MIL-PRF-89201B
3.3.1 Horizontal Datum. For new production, as charts are revised or
updated for periodic maintenance, the WGS 84 or NAD 83 datum shall be used.
Where appropriate, a revised Military Grid System, also based on WGS 84 or NAD
83, shall be depicted as the primary grid (purple), and if required, the
overlapping grid (blue). The old (local) datum and secondary grid will not be
shown. Charts on the Mercator projection will show interior ticks in addition
to border ticks, due to the curvature of grid lines on that projection.
3.3.2 Vertical datum. Features shown on nautical charts are referenced
to one of three vertical datums. Topographic features are referenced to Mean
Sea Level. Shorelines generally represent the land-water interface at Mean
High Water. The hydrographic features are referenced to a vertical datum
based on a low water tide level and is called the sounding datum or chart
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.
3.4 Security. The security classification of Harbor, Approach, Coastal
and General Charts shall be the lowest category practicable. When it becomes
necessary to assign a security classification to the product, it shall be in
accordance with established national procedures.
3.5 Chart classification by scale. There are four basic types of
nautical charts based on scale and function. The scale categories are not
rigid and may vary from area to area.
a. General charts are those charts with scales of 1:600,000 and
smaller. They show major ocean basins, major seas and bays. They are used
for oceanic route planning and deep-ocean navigation. They show minimal
detail, with the understanding that the mariner will use larger scale charts
for actual plotting of fixes and navigating near shore.
b. Coastal charts range in scale from 1:150,000 to 1:600,000. They
show coastal areas, and are intended for navigation along the coast, but not
close to or approaching it. For this reason, only major aids to navigation
are portrayed, especially if larger scale charts show the same area. Inshore
information may be greatly generalized. Offshore dangers are portrayed but
are generalized if larger scale coverage exists.
c. Approach charts, with scales of 1:50,000 to 1:150,000, are used for
inshore navigation and for approaching ports and harbors. They portray all
aids to navigation usable by the mariner in the area, as well as dangers, but
may generalize harbor information where a harbor chart exists. If there is no
harbor chart for a given place, the approach chart may show all such
information.
d. Harbor charts have scales of 1:50,000 and larger and are used for
navigation within and around ports and harbors. All aids to navigation and
dangers are shown, as well as information on the facilities and services of
the port, and detailed information on depths, especially in channels and
alongside berthing facilities.
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