Navigation & Nautical Chart Production Flashcards

What is the primary horizontal datum used for nautical charts produced by NOAA in the United States?

Answer: NAD 83 / WGS 84

NOAA nautical charts use NAD 83, which is nearly identical to WGS 84, as the horizontal reference datum for all positional information.

On a nautical chart, a sounding value shown in italics typically indicates:

Answer: A depth that is less reliable or obtained from an older survey

Italic soundings on NOAA charts indicate depths that are of lesser reliability, often sourced from historical or unverified surveys.

What does the chart symbol 'Wk' followed by a dashed outline indicate on a nautical chart?

Answer: A wreck whose position is approximate or unsurveyed

A dashed outline around a wreck symbol indicates the wreck's position is approximate and may not have been accurately surveyed.

The Mercator projection used for most nautical charts preserves which property?

Answer: True compass bearings (rhumb lines appear as straight lines)

The Mercator projection is conformal, preserving local angles so that rhumb lines (constant compass-bearing courses) plot as straight lines, which is essential for navigation.

Which vertical datum is used as the reference surface for depths (soundings) on NOAA nautical charts in US waters?

Answer: Mean Lower Low Water (MLLW)

NOAA uses MLLW as the chart sounding datum because it represents approximately the lowest water level, providing conservative (safe) depth values for mariners.

The term 'Least Depth' on a chart refers to:

Answer: The minimum depth over a specific feature such as a shoal or wreck

Least depth denotes the minimum sounding value recorded over a specific hazard or feature, representing the critical clearance depth for navigation.