Colour Aerial Photography for Riverbed Classification

Colour aerial photography has the potential to identify and classify the size characteristics of exposed riverbed sediments over large areas efficiently. This paper describes the findings of a pilot project that sought to assess this potential and consider the impact of photo-scale on the accuracy of classifications obtained. Colour multi-scale digital imagery was acquired of a test area (120 x 80m) at scales of 10,000, 5,000, 3,000, 2,000 and 1,000 using a helicopter and a hand-held Kodak DCS460 high-resolution digital camera. Intensive ground work obtained conventional grain size parameters (Wolman samples), requiring 15 person days of fieldwork. This was supplemented with the measurement of a 1 and 2 m high resolution digital elevation model (DEM) and photo-control, using a motorised Total Station. Conventional photogrammetric processing and the DEM were then used to create orthophotos of the test area at differing photo-scales. Supervised classification methods were adopted to classify each pixel into one of five classes: sand, sandy gravel, pebble, clean gravel and cobbles. Comparison with the known ground truth achieved a success rate initially of only 38% at 1:5,000 photo-scale, but developments enabled this to be increased to a more encouraging 49%. Similar tests were conducted using orthophotos at other scales (1:3,000 and 1:10,000) and similar improvements were achieved using the approaches developed. A key parameter that indicates bed roughness and is of significant biological interest, is the percentage content of sand. Further work was carried out to ascertain whether this simple parameter could be extracted from the imagery at differing photo-scales. The dataset derived by the supervised classification procedure was converted to percentage sand using a 5x5 convolution filter. It was hoped to assess the accuracy of the classification by comparing this percentage sand map with a similar map derived from the intensive fieldwork. However, the enormous improvement in spatial resolution demonstrated that the two datasets were not directly comparable. Despite this, it was evident that overall sand distribution was clearly revealed using both the 1:5,000 and 1:10,000 scale imagery. More significantly, it was apparent that significant savings in time and effort would be accrued if the methods developed in the study were to be used to map and classify large areas of dry river-bed using colour aerial photography. The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol. 34, Part XXX Figure 2DEM points and 10m sampling quadrats 2. PRACTICAL WORK