All the maps must be scanned in the .tiff or .png formats before they can be georeferenced. While it has not been possible to geo-reference the Chinese-Vietnamese maps in our possession, the thorough georeferencing of the arial photographs, and the French maps dating from the nineteenth and twentieth centuries, did, however, enable us to work on the river system of the region.
A vectorization of the hydrographic network (that is to say, tracing the waterways using a GIS software programme such as ESRI’s ArcGIS) allows to conduct fruitful comparisons by studying the evolution of the waterways. We compared vectorized maps dating back to 1883, 1889, 1893, the early, and mid-twentieth century.
The geographic information system process highlights the flexibility of the landscape. This helps to reconstruct the evolution of the landscape and to understand the population settlement process across the centuries. The superimposed layers on the vectorized maps reveal the densely populated and the barren areas and a comparison between these vectorized maps shows that in the timespan of 70 years, the main hydrological network did not change much, except in terms of the width of the waterways and of the movements of sandbanks; whereas the secondary network changed and became denser.
The GIS makes it possible to distinguish the changes related to human activity (roads, urbanisation) from those related to geological changes (soil subsidence, erosion). It therefore seems possible to extend the reasoning validated by the study of the vectorized maps to the ancient times, by carrying out an additional study of the texts though.
Thus, all the sites are located in plains prone to saltwater intrusion. GPS-based positioning of the ancient sites makes it possible to identify them easily on the maps. It appears that soil salinization was dealt with differently from one period to another: the Cham sites were usually built on elevated land, whereas, during the occupation by the Nguyễn Lords, the secondary water system intensified – wetland stabilization -probably because of the increasing population density. The development of networks of canals certainly helped to gain arable land and to desalinate the soil.
An in-depth comparative examination of the vectorized lines of the 19th– and 20th– century maps with the Chinese-Vietnamese and Western texts must therefore be conducted. It will probably reveal that environmental issues determine history as much as human issues.