Advanced Spaceborne Thermal Emission And Reflection Radiometer Global Digital Elevation Model Research Articles

Potential Impacts of Flooding on Alluvial Gold Mining Activities in Parts of Ilesha-Egbe Gold Field, Osun State, Nigeria.

The study mapped mining communities in parts of Ilesha-Egbe gold field in Osun State, South-West (SW) Nigeria, with their respective mining sites, their elevation above sea level and the average depth of mining pits in meters. This was done with a view to determining the potential impacts of flooding on mining activities in these areas. Geological map from Nigeria Geological Survey Agency (NGSA), Advanced Space borne Thermal Emission and Reflection Radiometer- Global Digital Elevation Model (ASTER-GDEM) from Global Land Cover Facility (GLCF) and topographic map from Office of the Surveyor General of the Federation (OSGOF) in Nigeria were used for this study. Geology and topographic maps were interpreted for drainage channels and rivers. These and other information were incorporated in mapping and interpreting the mine localities with their respective characteristics. ASTER-DEM was used to generate elevation information like DEM and slope. Stream ordering was done for the digitized drainages and buffering analysis was done for the respective mining points to determine how the mines will be affected in the event of a flood. Results indicate that the study area is well drained with abundant alluvial gold deposit with mines lying on drainage channel. In the events of flood, mine pit will be filled or partially filled with water and areas with pay-streaks easily flooded. Water runs down slope rather than infiltrates and run-off causes flooding at the lower courses of the water body and this provides for gold dust recharge downstream, opening up new gold exploitation areas.

Pyroclastic Density Current Hazard Assessment and Modeling Uncertainties for Fuego Volcano, Guatemala

On 3 June 2018, Fuego volcano experienced a VEI = 3 eruption, which produced a pyroclastic density current (PDC) that devastated the La Réunion resort and the community of Los Lotes, resulting in over 100 deaths. To evaluate the potential hazard to the population centers surrounding Fuego associated with future PDC emplacement, we used an integrated remote sensing and flow modeling-based approach. The predominate PDC travel direction over the past 15 years was investigated using thermal infrared (TIR) data from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument validated with ground reports from the National Institute of Seismology, Volcanology, Meteorology, and Hydrology (INSIVUMEH), the government agency responsible for monitoring. Two different ASTER-derived digital elevation model (DEM) products with varying levels of noise were also used to assess the uncertainty in the VolcFlow model results. Our findings indicate that the recent historical PDC travel direction is dominantly toward the south and southwest. Population centers in this region of Fuego that are within ~2 km of one of the volcano’s radial barrancas are at the highest risk during future large eruptions that produce PDCs. The ASTER global DEM (GDEM) product has the least random noise and where used with the VolcFlow model, had a significant improvement on its accuracy. Results produced longer flow runout distances and therefore better conveys a more accurate perception of risk. Different PDC volumes were then modeled using the GDEM and VolcFlow to determine potential inundation areas in relation to local communities.

Remote sensing signature analysis of ASTER imagery for geological mapping of Gasus area, central eastern desert, Egypt

The using of processed ASTER images and ASTER-GDEM (Advanced Spaceborne Thermal Emission and Reflection Radiometer-global digital elevation model) supplemented by extensive geological field work and spectral measurements of field samples enabled the creating of a new geological map of Gasus area. SVM (Support Vector Machine) method obtained 86% as overall accuracy of the classification and was verified by previous published maps and field verifications. Structure lineaments were automatically extracted and directional analysis of the automatically extracted lineament maps showing that the major trend of the lineaments are NW-SE and NNW-SSE, while NNE-SSW and NE-SW trends can also be recognized but in a less significant order. These trends are compatible and corresponding to the field verification.

Line Generalization Evaluation on Contour Map Generated From SRTM and ASTER GDEM

A contour map is one of many layers that composed Informasi Geospasial Dasar (IGD), which according to Act. No 4 2011 serves as a reference for any thematic map. The provision of contour map at a different level of scale is needed since mapping activities will always refer to map scale based on the mapping area. This research aims to analyze automated contour generation quality to produce 1:50.000 contour map, by means of using open access Digital Elevation Model (DEM) data, such as Shuttle Radar Topographic Mission (SRTM) and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Global Digital Elevation Model (GDEM).
 The automated contour generation was done by using contour interpolation in Quantum GIS software. Furthermore, simplification and smoothing algorithm was applied to both data, in order to improve their visual appearance. In this case, there are four algorithms used in the study, namely Douglas-Peucker, Visvalingam, Chaikin, and McMaster. Quality assessment, both qualitative and quantitative assessment, was done to each derived contour map to ensure the applicability of the procedure.
 The result shows that contour map generated from SRTM has a better quality than contour map generated from ASTER GDEM. Nevertheless, both data has a similar pattern on each topographical classes, which tends to produce bad quality contour line in the flat area. The more mountainous the area, the better the contour line. Meanwhile, of all generalization algorithm applied in this study, Chaikin’s algorithm is the best algorithm in terms of smoothing the contour line and improving visual quality, but still doesn’t significantly improved the metric accuracy. The contour line can be either directly added to the Digital Cartographic Model of Topographic Map (Rupabumi Map), or used as compliance data in a thematic map.

EVALUATION OF ASTER GDEM V3 USING ICESAT LASER ALTIMETRY

Abstract. We have used a set of Ground Control Points (GCPs) derived from altimetry measurements from the Ice, Cloud and land Elevation Satellite (ICESat) to evaluate the quality of the 30 m posting ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) Global Digital Elevation Model (GDEM) V3 elevation products produced by NASA/METI for Greenland and Antarctica. These data represent the highest quality globally distributed altimetry measurements that can be used for geodetic ground control, selected by applying rigorous editing criteria, useful at high latitudes, where other topographic control is scarce. Even if large outliers still remain in all ASTER GDEM V3 data for both, Greenland and Antarctica, they are significantly reduced when editing ASTER by number of scenes (N≥5) included in the elevation processing. For 667,354 GCPs in Greenland, differences show a mean of 13.74 m, a median of -6.37 m, with an RMSE of 109.65 m. For Antarctica, 6,976,703 GCPs show a mean of 0.41 m, with a median of -4.66 m, and a 54.85 m RMSE, displaying smaller means, similar medians, and less scatter than GDEM V2. Mean and median differences between ASTER and ICESat are lower than 10 m, and RMSEs lower than 10 m for Greenland, and 20 m for Antarctica when only 9 to 31 scenes are included.

EVALUATION OF ASTER GDEM V3 USING ICESAT LASER ALTIMETRY

We have used a set of Ground Control Points (GCPs) derived from altimetry measurements from the Ice, Cloud and land Elevation Satellite (ICESat) to evaluate the quality of the 30 m posting ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) Global Digital Elevation Model (GDEM) V3 elevation products produced by NASA/METI for Greenland and Antarctica. These data represent the highest quality globally distributed altimetry measurements that can be used for geodetic ground control, selected by applying rigorous editing criteria, useful at high latitudes, where other topographic control is scarce. Even if large outliers still remain in all ASTER GDEM V3 data for both, Greenland and Antarctica, they are significantly reduced when editing ASTER by number of scenes (N≥5) included in the elevation processing. For 667,354 GCPs in Greenland, differences show a mean of 13.74 m, a median of -6.37 m, with an RMSE of 109.65 m. For Antarctica, 6,976,703 GCPs show a mean of 0.41 m, with a median of -4.66 m, and a 54.85 m RMSE, displaying smaller means, similar medians, and less scatter than GDEM V2. Mean and median differences between ASTER and ICESat are lower than 10 m, and RMSEs lower than 10 m for Greenland, and 20 m for Antarctica when only 9 to 31 scenes are included.

The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) after fifteen years: Review of global products

The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is a 15-channel imaging instrument operating on NASA’s Terra satellite. A joint project between the U.S. National Aeronautics and Space Administration and Japan’s Ministry of Economy, Trade, and Industry, ASTER has been acquiring data for 15 years, since March 2000. The archive now contains over 2.8 million scenes; for the majority of them, a stereo pair was collected using nadir and backward telescopes imaging in the NIR wavelength. The majority of users require only a few to a few dozen scenes for their work. Studies have ranged over numerous scientific disciplines, and many practical applications have benefited from ASTER’s unique data. A few researchers have been able to mine the entire ASTER archive, that is now global in extent due to the long duration of the mission. Six examples of global products are described in this contribution: the ASTER Global Digital Elevation Model (GDEM), the most complete, highest resolution DEM available to all users; the ASTER Emissivity Database (ASTER GED), a global 5-band emissivity map of the land surface; the ASTER Global Urban Area Map (AGURAM), a 15-m resolution database of over 3500 cities; the ASTER Volcano Archive (AVA), an archive of over 1500 active volcanoes; ASTER Geoscience products of the continent of Australia; and the Global Ice Monitoring from Space (GLIMS) project.

Accuracy assessment of GDEM, SRTM, and DLR-SRTM in Northeastern China

This paper compares the accuracy of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Global Digital Elevation Model (GDEM), Shuttle Radar Topography Mission (SRTM) C-band and German Aerospace Centre (DLR)-SRTM X-band digital elevation models (DEMs) with the Ziyuan 3 (ZY-3) stereoscopic DEM and ground control points (GCPs). To date, the horizontal error of these DEMs has received little attention in accuracy assessments. Using the ZY-3 DEM as reference, this study examines (1) the horizontal offset between the three DEMs and the reference DEM using the normalised cross-correlation method, (2) the vertical accuracy of those DEMs using kinematic GPS data and (3) the relationship between the three DEMs and the reference ZY-3 DEM. The results show that the SRTM and DLR-SRTM have greater vertical accuracy after applying horizontal offset correction, whereas the vertical accuracy of the ASTER GDEM is less than the other two DEMs. These methods and results can be useful for researchers who use DEMs for various applications.

Ecological Niche Modelling using satellite data for assessing distribution of threatened species Ceropegia bulbosa Roxb.

Abstract. Ceropegia bulbosa Roxb. is a narrow endemic, tuberous twiner of Asclepiadaceae family. It is medicinally important: tubers are nutritive and edible, leaves are digestive and a cure for dysentery and diarrhea. Exploitation for its tubers and poor regeneration of this species has shrunk its distribution. In order to know its present status, we report here the results of its appraisal in Rajasthan, using remote sensing and ground truthing in the past five years (2009–14). A base map of C. bulbosa was prepared using Geographical Information System (GIS), open source software Quantum GIS, SAGA. The Landsat Enhanced Thematic Mapper (ETM) +Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), Global Digital Elevation Model (GDEM) Satellite Data were used in this study. ASTER and GDEM Data was clipped with district boundary and provided color range to get elevation information. A digital elevation model of Rajasthan physiography was developed from ASTER GDEM of 30-m resolution. GIS layers of Area of occurrences for C. bulbosa plant and elevation were created. This map along with topographic sheets of 1:50000 were used for field traversing and ground truthing as per GPS location inferred from map. Its geographic distribution was assessed using MaxEnt distribution modelling algorithm that employed 12 presence locality data, 19 bioclimatic variables, and elevation data. Results of this modelling predicted occurrence of C. bulbosa in the districts of Sirohi, Jalore, Barmer, Pali, Ajmer, Jhalawar, Dungarpur, Banswara, Baran, Kota, Bundi and Chittorgarh. Ground validation in these districts revealed its presence only at four places in three districts confirming its rarity. Analysis of dominance at their sites of occurrence revealed their poor populations and sub dominant status (RIV = 20–32) and very low density (2–12 plants per tenth ha).

Capacity assessment of the Qattara Depression: Egypt as a sink for the global sea level rise

One of the most prominent geomorphological features in the Western Desert of Egypt is the occurrence of the Qattara Depression (20,800 km2 or 2% of Egypt’s area), which has the deepest point in Africa. Topographic analysis of the depression was carried out using fine resolution digital elevation models acquired from the Advanced Spaceborne Thermal Emission and Reflection Radiometer-Global Digital Elevation Model (ASTER-GDEM) in order to assess the capacity of the depression. Results showed that the lower point in the depression occurs at −136 m and its capacity at the sea level is 1340 billion m3, which equals 10 times the live storage capacity of the Lake Nasser behind the Aswan High Dam. Although the depression has the aptitude to be a reservoir for any augmentation in the sea level, detailed environmental impact assessment studies should be implemented to address geologic, climatic, biologic as well as socio-economic impacts.

Assessment of the global digital elevation models ASTER and SRTM in Greece

This study evaluates the height accuracy of the free to download global digital elevation models (DEMs): advanced spaceborne thermal emission and reflection radiometer (ASTER) global digital elevation model (GDEM) and shuttle radar topography mission (SRTM) for the area of Greece. The analysis is done in two ways; using control points with known elevation and accurate DEMs. Quantitative and qualitative assessments are made both for the whole country, and for specific regions (test sites). Using the DEMs of SRTM with coverage from 56°S to 60°N and ASTER GDEM with coverage from 83°S to 83°N, the elevation information is available to users free of charge almost worldwide. In the regions where both data sets exist, the question arises which one is best to use; the decision is based on a series of parameters, such as accuracy, homogeneity, reliability, completeness, performance of morphological features, as well as the purpose of the work for which the elevation information will be used. These parameters are reviewed and analysed using data covering Greece. The data come from various providers and varying formats. The evaluation of the results highlights the need for a ‘fit for purpose’ use of each of these DEMs, taking into consideration the implementation constraints (technical and qualitative specifications of each project), along with the usefulness of these DEMs for a wide range of applications.

Estimation of forest canopy height using orthoimage-refined digital elevation models

Tree height is an important factor when making estimates for the management of disasters, the atmospheric environment, and forest biomass. However, there are limits to the accurate estimation of tree heights due to the inherent deviations in Shuttle Radar Topography Mission (SRTM) digital terrain elevation data (DTED) and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) global digital elevation model (GDEM) data, which are used to capture surface elevations in wide areas. There are also other problems, such as errors introduced during automatic terrain extraction from satellite images and when matching treetop areas with tree heights. In this study, we propose a method for producing refined DEMs from stereo orthoimages, which are subtracted from existing digital terrain models to calculate tree heights over a wide area. Experimental results showed that our proposed method yields tree heights that are closer to those measured manually than tree heights estimated via SRTM DTED or ASTER GDEM are. Our method also facilitates the generation of polygons by updating the average tree height in existing vector interim terrain data, thereby improving their accuracy.

Evaluation of ASTER GDEM using GPS benchmarks and SRTM in China

The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Global Digital Elevation Model (GDEM) has generated one of the most complete high-resolution digital topographic data sets of the world to date. The ASTER GDEM covers land surfaces between 83° N and 83° S at a spatial resolution of 1 arc-second (approximately 30 m at the equator). As an improvement over Shuttle Radar Topography Mission (SRTM) coverage, the ASTER GDEM will be a very useful product for many applications, such as relief analysis, hydrological studies, and radar interferometry. In this article, its absolute vertical accuracy in China was assessed at five study sites using ground control points (GCPs) from high-accuracy GPS benchmarks and also using a DEM-to-DEM comparison with the Consultative Group on International Agriculture Research Consortium for Spatial Information (CGIAR-CSI) SRTM DEM Version 4.1. It is demonstrated that the vertical accuracy of ASTER GDEM is 26 m (root mean square error (RMSE)) against GPS-GCPs, while for the SRTM DEM it is 23 m. Furthermore, height differences in the GDEM-SRTM comparison appear to be overestimated in the areas with a south or southwest aspect in the five study areas. To a certain extent, the error can be attributed to variations in heights due to land-cover effects and undefined inland waterbodies. But the ASTER GDEM needs further error-mitigating improvements to meet the expected accuracy specification. However, as for its unprecedented detail, it is believed that the ASTER GDEM offers a major alternative in accessibility to high-quality elevation data.

Technical Methodology for ASTER Global DEM

At the core of the technical methodology for creating the Advanced Spaceborne Thermal Emission and Reflection radiometer (ASTER) global digital elevation model (GDEM) is the procedure for generating a global set of 1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> latitude-by-1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> longitude tiles containing DEM data in geographic latitude and longitude coordinates and with one arc second postings from scene-based ASTER DEMs. The ASTER GDEM is comprised of all tiles, which include at least 0.01% land in them, each containing 3601-by-3601 elevation data points. The tiles are created by stacking all observed scene DEM data matched geographically to the tile container, selecting valid data for each pixel, removing abnormal data values, and then averaging the remaining selected valid data to assign as the tile elevation data. Valid Earth surface elevation values typically clump within a ± 40-m range and are assumed to be lower in elevation than residual cloud outliers. The filtering process, which assigns the tile elevation data, is one of the most important parts of the GDEM generation system. The median-based selection method is designed to efficiently select the valid data for each pixel. The combination of cloud-masked and non-cloud-masked data is another important part of the process to assign accurate elevation data for each pixel, because the cloud masking capability is not perfect. The algorithm used to combine both data is described. The postprocessing for inland water bodies is successfully carried out to yield a flattened elevation value. This postprocessing is essential to assign unique elevation values for each inland water body. GDEM tile elevation data include some residual anomalies, mostly in areas with fewer than three valid stacked input scenes. The correction method using existing reference data also is described.

SUMMARY OF THE VALIDATION OF THE SECOND VERSION OF THE ASTER GDEM

Abstract. On October 17, 2011, NASA and the Ministry of Economy, Trade and Industry (METI) of Japan released the second version of the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Global Digital Elevation Model (GDEM) to users worldwide at no charge as a contribution to the Global Earth Observing System of Systems (GEOSS). The first version of the ASTER GDEM, released on June 29, 2009, was compiled from over 1.2 million scene-based DEMs covering land surfaces between 83°N and 83°S latitudes. The second version (GDEM2) incorporates 260,000 additional scenes to improve coverage, a smaller correlation kernel to yield higher spatial resolution, and improved water masking. As with GDEM1, US and Japanese partners collaborated to validate GDEM2. Its absolute accuracy was within -0.20 meters on average when compared against 18,000 geodetic control points over the conterminous US (CONUS), with an accuracy of 17 meters at the 95% confidence level. The Japan study noted the GDEM2 differed from the 10-meter national elevation grid by -0.7 meters over bare areas, and by 7.4 meters over forested areas. The CONUS study noted a similar result, with the GDEM2 determined to be about 8 meters above the 1 arc-second US National Elevation Database (NED) over most forested areas, and more than a meter below NED over bare areas. A global ICESat study found the GDEM2 to be on average within 3 meters of altimeter-derived control. The Japan study noted a horizontal displacement of 0.23 pixels in GDEM2. A study from the US National Geospatial Intelligence Agency also determined horizontal displacement and vertical accuracy as compared to the 1 arc-second Shuttle Radar Topography Mission DEM. US and Japanese studies estimated the horizontal resolution of the GDEM2 to be between 71 and 82 meters. Finally, the number of voids and artifacts noted in GDEM1 were substantially reduced in GDEM2.

Performance Estimation of Aster Global DEM Depending upon the Terrain Inclination

A digital elevation model (DEM) is a source of immense three dimensional data revealing topographic characteristics of any region. The performance of a DEM can be described by accuracy and the morphologic conformity. Both depend upon the quality of data set, the used production technique and the roughness of the terrain. The global DEM of ASTER (Advanced Space-borne Thermal Emission and Reflection Radiometer) was released to public utilization as free of charge on June 2009. It covers virtually overall the globe using 1 arc-second posting interval. Especially easy availability renders ASTER Global DEM (GDEM) one of the most popular and considerable global topographic data for scientific applications. From this point of view, the performance of ASTER GDEM has to be estimated for different kinds of topographies. Accordingly, six test fields from Spain (Barcelona) and Turkey (Istanbul and Zonguldak) have been preferred depending upon the terrain inclination. Thus, the advantages and disadvantages of the DEM product have been proved by means of a group of advanced performance analysis. The analyses indicate that the performance of ASTER GDEM is quite satisfying at urban areas because of flat topography. On the other hand, terrain slope has negative effect on the results. Especially steep, mountainous, forestry topographic formations and the regions which have sudden changes at the altitude have lower accuracy.

ASTER GDEM validation using LiDAR data over coastal regions of Greenland

Elevation data from airborne Light Detection and Ranging (LiDAR) campaigns are used in an attempt to evaluate the accuracy of the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) global digital elevation model (GDEM) in Greenland. The LiDAR elevation data set is characterized by a high spatial resolution of about 1 m and elevation accuracy of 20–30 cm root mean square error (RMSE). The LiDAR data sets used were acquired during ice-monitoring campaigns carried out from 2003 to 2008. The study areas include ice-free regions, local ice caps and the ice sheet margin. A linear error of 15–65 m was derived, which is far greater than the 20-m product specification. This estimation is biased by both the seasonal and the climatic changes in local ice caps because the ASTER GDEM was computed from imagery acquired in the period 2000–2009. High sloping areas along the coastal regions of Greenland and the effect of the number of scenes used to generate the ASTER GDEM as well as relief are associated with the GDEM accuracy.

Accuracy assessment of the ASTER GDEM and SRTM3 DEM: an example in the Loess Plateau and North China Plain of China

The digital elevation model (DEM) produced by the Shuttle Radar Topographic Mission (SRTM) has provided important fundamental data for topographic analysis in many fields. The recently released global digital elevation model (GDEM) produced by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) has higher spatial resolution and wider coverage than the SRTM3 DEM, and thus may be of more value to researchers. Taking two typical study areas—the Loess Plateau and the North China Plain of China—as an example, this article assesses the accuracy of the SRTM3 DEM and ASTER GDEM by collecting ground control points from topographical maps. It is found that both the SRTM3 DEM and the ASTER GDEM are far more accurate for the North China Plain than for the Loess Plateau. For the Loess Plateau, the accuracy of the ASTER GDEM is similar to that of the SRTM3 DEM; whereas for the North China Plain, it is much worse than that of the SRTM3 DEM. Considering the negative bias of the ASTER GDEM for flat or gentle regions, we improve its accuracy by adding the difference of the mean value between the SRTM3 DEM and ASTER GDEM for the North China Plain; then, the root mean square error (RMSE) of ±7.95 m from the original ASTER GDEM is improved to ±5.26 m, which demonstrates that it is a simple but useful way to improve the accuracy of the ASTER GDEM in flat or gentle regions.

Assessment of ASTER global digital elevation model data for Arctic research

ABSTRACTA new source of digital elevation data, the advanced spaceborne thermal emission and reflection radiometer (ASTER) global digital elevation model (GDEM), has been freely available since 2009. It provides enormously greater coverage of the Arctic than previous satellite derived ‘global’ digital elevation models, extending to a latitude of 83 °N in contrast to 60 °N. The GDEM is described as a preliminary, research grade product. This paper investigates its accuracy in a number of specifically Arctic landscapes, including ice and snow, boreal forest, tundra and unvegetated terrain, using test sites in Svalbard, Iceland, Norway and Russia. Semivariogram analysis is used to characterise the magnitude and spatial correlation of errors in the GDEM products from the test sites. The analysis suggests that the horizontal resolution of the GDEM data is around 130 m, somewhat coarser than the sampling interval of 1 second of latitude and longitude. The vertical accuracy is variable, and the factors influencing it have not been systematically explored. However, it appears that the likely accuracy can be estimated from ‘stacking number’ data supplied with the elevation data. The stacking number is the number of independent digital elevation models averaged to generate the supplied product. Provided that this number is greater than around 6 the data have an rms accuracy of typically 5–10 m.

An evaluation of the accuracy of the ASTER GDEM and the role of stack number: a case study of Nisiros Island, Greece

A reference digital elevation model (RDEM) produced by the Hellenic Military Geographical Service (HMGS) (Athens, Greece) is used to assess the vertical accuracy of the ASTER (advanced spaceborne thermal emission and reflection radiometer) global digital elevation model (GDEM) for a small volcanic island, Nisiros, in south-east Greece. The overall vertical accuracy of the GDEM is within the root mean square error (RMSE) specified by the ASTER DEM Working Group. Despite this, ASTER GDEM is a product with variable vertical accuracy depending on the number of scenes (stack number (SN)) per grid point used for the computation of elevation. The exposure (SN) of a physiographic region to ASTER sensors seems to be elevation- and slope-dependent in this particular study area. The difference between the GDEM and RDEM may partly be explained by the fact that GDEM is a digital surface model and thus the variable landcover heights introduce additional error in accuracy estimates.