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Vertical stereographic aerial photographs obtained as photographic plates or, more recently, as digital images represent a primary source of data for desk studies on all engineering projects. New methods of data acquisition from both airborne and spacecraft-based scanners provide unparalleled opportunities to obtain high-resolution data. However, interpretation of the images remains as much an art as a science and is critically dependent on the skill and experience of the interpreter. Relevant experience can be obtained only by the practice of ground-truthing an interpretation that the interpreter has carried out. Although it has rarely been quantified there are important lessons for all engineering projects on the divergence between the interpreted image and the actual ground situation. Some of these are exemplified in a case study undertaken by the authors using large-scale aerial photographs for a landslide hazard investigation in Spain. Based on this investigation it can be assumed that up to 50% of landslide features of engineering geological significance might not have been identified by the aerial photograph interpretation. The alternative is to use the images to divide the landscape into terrain units that allow the landslide potential to be evaluated. It can be demonstrated that this approach is more effective than trying to create a landslide inventory based on aerial photographs alone. Given the potential expansion in the use of large-scale image analysis associated with the recent improvement in data acquisition systems this has implications for the use of images and the training of interpreters.

The definitions of hazard and risk in natural hazard studies are well established in the scientific literature. However, many examples of ‘landslide hazard assessment’ only identify the susceptibility of slopes to failure and make no statement on the frequency of occurrence that would be necessary for a complete hazard evaluation. In a research programme undertaken in SE Spain the issue of landslide susceptibility in a semi-arid, neotectonic environment was examined, with some attempt to evaluate the hazard. This work involved establishing the occurrence of landsliding within the 550 km 2 Rio Aguas catchment through remote sensing interpretation and field mapping. These data were compiled in an inventory containing the records of nearly 250 landslides that was analysed to establish the nature and extent of landslide susceptible situations. Within the catchment anticipated combinations of geological materials proved to be susceptible to failure, and relationships between landslide volume and travel angle were examined in relation to standard models. The highest incidence of contemporary landsliding appeared to be related to the proximity of a major river capture site, a geomorphological event that had been dated at 100 000 BP. This produced localized rapid incision, a ten fold increase in sediment removal and the creation of oversteepened slopes that were only recently degrading to their long-term angle of stability. In addition to contemporary landslides, field mapping identified anomalous geological structures that proved to be degraded erosional remnants of ancient landslides. Relating these remnants to the river terrace sequence in the region provided some control on the relative ages of these ancient or ‘fossil’ landslides. It was concluded that any assessment of landslide risk in the study area would need to take into account not only geological materials and the contemporary geomorphological environment but also the geomorphological history of the region.