Spatial Resolution and Algorithm Choice as Modifiers of Downslope Flow Computed from Digital Elevation Models

This research investigated the dependence of computing downslope flow from digital elevation models (DEMs) on two basic elements of the flow calculation: the spatial resolution of the DEM and the algorithm used to compute the downslope flow, specifically how it defines topographic slope and aspect. Six algorithms for downslope flow and downslope accumulation were implemented in MATLAB using different assumptions about components of the computation from the terrain analysis literature. We analyzed the results of downslope flow statistically, thresholded the values to yield streams, and compared the different results with the actual surface streams in the National Hydrographic Database. We repeated the computations using DEMs of 3-m, 30-m, and 90-m resolutions, covering a test area near Santa Barbara, California. We conclude that downslope flow computations are overestimates, and that they are fraught with critical algorithmic assumptions and scale effects. They should be approached with great caution in GIS-based analyses.     

Assessment of Shuttle Radar Topography Mission Elevation Data Based on Topographic Maps in Turkey

Depending on scale, topographic maps depicting the shape of the land surfaces of the Earth are produced from different data sources. National topographic maps at a scale of 1:25 000 (25K maps) produced by General Command of Mapping are used as the base map set in Turkey. This map set, which consists of approximately 5500 sheets, covers the whole country and is produced using photogrammetric methods. Digital Elevation Models (DEMs) created from these maps are also available. Recently, another data source, Synthetic Aperture Radar (SAR) interferometric data, has become more important than those produced by conventional methods. The Shuttle Radar Topography Mission (SRTM) contains elevation data with 3 arc-second resolution and 16 m absolute height error (90 percent confidence level). These data are freely available via the Internet for approximately 80 percent of the Earth's land mass. In this study, SRTM DEM was compared with DEM derived from 25K topographic maps for different parts of Turkey. The study areas, each covering four neighboring 25K maps, and having an area of approximately 600 km², were chosen to represent various terrain characteristics. For the comparison, DEMs created from the 25K maps were obtained and organized as files for each map sheet in vector format, containing the digitized contour lines. From these data, DEMs in the resolution of 3 arc-second were created (25K-DEM), in the same structure as the SRTM DEM, allowing the 25K-DEMs and the SRTM DEM to be compared directly. The results show that the agreement of SRTM DEM to the 25K-DEM is within about 13 m, which is less than the SRTM's targeted error of 16 m. The spatial distribution of the height differences between SRTM-DEM and the 25K-DEM and correlation analysis show that the differences were mainly related to the topography of the test areas. In some areas, local height shifts were determined.     

The Tenth International Conference and Sixth General Assembly of the International Cartographic Association: A Report

An invertible model is proposed which uses the basis of scale-independence and scale-dependence to capture some of the essential cartographic elements of topographic relief. The model, which uses Fourier and fractal methods in combination with local surface operators, is invertible, and is used to simulate topography. The uses of such simulated topography are discussed, as are the advantages of the particular model used. The model is developed mathematically. and seems in its application to produce terrain with some desirable cartographic traits.     

Recent literature in cartography and geographic information science

The National Imagery and Mapping Agency (NIMA) is at the forefront of using the techniques of Interferometric Synthetic Aperture Radar (IFSAR) for the production of large volumes of digital elevation models. Two NIMA-sponsored programs, one space- borne and one air-borne, have had very positive results in 2002. These are the Shuttle Radar Topography Mission (SRTM) and the Geographic Synthetic Aperture Radar (GeoSAR).