GIS stands for Geographic Information System - a combination of maps and database information. The quality and nature of implementations is widely variable.

Usually GIS maps show property boundaries overlaid onto on a map of the region.

In many applications it is also possible to turn on or off other information such as road names and aerial photographs.

As many property boundaries are still the same as the original surveys, this information is very useful for Family History research. You can often find the locations of properties given in electoral rolls etc.