Global Multi-resolution Terrain Elevation Data Research Articles

Suitability of spaceborne digital elevation models of different scales in topographic analysis: an example from Kerala, India

Digital elevation model (DEM), deriving conventionally from contour data of topographic maps, provides sufficient information regarding the continuously varying topographic surface of the Earth. Though spaceborne DEMs are increasingly being used in earth-environmental-applications, suitability of various freely available spaceborne DEMs [e.g., advanced spaceborne thermal emission and reflection (ASTER), shuttle radar topography mapping mission (SRTM), global multi-resolution terrain elevation data (GMTED)] for topographic and geomorphometric analyzes in tropical regions is yet to be ascertained. In this paper, comparability of these spaceborne DEMs among themselves and also with the DEM (TOPO) prepared from digital contour data of topographic maps is assessed. Results show that various primary and secondary derivatives of ASTER and SRTM DEMs provide relatively better precision and substantial agreement with the corresponding parameters derived from TOPO. Among the spaceborne DEMs, SRTM has relatively higher vertical accuracy (root mean square error = 17.05 m), compared to ASTER (24.09 m) and GMTED (32.85 m). The vertical accuracy of all the spaceborne DEMs strongly depends on the relief and ruggedness of the terrain as well as the type of vegetation. It is proposed that in the absence of other available and acceptable elevation datasets, SRTM and ASTER are equally competent for geomorphometric analysis in tropical regions, while GMTED shows significant loss of terrain information due to coarser spatial resolution.

External Validation of the ASTER GDEM2, GMTED2010 and CGIAR-CSI- SRTM v4.1 Free Access Digital Elevation Models (DEMs) in Tunisia and Algeria

Digital Elevation Models (DEMs) including Advanced Spaceborne Thermal Emission and Reflection Radiometer-Global Digital Elevation Model (ASTER GDEM), Shuttle Radar Topography Mission (SRTM), and Global Multi-resolution Terrain Elevation Data 2010 (GMTED2010) are freely available for nearly the entire earth’s surface. DEMs that are usually subject to errors need to be evaluated using reference elevation data of higher accuracy. This work was performed to assess the vertical accuracy of the ASTER GDEM version 2, (ASTER GDEM2), the Consultative Group on International Agriculture Research-Consortium for Spatial Information (CGIAR-CSI) SRTM version 4.1 (SRTM v4.1) and the systematic subsample GMTED2010, at their original spatial resolution, using Global Navigation Satellite Systems (GNSS) validation points. Two test sites, the Anaguid Saharan platform in southern Tunisia and the Tebessa basin in north eastern Algeria, were chosen for accuracy assessment of the above mentioned DEMs, based on geostatistical and statistical measurements. Within the geostatistical approach, empirical variograms of each DEM were compared with those of the GPS validation points. Statistical measures were computed from the elevation differences between the DEM pixel value and the corresponding GPS point. For each DEM, a Root Mean Square Error (RMSE) was determined for model validation. In addition, statistical tools such as frequency histograms and Q-Q plots were used to evaluate error distributions in each DEM. The results indicate that the vertical accuracy of SRTM model is much higher than ASTER GDEM2 and GMTED2010 for both sites. In Anaguid test site, the vertical accuracy of SRTM is estimated 3.6 m (in terms of RMSE) 5.3 m and 4.5 m for the ASTERGDEM2 and GMTED2010 DEMs, respectively. In Tebessa test site, the overall vertical accuracy shows a RMSE of 9.8 m, 8.3 m and 9.6 m for ASTER GDEM 2, SRTM and GMTED2010 DEM, respectively. This work is the first study to report the lower accuracy of ASTER GDEM2 compared to the GMTED2010 data.

Sensitivity of digital elevation models: The scenario from two tropical mountain river basins of the Western Ghats, India

The paper evaluates sensitivity of various spaceborne digital elevation models (DEMs), viz., Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), Shuttle Radar Topography Mapping Mission (SRTM) and Global Multi-resolution Terrain Elevation Data 2010 (GMTED), in comparison with the DEM (TOPO) derived from contour data of 20 m interval of Survey of India topographic sheets of 1: 50,000 scale. Several topographic attributes, such as elevation (above mean sea level), relative relief, slope, aspect, curvature, slope-length and -steepness (LS) factor, terrain ruggedness index (TRI), topographic wetness index (TWI), hypsometric integral (Ihyp) and drainage network attributes (stream number and stream length) of two tropical mountain river basins, viz., Muthirapuzha River Basin and Pambar River Basin are compared to evaluate the variations. Though the basins are comparable in extent, they differ in respect of terrain characteristics and climate. The results suggest that ASTER and SRTM provide equally reliable representation of topography portrayed by TOPO and the topographic attributes extracted from the spaceborne DEMs are in agreement with those derived from TOPO. Despite the coarser resolution, SRTM shows relatively higher vertical accuracy (RMSE = 23 and 20 m respectively in MRB and PRB) compared to ASTER (RMSE = 33 and 24 m) and GMTED (RMSE = 59 and 48 m). Vertical accuracy of all the spaceborne DEMs is influenced by relief of the terrain as well as type of vegetation. Further, GMTED shows significant deviation for most of the attributes, indicating its inability for mountain-river-basin-scale studies.

Assessing the spatiotemporal dynamics of vegetation cover as an indicator of desertification in Egypt using multi-temporal MODIS satellite images

Desertification is the major environmental threat in the arid and semiarid regions. The soil-adjusted vegetation index (SAVI) was used as an indicator to monitor the desertification change in Egypt. A multi-temporal satellite data of moderate-resolution imaging spectroradiometer were used to estimate SAVI and land surface temperature. Also, Global Multi-resolution Terrain Elevation Data 2010 and climatic data were used for the analysis. This research focuses on assessing the trend of the vegetation cover change in the seasons of January, March, June, September, and December for the years 2002, 2005, 2008, and 2011. The magnitude of the vegetation cover change in periods 2002–2005, 2005–2008, and 2008–2011 at ≤100 and >100 m elevation was analyzed. A major increase in the vegetation cover that occurred in the period 2002–2005 was about 3,400 km2, as a result of two national megaprojects (Toshka Project and El-Salam Canal). In contrast, vegetation cover decreased by 5,500 km2 in March during the period 2005–2008, coinciding with the period when the management of the megaprojects failed. Vegetation cover changed again by 1,500 km2 in the period of 2008–2011, and the vegetated areas in the Nile Delta were affected by the sea level rising which was responsible for the soil salinization. Three sites were chosen in this investigation (Kom Ombo, El-Oweinat, and Nile Delta) in order to observe the difference of desertification dynamics and to understand the relationship between the vegetation cover distribution and other environmental variables. Anti-desertification policies and advanced agricultural management are highly required in Egypt to decrease any environmental crises and food shortage.