How to import ArcGIS flow direction into GRASS GIS
As shown in Figure 1, ArcGIS uses powers of two to represent the flow direction starting with \(2^0\) from East in the clockwise direction. Figure 2 shows how the r.watershed module in GRASS GIS encodes similar information in a raster map. This module measures the aspect of each cell in the counter-clockwise direction from East and divides the aspect angle by 45 so that East becomes 8, which is \(8\times 45^\circ=360^\circ\). East is not 0 because this module uses 0 to indicate depression areas.
Using this information, you can import the flow direction raster from ArcGIS into GRASS GIS and convert it to the drainage raster.
- In ArcGIS, right click on the flow direction layer (fdr) => Data => Export Data
- Export fdr as GeoTiff (TIFF format). Output filename fdr.tif in this example.
- In GRASS GIS, import fdr.tif first:
r.in.gdal input=fdr.tif output=fdr
- Convert fdr to drain: