Measuring beach loss and gain from archives of drone orthophotos calibrated with fixed structures

Technical note (not peer reviewed) describing a method to measure beach loss and gain from heterogeneous archives of drone orthophotos flown without RTK. All orthophotos are co-registered against a master image using only tie points on fixed structures (buildings, swimming pools, piers, roundabouts, rock groynes) as ground control; the instantaneous water line is then measured on transects every 5–10 m, DSAS-type indicators (NSM, EPR, LRR, area) and a per-transect trend using all flights are computed, and results are validated with an independent QGIS-equivalent chain (Otsu threshold). The method was applied to four real Caribbean beaches, presented anonymously (sheltered bay with groynes, open coast, small island, very clear water with a nearby reef; 20–39 flights each, 2017–2026). Position error on the structures drops from up to 10 m to 0.2–0.3 m. Independent validation: r = 0.81, 0.89 and 0.64 in three cases and 0.41 (interannual changes) in the clear-water case. The note documents where the method fails (clear water and reefs, tide, seasonality, partial coverage, rocky coasts, works, no volume) and the known errors found with the fixes applied. All figures are schematics or synthetic data; no real imagery, names or locations are included. Real cases will be added in later versions once the data owners authorise it. Files: PDF of the note in English and Spanish, and a short technical video in each language (MP4, about 3 minutes, with SRT subtitles). Code available on request.