The use of image processing systems for the analysis of digitised Aerial Photography
N.C. Coops, D.D. Fraser, N.M. Rollings, G.P.Ellis
RMIT Centre for Remote Sensing
Victoria University of Technology
GPO Box 2476V
Melbourne
Victoria 3001 Australia
Abstract
The use of image processing systems in the analysis of remotely sensed data such as that derive from satellites is well documented. There remains, however, a substantial amount of data in the form of aerial photography, analogue satellite imagery and maps, the analysis of which can be greatly improved using image enhancement and geographic manipulation techniques. Aerial photography provides high resolution information and offers an historical archive dating back many years. Mapped data provide useful ancillary information, which can be used to assist in the analysis.
Image enhancement techniques may be applied to digital data derived from analogue sources enabling subtle features to be extracted that were not apparent in the original data set.
This principle of digitising analogue data has been applied to several data sets relating to geological and land use mapping applications. The use of techniques such as contrast enhancement, histogram equalisation, principal component analysis, decorrelation stretching and classification has allowed significant feature extraction.
The digital nature of these data makes them amenable to further analysis in a Geographic information system (GIS). The use of automated cartographic software packages also permits the output of useful and attractive map products from such integrated data sets.
Introduction
Technological advancement in microcomputer hardware has meant a substantial decrease in their cost with a corresponding increase in their computing power. This has led to an increasing use and awareness of personal computers in their application to tasks previously restricted to expensive mainframes. More recently, the development of low cost scanners (automatic raster digitisers) has emerged as a significant image processing tool for the mapping and graphics industry.
Civil and Municipal Engineers GIS specialists and other workers in the spatial data field all hold archives of large-format maps that require regular updating and revision, and from which copies are regularly made. It is only recently that these groups are recognising the advantages of raster data as an archive and raster editing for updating (Bosma et al, 1989).
Even with this increase in the use of raster data by a wide variety of users, the purchase of remotely sensed imagery from satellites and airborne scanners at a suitable mapping resolution is still an expensive venture (corblet,1990). As an alternative some users are turning to existing data sources as a means of obtaining high resolution data over particular regions.
Aerial photography has been flown in many countries as an integral part of topographic mapping programs. These data, apart from providing topographic information to allow the production of maps, provide a continuous, large photographic scale archive in some cases dating back many years.
The digitising of aerial photographs and subsequent entry into an image processing system allows digital manipulation of the data and consequently provides a powerful means of interpretation.
The image processing techniques used on digitised analogue images will be discussed in relation to three pilot study areas.
