A historical GIS is a geographic information system that may display, store and analyze data of past geographies and track changes in time. It can be regarded as a subfield of historical geography and geographic information science. GIS is very essential for Historian especially for those who dealing with world history or the history of large geographic regions for their research and teaching.
Historians are noticing GIS because they normally deal with processes in complex, dynamic, nonlinear systems and, therefore, demand a means to organize a large number of variables and identify those variables most likely implicated in the stability and transformation of such systems. However, GIS remains largely unknown among the vast majority of professional historians, and a significant percentage of those who believe they know about the technology think it is something they can buy with their next car so that they will not become lost.
There are so many techniques used in Historical GIS, out of which consider the following techniques:
Digitization and dereferencing of historical maps. Old maps may contain valuable information about the past. By adding coordinates to such maps, they may be added as a feature layer to modern GIS data. This facilitates comparison of different map layers showing the geography at different times.
Reconstruction of past boundaries. By creating polygons of former administrative sub-divisions and borders, aggregate statistics can be compared through time.
Geo-referencing of historical microdata (such as census or parish records). This enables the use of spatial analysis to historical data.