Comparisons between global and Helidem transalpine models : problem identification

Digital Elevation Models (DEMs) provide a mathematical description of the terrain surface and have the trait to represent it in a visual way as well. Various tools using computer technologies have been developed since the middle of the 20th century and one of the most important and powerful technique to represent terrain data is the digital modeling. It calculates the elevation of a point of the surface through different interpolation methods and the obtained altitudes of the terrain are set down to a certain reference frame. Depending on the data set that DEMs represent they could be distinguished in Digital Surface Models (DSMs) and Digital Terrain Models (DTMs). The first one refers to the Earth’s surface including all the objects that belong to or are built on it (buildings, vegetation and other features elevated above the land). DTMs represent for the elevations of the bare ground without any objects. Other differentiation could be done regarding the specific need, the size of the area that the user has to describe and the level of accuracy that he wants to obtain. If a big portion of the Earth surface has to be studied, like an entire region or an entire country, but with low details, a global DEM can be used; on the contrary local DEM can be adopted when there is the need to be analyzed a small portion of the surface but with high details. Another important feature that has to be taken into account is that the accuracy depends on the equipment performing the acquisition. There are a lot of acquisition techniques for generating data with the aim to create DEMs but for sure one of the most advantageous is using satellite sensors for obtaining remotely-sensed elevation data. The topic of this study is based on all these facts. The present work aims to compare the different most well-known global Digital Elevation models namely Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), Shuttle Radar Topography Mission (SRTM) and Global Multi-resolution Terrain Elevation Data 2010 (GMTED2010) with the local transalpine Helvetia-Italy Digital Elevation Model (HELI-DEM), and then to identify the existing problems. The ASTER GDEM was developed in cooperation of the U.S. National Aeronautics and Space Administration (NASA) and Japan’s Ministry of Economy, Trade, and Industry (METI) and covers land surfaces between 83°N and 83°S which is 99% of Earth's landmass. ASTER data are posted on a 1 arc-second (approximately 30 m) grid. The SRTM is an international joint project developed by The National Aeronautics and Space Administration (NASA) and the National Geospatial-Intelligence Agency (NGA). The collected data covers about 80% of the Earth's land surface between 60°N and 56°S latitude. The derived products are two DTMs at resolution level of 1 arc-second (approximately 30 m) at the present available only for USA area and 3 arc-second (approximately 90 m) for worldwide coverage. The GMTED2010 is a global elevation model developed by the U.S. Geological Survey (USGS) in collaboration with the National Geospatial-Intelligence Agency (NGA) with the aim to replace the Global Topographic Data GTOPO30. The previous model has horizontal resolution of 30 arc-seconds for the entire Earth and provided elevation data that widely vary in quality. The new model has been generated at three separate resolutions of 30 arc-second (approximately 1 km), 15 arc-second (approximately 500 m) and 7.5 arc-second (approximately 250 m). GMTED2010 data covers all land areas from 84°N to 56°S latitude for most products, and coverage from 84°N to 90°S latitude for several products. Each of the three GDEMs is posted on a Lat/Lon grid and referenced to the 1984 World Geodetic System (WGS84) horizontal datum and 1996 Earth Gravitational Model (EGM96) vertical datum. All the data are available in GeoTIFF format as SRTM could be downloaded in Arc-Info ASCII format as well. The models are considered as Open data so they are available online and could be obtained free of charge from users worldwide. The local HELI-DEM has been developed with the aim to create a unified DEM for the area of Italian and Swiss Alps. HELI-DEM covers two regions in Italy – Lombardy and Piedmont and two cantons in Switzerland – Ticino and Grisons. The model covers the ETRF2000 rectangle comprised by the following boundaries: from 45.10° N to 46.70° N latitude and from 7.80° E to 10.70° E longitude. It is gridded in geographical coordinates (as the global ones) and has a spatial resolution of 2 × 10-4 degrees.