Topography is described by a Digital Terrain Model (DTM). This is a representation of the altimetry of the ground surface in the form of a raster mesh of a given resolution. For each mesh, the ground elevation is given in metres NGF.
The DTM is the primary input to the WaterSed model. Different DTM resolutions can be used, depending on data availability and modelling objectives.
IGN covers the whole of France with its two products :
- The BD ALTI with a resolution of 75, 50 and 25 m
- The RGE ALTI with a resolution of 5 m and 1 m (or 0.5 m in some areas)
The RGE ALTI is distributed in the form of tiles defined by :
- A point of origin (North-West corner) with its X and Y coordinates (generally in Lambert 93)
- A number of rows and a number of columns, both identical (generally 1000 rows * 1000 columns)
- A step (or resolution): 5 m, 1 m
The X and Y coordinates of this point of origin are indicated on the name of the tile (in Lambert 93 in the example below).
For example, for the highlighted tile, the X and Y origins are 649500 and 7018500, respectively.
It is generally necessary to merge several tiles to cover the study area. Once merged, the DTM produced cannot be used directly in the model. The DTM should be hydrologically corrected to (1) ensure a continuous gradient from upstream to downstream and (2) integrate the various hydraulic features (e.g. ditch, road, path, etc.) modifying the flow direction .
The various steps involved in building and processing a DTM are detailed in the following five tutorials (to be done in order):
Before you start :
- Create a TUTORIEL folder, a sub-folder PREPROCESSING and a sub-sub-folder TOPOGRAPHIE
- Download the tutorial dataset and unzip the file in TUTORIEL/PREPROCESSING/TOPOGRAPHIE
- Create a MODELE folder in the TUTORIEL folder
- Download the WaterSed model and the associated tools (WaterSed.zip) from the Téléchargement and unzip WaterSed.zip in TUTORIEL/MODELE
WaterSed 