5.A.vii. Digital Chart of the World

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Primary Developer:		Defense Mapping Agency (now National Imagery and Mapping Agency)
Title:		Digital Chart of the World
Publication Date:		1992
Bibliographic Citation:	**	Defense Mapping Agency (DMA), 1992. Digital Chart of the World. Defense Mapping Agency, Fairfax, Virginia. (Four CD-ROMs.)
Post-processing:		U.S. Geological Survey (USGS) for GTOPO30.
Bibliographic Citation for Post-processed DEM:	*	DMA and USGS, 1996. 30" DEM from Digital Chart of the World (in USGS, ed., 1997).
Source/Lineage Category:		14

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* Primary reference citation for all data from this source
** Primary reference citation for Digital Chart of the World

Digital Chart of the World (DCW) is a vector cartographic data set based largely on the 1:1,000,000-scale Operational Navigation Chart (ONC) series of aircraft navigation charts. ONC has been cited as the largest scale base map source with global coverage (Danko, 1992). ONCs were products of the Army Mapping Service (AMS), then DMA, now National Imagery and Mapping Agency (NIMA). They were based on photogrammetric analyses of Department of Defense Corona imagery acquired in the 1960s.

ONCs contain vector base map information such as political boundaries, transportation infrastructure, waterways, and coastlines, as well as raster-like shading for selected categories of land cover type. The objective of this information is to facilitate navigation by pilots. In addition, for much (but not all) of the world, ONCs contain topographic contours. The primary contour interval is 1000ft (305m), with supplemental contours at 250ft (76m) intervals in areas below 1000ft elevations. Limited supplemental contours at 500ft (152m) intervals exist above 1000ft elevations.

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.

ONCs (and DCW, which is derived from ONCs) use World Geodetic System 84 (WGS84) for horizontal reference, and Mean Sea Level as vertical reference. However, this may not always be the case, as cartographic sources (for example) may not be completely or accurately described, and materials from such sources may not be accurately converted to WGS84 and Mean Sea Level.

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. The resultant data base occupies four CD-ROMs, which is distributed by USGS for DMA.

Absolute horizontal accuracy of the DCW hypsography is reported to be 2040m rounded to the nearest 5m at 90% circular error. Vertical accuracy is considered to be 610m for contours, and 30m for spot elevations (DMA, 1990).

Errors can exceed such figures locally. For example, the ONC for northern Greenland contains notices such as: "CAUTION: Arctic Institute of North America Project Nord (Control Data Corp.) indicates position discrepancies in excess of 11 nautical miles (Nov. 68)."

Topographic contours and point elevations are the primary data for deriving DEMs from DCW. Supplementary data include drainage information. Streams were used to guide the contour-to-grid process using the ANUDEM program of Hutchinson (1989, 1996). 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. University College London was active in early development of DCW conversion techniques; the Jet Propulsion Laboratory used contour-to-grid techniques to make much of its 30" DEM noted in Section 1 and in Section 5.A.vi. However, all of DCW contour-to-grid data used in GLOBE were produced by USGS.

The following is USGS, 1997b, description of its vector data processing techniques. These apply to DCW, as well as the other cartographic source materials listed in Sections 5.A.viii, 5.A.ix, 5.A.x., and 5.A.xi:

"The topographic information from the vector cartographic sources, including the DCW, the ADD [Antarctic Digital Database], and the Army Map Service, International Map of the World, and Peru 1:1,000,000-scale maps, was converted into elevation grids through a vector-to-raster gridding approach. Contours, spot heights, stream lines, lake shorelines, and ocean coastlines were input to the ANUDEM surface gridding program developed at the Australian National University (Hutchinson, 1989). ANUDEM, specifically designed for creating DEM’s sic from digital contour, spot height, and stream line data, employs an approach known as drainage enforcement to produce raster elevation models that represent more closely the actual terrain surface and contain fewer artifacts than those produced with more general purpose surface interpolation routines. Drainage enforcement was performed for all areas covered by vector source data except Antarctica and Greenland.

A significant amount of preprocessing was required to prepare and format the vector source data for input to ANUDEM. This processing included editing and updating the vector stream lines so that the direction of each was oriented downstream (a requirement of ANUDEM). Further preprocessing involved detection and correction of erroneous contour and point elevations (Larson, 1996). Ocean coastlines were assigned an elevation of zero for input as contours. Also, shorelines of lakes for which the DCW included elevations were tagged and used as contour input. The output from ANUDEM was an elevation model grid referenced in the same horizontal coordinate system as the generalized raster source data. The output grid spacing of 30 arc-seconds has been shown to be appropriate for the information content present in the DCW hypsography layers (Hutchinson, 1996; Shih and Chiu, 1996)."

The following passage is from another part of USGS (1997b):

"Prior to merging with the generalized raster data, lakes for which the DCW did not indicate an elevation were updated on the DCW grid with the lowest grid cell elevation found along the shoreline. When each of the vector sources was gridded, an overlap area with the adjacent raster sources was included so that smoothing could be performed to minimize the elevation discrepancies among the sources. Also, additional point control was input into the ANUDEM gridding process so interpolated elevations in the overlap region would more closely match the raster source elevations. The additional control was derived from the generalized raster sources within a 1-degree buffer surrounding the vector source areas."

Cell-centered registration; contour-to-grid value computed. Graphic describing georeferencing and sampling for DCW conversion.

Analysis of Histograms: As the Digital Chart of the World was the source for about 23% of GLOBE, we performed histogram analyses on DEMs from DCW for the same areas as NIMA discrete data. Note that only data used in GLOBE were run through the histogramming process, so DTED- and DCW-based histograms should be different:

Plate 22. Click on image to view larger size.

Africa/Europe: Plate 22 plots DCW coverage of Africa and Europe (0o to 60oN latitude and 0o to 60oE longitude) is largely similar to that of DTED discrete data for the same region. A similar trough at low elevations (including some elevations below sea level), peak about 450m, and gradual quasi-linear decline to 4500m is apparent. The DCW-based DEM is, however, punctuated by spikes at approximately 300m (1000ft) intervals, with subsidiary spikes at lower elevations, as might be expected from its source, Operational Navigation Charts. A spike at 1200m is superimposed upon the broader peak at the same elevation that is more apparent in the histogram of DTED discrete for Africa and Europe. However, that elevation appears to exhibit a slightly more pronounced peak around 1200m in the DCW-derived DEM, as well.

Plate 23. Click on image to view larger size.

Asia: Plate 23 plots DCW coverage for Asia (0o to 90oN latitude, 60o to 180oE longitude) shows a similar pattern of steady decrease in frequency of occurrence, with a broad peak at about 5000m elevation. As for Asia, the DCW-based DEM shows prominent spikes at 300m increments, with spikes at subsidiary contour intervals for lower elevations.

Plate 24. Click on image to view larger size.

North America: A similar broad pattern to that shown for DTED discrete data for North America is apparent in Plate 24. Spiking at contour intervals in source maps (with associated broader peaks probably derived in gridding) is typical of this series of DCW-based DEMs. There appear to be fewer higher elevations than in Plate 4, as might be expected considering the coverage of these data.

Plate 25. Click on image to view larger size.

South America: In Plate 25, the DCW-based DEM has a similar pattern to that of DTED discrete data, with the superposition of more spikes (at about 300m increments, with intermediate spikes at lower elevations). The broad peak at 4000–5000m apparent in the DTED-based data is not so apparent in the DCW-based DEM. This is because the Altiplano is covered more by DTED (and, to a lesser degree, by AMS and Peruvian maps) than by DCW.