surface op• Data kinds: depth → image2d
• Call: import fullseye as fs; fs.ledger.dem_slope(dem, cell_size, method='horn', units='degrees') (to call the implementation directly, import demops; demops.dem_slope(dem, cell_size, method='horn', units='degrees'); from the registry, opsdem.get("dem_slope"))
Slope angle. units is "degrees" / "radians" / "percent".
On a plane it agrees with the closed form atan(|grad|) (measured 1e-13 or less).
Procedure: from the 3x3 neighbourhood take dz/dx (eastward) and dz/dy (northward = direction of decreasing row), then g = hypot(dz/dx, dz/dy). "degrees" gives atan(g) in degrees, "radians" in radians, "percent" gives 100 g (45 degrees = 100 %). Edges are padded by replicating the border values (pad mode="edge"), so the outermost 1 cell comes out gentler than the interior.
• dem: (H, W) elevation [m], 3x3 or larger, real, no inf. Pass missing data as nan ―― if sentinel values like -9999 are left in (<= -9000 present and no nan), it is rejected. nan propagates over the 3x3 range.
• cell_size: cell edge length [m], positive finite. bool / strings rejected. For lat/lon grids use dem_geodetic_slope instead.
• method: "horn" (Horn 1981 3x3 weighted difference, default) / "central" (Zevenbergen–Thorne central difference).
• Return value: (H, W) float64. degrees is in [0, 90).
• All failures are ValueError (shape, sentinel values, cell_size, choices).
Used together with dem_aspect. dem_hillshade builds shading from these 2.
• dem_terrain_analysis family guide
• Sample-data catalog (download URLs / licences) — 2-D uses skimage.data (BSD/public domain) plus synthetic images; 3-D lists download URLs for real data sources (Stanford, PDS, …).
• Operator provenance and references — the sources of the research/methods this op family came from.
• The canonical algorithm (author, year) and its uses are named in the family usage guide above.
• dem_geodesy_tour — py -3.11 examples/dem_geodesy_tour.py
• poc_crop_phenotyping — py -3.11 examples/poc_crop_phenotyping.py
• poc_dem_terrain — py -3.11 examples/poc_dem_terrain.py
• poc_geodetic_height_frames — py -3.11 examples/poc_geodetic_height_frames.py
• poc_lidar_terrain_change — py -3.11 examples/poc_lidar_terrain_change.py
• poc_multibeam_bathymetry — py -3.11 examples/poc_multibeam_bathymetry.py
• poc_stockpile_volume — py -3.11 examples/poc_stockpile_volume.py
image2d as input)—
surface)dem_aspect · dem_curvature · dem_roughness · dem_tpi
*Provenance: demops.py — DEM operator registry. This per-op note is generated by tools/opdocs.py md (do not hand-edit).*
© 2026 Kazufumi Furuse — Fullseye operator documentation. Licensed under Apache-2.0.