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5.B. Assembly of GLOBE Version 1.0

DEMs from various sources were merged several times during the life of the GLOBE project:

  • Twenty-six DEMs and Digital Bathymetric Models from various sources were merged at NGDC to form TerrainBase. TerrainBase was primarily a lower-resolution data set. However, DEMs for the U.S. and environs, and for Italy, were available at 30" gridding for GLOBE.
  • DEMs for various sources were merged at JPL for its MISR DEM. The JPL contribution to GLOBE consists of coverage of Greenland, where DTED are not available.
  • DEMs from many sources were merged by USGS, during the creation of its GTOPO30. These merges included USGS's selection of its preferred raster-derived DEMs, then its selection of preferred DEMs from cartographic sources. The following text is USGS's (1997b) description of its process of "learning as the project progresses":
"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).

Data processing was accomplished using commercially available geographic information system software, public domain image processing software, vector-to-raster gridding software, and utilities developed specifically for this project. To more efficiently handle the numerous input data sets and to standardize the proper sequence of processing steps, the production procedures were automated to a great extent by employing preset parameter values, scripted command files, and consistent naming schemes for input and output data files."

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).

A final processing step performed on the mosaicked and blended product involved "clipping out" the land (as defined by vector coastline data) and setting the ocean areas to a constant background value. Use of vector coastline data resulted in a more consistent portrayal of the land/ocean interface, especially in areas where raster source data (which had an implied coastline) met with vector source data. The DCW coastline was used to clip the following areas: Africa, Eurasia, South America, Australia, New Zealand, Greenland, and isolated ocean islands. The World Vector Shoreline (WVS), a vector shoreline data set from NIMA, was used for North America, including Hawaii, the Caribbean islands, and Central America. The islands of Borneo and Sulawesi in southeast Asia were clipped with the coastline digitized from the 1:1,000,000-scale map source. Antarctica was defined by the coastline as portrayed in the ADD."

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.

  • The MISR and GTOPO30 DEMs were made by their creators to meet their own objectives. Finally, NGDC selected and merged available data into GLOBE. This effort used DEMs generated by Zwally (and others)/NSIDC/JPL for Greenland. Many parts of GTOPO30 were used, including the DEM contribution for New Zealand, the SCAR vector data, and the maps cooperatively selected and converted to DEMs by GSI and USGS. Additional input data came from AUSLIG, GSI, NIMA, and SGN. The merging process is described below.

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
5.B.ii. Global Data Set Assembly
5.B.iii. Georeferencing of GLOBE
5.B.iv. Comparison of GLOBE with Other Available DEMs
5.B.v. GLOBE's Development as Additional DEMs Are Created


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