The open data HELI-DEM DTM for the western alpine area: computation and publication

Two problems can arise in analyzing Digital Elevation Models (DEMs). Firstly, one interest region could be patched by several partly overlapping models that present similar accuracies and spatial resolutions; they should be merged in one unified model. Moreover, even when the interest region is completely covered by one model, local data with better accuracy could cover partial areas and should be properly merged. All these problems have been addressed within Helvetia-Italy Digital Elevation Model, a project funded by the European Regional Development Fund (ERDF). One specific aim of the project was the creation and the publication of a unified DEM for a part of the Alps between Italy and Switzerland. The interest area is prevalently mountainous, with elevations that range from about 200 to 4600 m. Various elevation datasets were available that were gridded with different spatial resolutions, in different reference frames and coordinates. Moreover, one high resolution LiDAR model was available for some areas. Firstly, a validation was performed that allowed the identification of few blunders and confirmed the general accuracy of the input data. Two DEMs have been produced. Both of them cover the whole project area (boundaries λ = 7.80° East and λ = 10.70° East, φ = 45.10° North and φ = 46.70° North) and are gridded in ETRF2000 geographical coordinates, with a spatial resolution of 2 × 10−4 degrees. The former (named HD-1) has been obtained by interpolating and merging all the low resolution models on a new common grid. HD-1 has been locally corrected by the LiDAR model, where it was available; to avoid sharp discontinuities, the corrections have been filtered by Fast Fourier Transform before applying them. The resulting model has been called HD-2. HD-1 and HD-2 are published by an open access geoservice.