Reassessing megalithic spatial densities in Southwestern Iberia through Airborne Laser Scanning-based landscape analysis

Abstract

Megalithic landscapes in southwestern Iberia have long been reconstructed from heterogeneous regional inventories, often treating megalith spatial density as a proxy for demographic intensity. Here we present a probabilistic, auditable approach to reassessing megalithic densities and landscape dynamics by integrating conventional inventories with high-resolution Airborne Laser Scanning (ALS) and explicit uncertainty modelling. We compile the Western Iberia Megalithic Dataset (WIMD) (n = 3,069 sepulchres) from Spanish and Portuguese sources, curate a subset through bibliographic and field checks, and evaluate inventoried sites against ALS -derived visualisations to estimate apparent detectability. We then classify ALS -only anomalies using a repeatable certainty scale and apply a probabilistic estimation framework to infer the total underlying number of megaliths and omissions in both inventories and ALS detection. Two case studies—(1) the transboundary International Tagus region (Portugal, Spain) and (2) the intensively surveyed eastern Andévalo River basin (Huelva, Spain)— show that ALS can increase inferred monument totals by 35–50% under conservative assumptions in the analysed areas, while 35–46% of inventoried mounds remain non-recognisable in ALS products. Finally, ALS also identifies enclosure-like settlement features that co-occur with megalithic clusters, strengthening interpretations of densely structured landscapes and providing transferable priorities for targeted ground verification.