2.3.1. Dataset

The first stage of a drainage-based analysis is to procure a Digital Elevation Model (DEM) of the study area. As currently available stream data for the study area are insufficient to directly yield the required stream drainage information, it was determined that a DEM or Digital Surface Model (DSM) of high resolution would be required to achieve the project objectives. In this initial exploratory work, a 1 arc-second DEM supplied by USGS [21] with 30 m resolution was utilized. Efforts to procure a DEM of higher resolution, though preferred, have proven difficult to obtain. The Shuttle Radar Topography Mission (SRTM) starting in February 2000 generated 1 arc second DEMs with 30 m horizontal resolution ranging from 56◦ south latitude to 60◦ North Latitude in gridded files encompassing 1◦ longitude by 1◦ latitude. The Shuttle Radar Topography Mission (SRTM) dataset has been shown to provide more accurate modelling over other datasets [22]. In many cases, a study area will include several of these files. In this study, the Lae region of Morobe Province is covered by two DEM files (S06E146.hgt.zip and S06E147.hgt.zip). After expansion and importation of the available band interleaved (BIL) formatted files into Idrisi formatted raster files (RST) in the GIS, the two rasters were merged into a single DEM. Visual examination of Google aerial photography of the study area around Lae suggested a suitable window of the expansive DEM to create a more manageable raster for further analysis.

**Figure 3.** Flow chart depicting different anthropogenic and environmental factors influencing runoff and their characterization based on point-, line- and raster-based GIS layers.
