An alternative method for estimating total impervious area in catchments using high-resolution colour aerial photography

Estimation of total impervious area (TIA) is a pre-requisite for ecohydrological research to allow for a direct prediction on stream ecosystem health within catchments. This paper presents an alternative to using multi-spectral imagery for estimating TIA at a catchment scale, by using high-resolution colour aerial photography. The method was applied to a number of catchments in South East Queensland, Australia, some of which were gauged and some of which were part of an Ecosystem Health Monitoring Program (EHMP). The results from this method were compared to TIA estimates, for some of the same catchments, that were derived through three other techniques, i.e. manual digitization of geo-referenced aerial photos, Brisbane City Council data derived from image analysis using Landsat TM imagery and rainfall runoff depth relationship. The high-resolution colour aerial photography method compared favourably to the other techniques with standard deviations of TIA (%) ranging between 0.8% and 8%. The major constraints were shading effects, particularly on roads and grassed areas, and from the similarity in colours between some surface types, some of which can be reduced by appropriate selection of signature colours and multiple iterations of a supervised classification. It was concluded that while infra-red spectral wave bands could help considerably, the high-resolution colour photography could be applied with confidence to derive catchment-scale TIA estimates.