Simplified teaching model. This is not a validated analysis of any property.

Where does rainwater go on this small grid?

terrain_engine.fill_depressions and terrain_engine.flow_accumulation

Assumptions

• Each grid is a synthetic 3 by 3 DEM in meters with 10 m cells.
• Accumulation is run on the filled DEM, which is the input flow_accumulation expects.
• This implementation includes each finite cell itself in its accumulation count; cells times 100 gives contributing area in square meters, not a water volume.
• Only positive downhill slopes receive flow. Equal slopes keep the first neighbor in the implementation's fixed iteration order. Edges cannot route outside this small grid; missing cells are not counted.
• The center cell is row 1 column 1 and the southeast cell is row 2 column 2.

Limitations

• flow_accumulation does not return flow direction.
• Epsilon-fill can impose drainage on a flat or a pit, so the result is not the unfilled surface.
• Simplified teaching model, not a property drainage plan.

Counterexample

Epsilon-fill can impose drainage on a flat or a pit, so the result is not the unfilled surface.

Sources

• USGS 3D Elevation Program (3DEP), https://www.usgs.gov/3d-elevation-program
• Implementation reference: terrain_engine.py

Floats are rounded to 6 significant digits.

Scenario outputs

worked-example

pit

flat

Open the parcel snapshot