geodesy op• Data kinds: depth → image2d
• Call: import fullseye as fs; fs.ledger.dem_geodetic_slope(dem, lat0_deg, d_lat_deg, d_lon_deg, method='horn', units='degrees') (to call the implementation directly, import demops; demops.dem_geodetic_slope(dem, lat0_deg, d_lat_deg, d_lon_deg, method='horn', units='degrees'); from the registry, opsdem.get("dem_geodetic_slope"))
Slope of a DEM on a latitude-longitude grid (equal angular spacing). The cell dimensions vary with latitude.
Many public DEMs are equally spaced in angle, like a "1-arc-second mesh". Passing this grid as is to dem_slope with a fixed cell size ignores that the real distance in the longitude direction is shorter by a factor of cos(latitude). Using the latitude-direction size for both axes makes east-west slopes too small (half at 60 degrees north), and using the longitude-direction size for both axes makes north-south too large instead. Which way it goes depends on what was taken as constant, so rather than memorising "too large/too small", computing the size per latitude is the right answer.
★ At first this docstring said "east-west becomes too large", but when measured in a test it came out half (too small). Measure claims about sign or direction before writing them.
Here the east-west cell size is computed per latitude, and the grid is converted to equal distances before measuring.
Args:
dem: (H, W). Row 0 is north.
lat0_deg: latitude of the north-west corner [degrees].
d_lat_deg / d_lon_deg: latitude/longitude step per cell [degrees]. Positive values.
method / units: same as dem_slope.
• 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
image2d as input)—
geodesy)dem_geodetic_to_ecef · dem_ecef_to_geodetic · dem_geocentric_grid · dem_earth_curvature_drop · dem_cell_size_webmercator · dem_geoid_height · dem_height_frame_convert · dem_height_frame_residual
*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.