5.B. Assembly of GLOBE Version 1.0
DEMs from various sources were merged several times during the life of the GLOBE project:
"GTOPO30 was developed over a 3 year period during which continental and regional areas were produced individually. As such, processing techniques were developed and refined throughout the duration of the project. Although the techniques used for the various continental areas are very similar, there were some differences in approach due to varying source material. More details about data development for several of the continental areas are reported by Verdin and Greenlee (1996), Bliss and Olsen (1996), and Gesch and Larson (1996).
The following is USGS's (1997b) description of its merging of raster- and vector-derived DEMs within GTOPO30:
"Merging of the generalized raster sources and the gridded vector sources was accomplished by mosaicking the data sets. The generalized raster sources had the highest priority so coverage of the data with the greater topographic detail and accuracy was maximized. The grid derived from DCW data had the highest priority among the vector sources, and the other digitized map data was used when DCW hypsography was unavailable. The merging procedure including blending of the generalized raster sources and the vector- derived grids within an approximate 1-degree overlap area along the irregular boundaries. The blending algorithm computes a weighted average with the weights for each data source determined on a cell-by-cell basis according to the cell's proximity to the edges of the overlap area (Franke, 1982).
The following is USGS's (1997b) description of its final data assembly to make GTOPO30:
"The global product was assembled from the continental and regional DEMs. Several areas of overlap due to different production stages of the project were addressed and eliminated, most notably between the Africa and Eurasia data sets. The global source map was generated from masks of source data coverage, and was verified to register with the DEM precisely. Finally, the entire data set was packaged into tiles for easier electronic distribution."
Categories 2, 3, 5 - 7, 11, and 14 - 18 in the GLOBE source/lineage map were originally compiled by USGS into GTOPO30. (GTOPO30’s source map does not differentiate lineage, thus does not differentiate its various methods of handling data
from DTED-based sources.) NGDC reassembled the USGS/GTOPO30 tiles before assembling GLOBE. In addition, it edited the USGS/GTOPO30 source file to represent the various methods of resampling used in GTOPO30 (that were not all documented in
USGS's source file). This last editing is representational, to help make users aware of these resampling techniques. The actual boundaries depicted between resampling techniques are estimates, based on verbal descriptions by USGS scientists.
Please note that peer review for this merging continues. Future versions of GLOBE will incorporate peer review comments. If appropriate, substitutions of data will be made.
5.B.i. Quality Assessment