I am working on some flood maps of a given region. As the data is confidential, I will try to stay as general as possible without any loss of relevant information. The truth is, my knowledge on GIS is limited.
Available databases / files for my analysis
Some data on a river and its bank:
- A precise digital terrain model (.adf / ArcInfo binary grid format). The pixel size is 1m x 1m.
- a polygon (one feature in an ESRI shapefile). This polygon contains the 2D information on the flood boundary (it delimits where a river will overflow). No altitude or Z elevation is contained in this file.
Even though that information is not contained in the polygon, the elevation at each point on the boundary polygon is basically the same.
The problem I am facing
I would like to deduce the flood depth (the water height) observed in each cell of a grid (of the same resolution as the DTM). And then map it.
I am assuming this would be simple if we had the information on the elevation of the data contained in the shapefile. In this case, I would convert that shapefile to a raster and do a little subtraction. But I do not have it and I must deduce it.
I am comfortable with any coherent approximation. I must know this information at any pixel inside the polygon, not just on its boundary.
The tools I would like to use
I exclusively work with QGIS (in its latest version - Las Palmas). I do not have an ArcGIS licence.