DEM-based terrain-adjusted footprints

[NewGraphEnvironment]

Context

fly_footprint() assumes flat terrain beneath the aircraft. On slopes the true ground coverage differs — downhill slopes produce a larger actual footprint, uphill slopes a smaller one. In steep BC terrain this can be meaningful. This limitation is documented in fly_footprint(), fly_overlap(), and the vignette.

Enhancement

Allow users to provide a DEM to improve footprint accuracy.

Approach options (simplest first)

1. Terrain-adjusted area — drape the flat footprint polygon onto the DEM and report actual ground surface area (larger than planimetric area on slopes). Doesn't change the footprint shape, just gives a better area estimate.

2. Slope-adjusted footprint — compute average terrain slope under each footprint and stretch the rectangle accordingly. Assumes nadir (straight-down) camera, which is already our assumption.

3. Ray-cast footprint — derive flying height from scale + focal length (altitude = focal_length * scale_num), project corner rays from the camera onto the DEM surface. Produces an irregular quadrilateral matching actual ground coverage. Most accurate but requires nadir assumption since camera tilt/orientation aren't in the BC catalogue.

Interface sketch

fly_footprint(centroids, dem = raster_or_path)

• dem defaults to NULL (current flat-terrain behavior)
• When provided, applies terrain correction
• Would likely add terra to Suggests

Limitations

• BC air photo catalogue doesn't include camera tilt, roll, or exact altitude — must assume nadir
• DEM resolution matters: coarse DEMs (e.g. 30m CDEM) won't capture fine terrain detail
• Flying height derived from reported scale assumes the scale was calculated at a reference elevation

Acceptance criteria

• fly_footprint() accepts optional DEM
• Flat-terrain behavior unchanged when no DEM provided
• At minimum, terrain-adjusted area reported
• Document which DEM sources work well (CDEM, BC TRIM, LiDAR)
• Tests comparing flat vs terrain-adjusted footprints on sloped terrain