Impervious cover proportion is recognised as an important parameter in hydrologic modelling. Recent advances in the spatial sciences provide new capabilities for estimation of this value. This paper describes an assessment of seven estimation techniques, including human and computer based methods requiring a range of different input data and skill levels. In order to quantify the value of any gain in accuracy, the results from each method were applied to a real catchment using event based (WBNM) and continuous simulation (MUSIC) hydrologic models. The results showed that use of high resolution satellite imagery enabled the most accurate measurements of impervious cover proportion to be made, however in general human and computer based methods are comparable in terms of accuracy. Also it was found that in some circumstances the sensitivity of peak discharge and runoff volume estimates to error in impervious cover proportion can be very high, confirming the need for accurate measurement.