Point elevations also appear in ONCs, often at airports, cities, or topographic features such as selected mountain peaks. In addition, some lake surfaces are labeled with typical elevations. In many areas lacking topographic contours, sparse point elevations are still available. In the late 1980s, DMA instigated a design and contracting process to digitize the contents of ONCs. The work was contracted to a private company.
Errors can exceed such figures locally.
Supplementary data include drainage information. Lake shorelines and ocean coastlines provide further guidance. DCW was used as the primary source for filling gaps in the DTED coverage, including all of the unrestricted DEM for Australia, and large areas of Africa, South America, and Canada. The following is USGS, 1997b, description of its vector data processing techniques. Drainage enforcement was performed for all areas covered by vector source data except Antarctica and Greenland.