DEM generation from cartographic basis and their Validation of by GPS Kinematic Survey

A relevant task for several applications in geodetic sciences is the validation of Digital Elevation Models (DEM) generated by whichever technique, mainly by analytical and digital photogrammetry, digitalization of available maps and airborne laser scanning. The validation procedure can be divided into two steps: the internal one, that is the inner checking of the data useful to identify and remove outliers by suitable statistical techniques; the external, that is the comparison of the (interpolated) heights derived from the DEM with those estimated by a direct survey in several and possibly well distributed points. An efficient way to perform this survey can be represented by a GPS kinematic survey. Of course, the usage of GPS forces to georeference the DEM into a realization of the WGS84 reference system (e.g. ITRF97) and requires the knowledge of a geoid model at a proper level of precision. Moreover, in order to gain the required precision (generally not higher than few meter for DEM at medium and large scale as those considered in this paper) with a good internal and external reliability of the survey, it is necessary to use three or more base permanent stations. In this paper we discuss the encountered problems and the results of the application of the outlined DEM validation methodology to four DEM at different nominal scales (1:10000, 1:25000, 1:50000 and 1:100000) attaining to the Nemi lake area (Rome district, Central Italy), relevant also on the volcanological and geophysical point of view.