5.A.iv. DEM for Italy
* Primary reference citation for all data from this source
The DEM for Italy was derived at NGDC, from a 7.5x10-second DEM developed by the Servizio Geologico Nazionale (SGN) of Italy. This section summarizes documentation prepared by Row and others (1995, pp. 4-91 through 4-95) for TerrainBase (Row and Hastings, 1994), and is also described by Carrozzo and others (1985).
The 7.5x10-second DEM developed by SGN was developed for use in gravity terrain corrections. Organizations involved with either developing, analyzing, or financing the data set included AGIP (a major petroleum company in Italy), the National Research Council, National Group of Solid Earth Geophysicists (GNGTS), Ministry of Education (MPI), and the University of Lecce. Source materials included several topographic map sources cited in Row and others (1995), at scales of 1:25,000 to 1:50,000 for land, plus hydrographic maps at 1:100,000.
Mean height values were manually extracted from the topographic maps at regular grid intervals. The size of the grid was selected to maximize the amount of reliable detail obtainable from the source maps, while keeping the resolution from becoming so fine as to overly burden the developers. Grid spacings of 7.5x10 seconds were used for 1:25,000 scale maps, 15x20 seconds for 1:50,000 maps, and 30x40 seconds for the 1:100,000 scale maps. A map of sources is presented in Carrozzo and others (1985), and reproduced in Row and others (1995, p. 4-94).
Grid lines were printed on transparent acrylic sheets in 20-minute latitude sections, and overlain on the maps to guide data collection. For each grid cell, a mean height was manually estimated from the contours and point heights that appeared within and adjacent to each cell.
The 6,210,000 elevation values were derived from 3450 maps, 1,878,000 heights in coastal regions from 130 coastal maps, and 3,456,000 depth values from 120 maps. The coarser grids were then interpolated to 7.5x10-second gridding by an undocumented technique. SGN then tiled the data into 20x30-minute areas.
Data were analyzed for errors by SGN using the following procedures:
SGN and NGDC jointly developed a method for compiling these proprietary data into an unrestricted 30" grid. According to this method, NGDC computed the mean value of all 7.5x10-second grid values contained in each 30" output grid cell. Areas containing significant errors were removed from the model (these were filled by NIMA DTED Level 0 discrete values during the patching process making GLOBE Version 1.0).