not correctly matched, they will not align with each other in a GIS.
For heritage GIS work in India, the most commonly used systems are: WGS84 (EPSG:4326) — the global GPS datum, used for data sharing and as the base for web mapping; UTM (Universal Transverse Mercator) in the relevant zone — for accurate local mapping and distance measurement; and the Survey of India national grid (Everest spheroid) used for historic Indian topographic maps.