To compare, overlay or cross analyze two maps in GIS:

Concept: Overlay analysis in a Geographic Information System combines two or more thematic layers so that the same ground location can be read from all of them at once. The combination is meaningful only when the layers are co-registered: the software must be able to read them as data, their coordinates must resolve to one common spatial reference frame, and they must describe the ground at a comparable level of generalisation so that the comparison is fit for purpose.

Application to this item: the three conditions listed in the options correspond exactly to those three requirements, and each guards against a different kind of failure.

1. Digital form. Overlay operators — intersect, union, clip, erase, and raster map algebra — act on stored vector geometries or on raster cell values. A printed paper sheet holds no such stored geometry or cell values, so it must first be scanned, georeferenced and digitised into vector or raster data before any overlay operator can be applied to it.

2. Equivalent scale. The source scale of a layer fixes how much its features were generalised at the moment of capture. A coastline digitised at 1:10,000 carries far more vertices than the same coastline at 1:250,000. Combining layers captured at very different source scales produces sliver polygons and spurious mismatches along shared boundaries, so a comparable source scale is what makes the overlay result fit to use.

3. Same coordinate system. A coordinate system is a datum plus a projection: it is what turns a position on the earth into an x, y pair. The identical ground point takes different x, y values under, say, UTM Zone 43N and under a geographic latitude/longitude frame. The layers must therefore resolve to one common frame before features that share a location can be paired correctly.

Cross-check — the three conditions guard against three different failures:

None of the three substitutes for another: two layers can satisfy any two of them and still overlay incorrectly. The answer is therefore "All of the above", which is also the answer recorded in the UGC NET December 2011 Paper II official key.

Worth knowing in practice: modern GIS packages will reproject layers into a common frame on the fly, so the layers need not already be stored in the same coordinate system provided they can be brought into one; and a difference in source scale does not make an overlay impossible, it limits how far the result can be trusted. The exam item states these as the standard prerequisites for a reliable overlay.