5.A. DEM Sources and Processing
GLOBE version 1.0 has 11 broad sources of information. Many of these sources themselves have multiple sources, such as different series of imagery, maps, cadastral surveys, etc. In addition, some of these 11 broad sources of information have been reprocessed in various ways, by various organizations, before being combined into GLOBE Version 1.0.
GLOBE is not monolithically uniform in its characteristics. In fact, no monolithically uniform DEM of any great coverage area has yet been demonstrated to the GLOBE Task Team.
GLOBE cites data sources by original source and subsequent lineage. The importance of this for data stewardship is apparent.
Besides DTED Levels 1 - 5, distributed only by NIMA, and DTED Level 0, distributed via NIMA's Web site and on CD-ROM by NGDC, several other data sets developed from DTED or its precursors are distributed by other organizations. For example:
We thus prefer to cite the full lineage of the data in GLOBE documentation, including source and intermediate steps in the lineage of the data.
When citation of original source plus subsequent lineage is done, the contribution of the National Imagery and Mapping Agency and its predecessors (via DTED, Digital Chart of the World, and their derivatives) is noteworthy. A preponderance of the data began at this source. In addition, several previously copyright DEMs have been adapted for inclusion on an unrestricted or otherwise highly open basis. Also, GLOBE borrowed procedures, and in some cases data, from previous global DEM efforts, listed in Appendix A. The GLOBE project is indebted to these initiatives.
Various combinations of data source and lineage have resulted in subtle differences in georegistration and the actual type of elevation values used in different parts of GLOBE. The decisions that led to these differences were often based on scientific/technical analyses by diverse teams of data producers. However, some decisions may appear arbitrary to the user of GLOBE data. Considering the wide variety of possible influences on a DEM, the variety of georeferencing and sampling techniques incorporated into GLOBE is far from its greatest possible source of error.
Table 1. Percentage of the Global Land Surface Derived from Each Source % of global appx. % of number of 30" Source 30" grid cells global area grid cells Oceanic coverage 66.8 71.1 623,565,996 Land coverage 33.1 29.9 309,554,004 % of non-oceanic appx. % of number of 30" Source 30" grid cells land area grid cells Digital Terrain Elevation Data* 46.6 57.5 144,162,978 Digital Chart of the World 16.5 22.6 51,368,199 Australian DEM** 3.2 5.2 10,031,254 Antarctic Digital Database 28.1 8.3 86,973,518 Brazil 1:1,000,000-scale maps 2.0 3.5 6,027,490 DEM for Greenland 2.6 1.0 7,344,478 AMS 1:1,000,000-scale maps 0.67 1.1 2,088,224 DEM for Japan 0.18 0.26 556,763 DEM for Italy 0.16 0.21 490,585 DEM for New Zealand 0.14 0.18 419,894 Peru 1:1,000,000-scale map 0.03 0.05 90,625
* This line shows percentages of GLOBE derived from DTED sources, by all intermediate steps used. This includes DTED Level 0, and other 30" derivations from DTED and its prototypes distributed by the National Oceanic and Atmospheric Administration, and the U.S. Geological Survey.
** There are two versions of GLOBE. One version is completely unrestricted, and uses a DEM developed from Digital Chart of the World (DCW). Another version contains a licensed DEM from the Australian Surveying and Land Information Group. The figure given for Australia covers the area of that country/continent. If you are using the version of GLOBE that incorporates the DCW-based DEM for Australia, the total percentage of global land area with DCW source combines this line with the line above, for a total of 27.7% of the global land surface, 19.6% of non-oceanic 30" grid cells, and 60,666,875 total 30" grid cells. The unrestricted (G.O.O.D.) version of GLOBE has slightly less land area than the best available (B.A.D.) version of GLOBE in Australia. Table 1 shows the B.A.D. version.
Note that in source/lineage category 13, the blend between categories 12 and 14 contains 1,435,022 grid cells. Half this number (717,511 grid cells) has been assigned each to categories 12 (JPL model for Greenland) and 14 (DCW). This is because Table 1 treats sources, and the blending process used to make category 13 is dominated by category 12 in half its area, and by category 14 in the other half of its area.
5.A.i. Digital Terrain Elevation Data (DTED)