Estimate photo footprint polygons from centroids and scale

Creates rectangular polygons representing the estimated ground coverage of each airphoto, based on film negative dimensions and the reported scale.

Usage

fly_footprint(centroids_sf, negative_size = 9)

Arguments

centroids_sf: An sf point object with a scale column (e.g. "1:31680").
negative_size: Negative dimension in inches (default 9 for standard 9" x 9").

Value

An sf polygon object in the same CRS as input, with footprint rectangles.

Details

Ground coverage is computed as negative_size * scale_number * 0.0254 metres per side. Rectangles are constructed in BC Albers (EPSG:3005) for accurate metric distances, then transformed back to the input CRS.

The scale denominator is parsed from the scale column string (e.g. "1:12000" becomes 12000).

9x9 assumption: the default negative_size = 9 (inches) reflects the standard 228 mm format used by BC aerial survey cameras (e.g. Wild RC-10, Zeiss RMK). The BC Air Photo Database records camera focal length per roll (Type 02 field 3.2.2) but this is not available in the simplified centroid data from the catalogue. If working with non-standard format photography, override negative_size accordingly.

Flat-terrain assumption: footprints are estimated assuming flat ground beneath the aircraft. In reality terrain slope changes the actual ground coverage — downhill slopes increase the true footprint (ground falls away from the camera), while uphill slopes reduce it. In steep terrain typical of BC valleys, true footprints may differ meaningfully from these estimates. Coverage and overlap calculations downstream (e.g. fly_coverage(), fly_overlap()) inherit this limitation.

Examples

centroids <- sf::st_read(system.file("testdata/photo_centroids.gpkg", package = "fly"))
#> Reading layer `photo_centroids' from data source 
#>   `/home/runner/work/_temp/Library/fly/testdata/photo_centroids.gpkg' 
#>   using driver `GPKG'
#> Simple feature collection with 20 features and 9 fields
#> Geometry type: POINT
#> Dimension:     XY
#> Bounding box:  xmin: -126.7631 ymin: 54.34512 xmax: -126.449 ymax: 54.47635
#> Geodetic CRS:  WGS 84
footprints <- fly_footprint(centroids)
plot(sf::st_geometry(footprints))