Evaluation of the Wetland Mapping Methods using Landsat ETM + and SRTM Data

Overarching goal of this paper was to evaluate automated and semi-automated methods of mapping wetlands using Landsat ETM+ and SRTM data. Automated methods consisted of: (a) slope derived from SRTM, (b) Tasseled cap Wetness Index (TCWI), (c) Normalized Difference Water Index (NDWI), (d) multi-band vegetation indices (MBVIs), (e) two band vegetation indices (TBVIs), (f) normalized difference vegetation index (NDVI), and (g) data fusion involving ETM+ and SRTM and then classifying the same. The best of these indices or methods provide an accuracy of less than 30 percent with high errors of omissions and\or commissions. Semi-automated methods consisted of 3 key techniques: (a) image enhancements to highlight wetlands, (b) image display to discern precise boundaries of wetlands, and (b) digitizing directly off screen to separate wetlands from their neighboring landscape. The most useful displays of ETM+ image enhancements (e.g., ratios) and band combinations, displayed as false color composite (FCCs) of RGBs were: (a) NIR/SWIR2, NIR/red, NIR/green; (b) NIR, Red, SWIR1; and (c) red, green, blue. The near-infrared (NIR) is centered at 0.825 μm and the short-wave infrared bands 1 and 2 (SWIR1 and SWIR2) are centered at 1.650 μm and 2.22 μm. The SRTM slope threshold of less than 1 percent was also very useful in delineating higher-order floodplain wetland boundaries. The wetlands were delineated with an accuracy of 86.4 percent using the semi-automated methods. The total wetland area in the Limpopo river basin was 12.5 percent of the total basin area of 41.5 million hectares. The overall accuracy of the 4 aggregated wetland classes in the basin was 82 percent with reasonable errors of omissions (20 percent) and low errors of commissions (12 percent).