Here is a threat that already deals with the TilesetIndex in the tileimp.txt:
http://apolyton.net/forums/Forum44/HTML/000749.html?17
The TilesetIndex will display the finished tile improvement on the map.
The ConstructionTiles will display a not finished tile improvement on the.
TilesetIndex and ConstructionTiles use the same numbers.
Actually there are the ConstructionTiles but only the middle one is used. By the way it is no problem to distinguish between an advanced type improvement and one yet to be upgraded, because you can build an advance tileimprovement on a yet to be upgratded tileimprovement. But Fuzzball is also correct, because it is boring to have it in the mind which tile improvement must be upgraded and to see no different between the two types of tile improvements.
-Martin