An investigation into the use of aerial digital photography for monitoring coastal sand dunes
The coastal zone is a highly dynamic entity both spatially and temporally and when shoreline changes (and in particular retreat) occur on a hmnan time-scale, measurement of the rate of change becomes a pressing issue. This dynamism presents an excellent scenario for monitoring change using remote sensing techniques, and in the case of coastal sand dunes, where the requirement is to measure small scale changes such as erosion or accretion in the region of 10 or 20 m, aerial photography is the preferred source of remotely sensed data. The rapid developments in digital camera technology and real time satellite differential Global Positioning Systems have yielded new opportunities for mapping and monitoring environmental change when used with image processing and mapping software and state-of-the-art digital photogrammetric workstations. Despite the progress in digital technologies, however, there is still considerable lack of awareness on the part of potential users, and it is in response to this that the processing chain for data collection through to orthophoto production described here has been developed. This study explores the major issues that affect quality, mission logistics and cost and will demonstrate the methodology and application of digital techniques for producing georectified imagery and contoured orthophoto maps of coastal environments. This will be achieved through a series of case studies of dynamic dune environments in south-west England and France. Digital imagery was captured using a colour infrared Aerial Digital Photographic System and ground control was collected using differential Global Positioning Systems. This study seeks to assess the application of this imagery to coastal dune monitoring, putting these new techniques within the grasp of coastal dune managers, enabling them to make use of digital imagery captured to different specifications depending on the accuracy requirement of the end product. The results indicate that this type of imagery and the techniques used can provide the dune manager with information which would otherwise be too costly or time consuming to acquire. 2D rectification of the imagery provided maps of dune retreat and accretion with errors in the region of± 1.5m, and rectification to a higher order using 3D photogrammetric correction provided 1 :5000 contoured orthophotographs with mean xy errors in the region of 2. 5 m and mean elevation errors in the region of 1.5m.