Digital Nautical Chart (DNC) - NGA

MIL-PRF-89023
Class DNC CE DNC CE DNC CE DNC CE
HARBOR APPROACH COASTAL GENERAL
1 25m 50m 250m
2 50m 100m 500m >500m
3 100m 200m 1000m
4 >100m >200m >1000m
3.2.2 Absolute vertical accuracy. The difference between an assigned
elevation and the true elevation at a specific point. In this comparison,
both elevations must be referenced to MSL. A point's elevation may be
determined through interpolation of the digital contour file or it may be
listed as a vertex coordinate of a feature. Vertical accuracy is expressed at
90% probability (.9p) linear error as a proportion of the contour interval.
The following lists the vertical accuracy categories:
Class DNC LE
(Contour Interval)
1 0.5
2 1.0
3 2.0
4 >2.0
3.2.3 Relative accuracy. NIMA does not have a formal relative accuracy
objective for this product.
3.2.4 Hydrographic accuracy. NIMA strives to compile hardcopy nautical
charts with the most accurate depth information available. The charts are
compiled from a variety of sources with varying accuracies. The most accurate
data is taken from hydrographic surveys which meet IHO survey vertical
accuracy standards. Information regarding the source data used for each chart
comprising the DNC can be found in the Data Quality coverage of each library.
3.3 Datum.
3.3.1 Horizontal datum. The horizontal datum shall be referenced to
WGS84. If the source map/chart sheets are not referenced to WGS84, then they
will be converted from their original horizontal datum to WGS84.
3.3.2 Vertical datum. Charts generally have three vertical datums.
Topographic features are referenced to Mean Sea Level. Shoreline is
referenced to Mean High Water. The hydrographic features are referenced to a
vertical datum based on low water tide level and is called the Sounding Datum
or Hydrographic Datum. The specific low water datum used depends on the type
of tide in the area or on the number and magnitude of high and low tides in
one tidal cycle. Hydrographic datums used in the DNC will be specified in the
Data Quality coverage for each library.
3.4 Units of measure. Units of measurement in this specification are
generally given in the metric system. With very few exceptions, units of
measurement for the DNC will employ the metric system (see Appendix section
A.3.1.2).
3.5 DNC product description. This section provides information about
the DNC databases.
3.5.1 Database sources. The DNC is based on the feature content of the
hardcopy Harbor, Approach, Coastal and General charts produced by NIMA.
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