Why is the catchment smaller than expected?
Real DEMs contain flats and depressions. D8 flow stops there unless terrain is hydrologically conditioned first. The selected outlet and raster extent also constrain the result.
Free · Local file processing · No account
D8 Flow & Watershed Online
Compute downslope flow accumulation and the cells draining to a chosen outlet.
Loading the local file tool…
Loading the local file tool…
How to use this tool
- 1. Load a local raster, try the sample, or get a public DEM for a small area.
- 2. Use the map or raster grid to choose points; numeric coordinates remain editable. Run the analysis and compare input/result overlays.
- 3. Download results or use the compatible next-operation selector. Continue terrain analyses using the original elevation DEM.
Limits and checks before sharing
- Local calculations run in a cancellable worker. Input size and cell limits are shown by the tool; use a cropped DEM for large areas.
- NoData cells are excluded or propagated, not treated as zero elevation. Results depend on DEM resolution and source accuracy.
- Uses single-direction D8 steepest descent. Sinks and flats stop flow; this tool does not fill depressions or infer drainage across flats.
- Choose an outlet in native grid coordinates. Boundary cells can receive flow from outside the raster that this calculation cannot know about.
If processing fails, check the file extension and source contents. Use a smaller batch if memory runs low. The original files are never overwritten. Inspect known locations and attributes before sharing a result.
Frequently asked questions
Are these calculations local?
Yes. Files and calculations stay in your browser. Page resources require an initial network load. Map previews can request external map tiles. No account is needed.
More analyses with your source DEM
Use your original elevation DEM for the terrain analyses below. Slope, aspect, visibility and flow grids describe derived values; they are not elevation inputs. Download the raster reference JSON with ASCII outputs to retain their CRS and cell metadata.
Elevation Profile from DEM Online
Sample terrain along a route and export elevation against distance.
Slope & Aspect from DEM Online
Calculate terrain steepness and downslope direction from an elevation grid.
Raster Statistics Online
Summarize valid raster values and inspect the distribution without uploading a GeoTIFF.
DEM Viewshed Online
Estimate which terrain cells are visible from an observer using a local DEM.