Overland Flow Distance to Point¶
Computes the directional downslope flow-path distance across the land surface
from every cell to the nearest of one or more target points, using
WhiteboxTools' DownslopeDistanceToStream over a rasterised point set.
Unlike a straight-line or cost-distance surface, this follows the D8 downslope flow direction derived from the DEM: water only travels downhill, so cells that do not drain to a target point receive no distance (NoData) rather than an omnidirectional distance that would imply they could reach it.
Where to find it: Processing Toolbox → SCIMAP → Overland Flow Distance to
Point; toolbar icon; &SCIMAP menu; or the
SCIMAP Panel's Flood tab, via the Calc. OFD button
after picking impact points on the map.
Screenshot:
The Overland Flow Distance to Point Processing dialog.
Parameters¶
| Parameter | Description | Default |
|---|---|---|
| Digital Elevation Model (DEM) | The DEM to route flow over. | — |
| Target Point(s) | A point vector layer — one or more targets. | — |
| Channel Network (optional) | A line layer; when supplied, each target point is snapped onto the nearest channel first. | Optional |
| Maximum snap distance to channel (map units) | Points further than this from any channel are left unsnapped. Ignored with no channel network. | 100 |
| Snap target point(s) to the channel network by flow accumulation | An alternative snap mode, as used in Delineate Catchment, based on nearby flow accumulation rather than the supplied channel layer. | Off |
| Snap search radius (map units) | Ignored unless the accumulation-based snap above is enabled. | 250 |
| Minimum contributing area to snap onto (m²) | Ignored unless the accumulation-based snap above is enabled. | 800,000 |
| Minimum accumulation ratio vs the clicked cell | Ignored unless the accumulation-based snap above is enabled. | 100 |
| Breach DEM depressions before computing flow direction | Recommended — avoids broken flow paths at sinks. | On |
| Colour ramp | Applied to the output. | SCIMAP default |
| WhiteboxTools executable | Advanced; override the auto-detected binary. | Auto-detected |
Outputs¶
| Output | Description |
|---|---|
| Overland Flow Travel Distance | Downslope distance (map units) from each cell to its nearest target point; NoData where a cell's flow path never reaches one. |
Notes¶
- Two independent snapping options are available and can both be used: snap to a supplied channel network (nearest line, within a maximum distance), and/or snap by flow accumulation (the same logic Delineate Catchment uses). Neither is required — points can also be used exactly as supplied.
- This tool's output is one of SCIMAP Flood's required inputs; the SCIMAP Panel's Flood tab runs it directly from picked impact points via Calc. OFD.
Next: Export SCIMAP Results.