4.1 Nautical Charts, Scale, Lat/Long & Chart No. 1

Charts Are the Working Surface of Q162

Q162 Navigation General Near Coastal (50 questions, 70% pass) expects you to read a chart the way a master reads a COI: as a legal and operational document, not as pretty wallpaper behind the helm. On a Master Less Than 100 GRT near-coastal or inland passenger trip, the chart answers depth, hazards, aids, channels, and the scale at which you can safely measure distance and plot a course. Chart Plot (Q163) will make you draw on charts; this section makes you understand them so those plots mean something.

Mercator Projection — What Masters Use on Coastal Sheets

Most U.S. coastal and harbor charts you will handle day-to-day are Mercator projections (or electronic displays that present Mercator-style grids). On a Mercator chart:

• Meridians (longitude lines) are straight and parallel, running top to bottom.
• Parallels (latitude lines) are straight and parallel, running left to right.
• A rhumb line (constant compass course) plots as a straight line — convenient for coastal courses and chart-plot problems.

Mercator distorts area and scale as you move away from the equator: landmasses stretch north–south at high latitudes. For near-coastal Master 100 work in U.S. waters that distortion is manageable if you always measure distance correctly (next subsection). Do not assume a centimeter on the chart equals the same ground distance everywhere without using the proper scale.

Latitude, Longitude, and Distance

Latitude is north–south position from the equator (0° to 90° N/S). Longitude is east–west position from the prime meridian (0° to 180° E/W). Positions are written in degrees and minutes (and seconds or tenths of minutes on larger-scale charts).

The distance rule every exam hits

• One minute of latitude ≈ one nautical mile (NM) anywhere on Earth (by definition of the nautical mile as related to the meridian).
• Therefore: measure distance on the latitude scale adjacent to your working area — the minutes printed on the left/right borders of the chart.
• Do not measure distance on the longitude scale. One minute of longitude equals one NM only at the equator; it shrinks as you go toward the poles. Using the top/bottom longitude border for distance is a classic wrong answer.

Practical measuring technique on paper charts:

1. Open dividers to the track length between two points.
2. Walk the dividers along the nearest latitude scale (or the bar scale if provided and appropriate).
3. Read nautical miles (and tenths) from that latitude scale.

On electronic charts, use the tool’s range/distance function rather than eyeballing screen millimeters.

Large Scale vs Small Scale

Chart scale is a ratio such as 1:20,000 or 1:80,000. The first number is always 1 (chart distance); the second is ground distance in the same units.

Memory aid: large scale = large detail (you zoom in). Exam traps reverse the words: a 1:80,000 chart is smaller scale than a 1:20,000 chart even though the second number is bigger.

Master practice: for docking, narrow channels, and hazard-dense bays, shift to the largest-scale chart covering the area. Use small-scale charts for route planning and big-picture traffic awareness, then step down in scale as you approach the pier or inlet.

Chart No. 1 — Symbols, Not a Place

U.S. Chart No. 1 (Nautical Chart Symbols, Abbreviations and Terms) is the joint NOAA/NGA reference that explains what chart symbols mean: wrecks, rocks, cables, lights, buoys, depth contours, quality of bottom, restricted areas, and more. It is not a chart of a geographic area and not something you plot a course on.

When Q162 asks where to find the meaning of an unfamiliar symbol, the answer is typically Chart No. 1 (or the chart’s own legend for a subset). Do not confuse Chart No. 1 with:

• A harbor chart number such as 13270
• The Light List volume number
• Coast Pilot chapter numbers

Soundings and Chart Datums

Soundings printed on the chart are depths below chart datum, not “depth under your keel right now.” On most U.S. coastal charts, the tidal datum for depths is Mean Lower Low Water (MLLW) — a low-water reference so that actual depths are usually equal to or greater than charted soundings when tide is considered. Heights of bridges and overhead clearances use a high-water-related datum (often Mean High Water or a local equivalent as noted on the chart).

Master-level clearance thinking:

• Charted depth + height of tide (from Tide Tables, adjusted for your location) − draft − safety margin = estimated under-keel clearance.
• Never treat a green-shaded “deep” area as guaranteed water without checking date of survey, notes, and obstructions.
• Units matter: U.S. charts may show feet or fathoms and feet; metric charts use meters. Read the title block every time you open a new sheet.

Title Block, Notes, and Source Diagrams

Before trusting a chart for passenger work, read:

• Title, scale, projection, sounding units, and datum
• Edition date and last correction information
• Notes on regulated areas, cables, fish havens, and local hazards
• Source diagram / reliability notes (older surveys may be sparse)

Electronic Navigation Charts (ENCs) and raster charts still require the same mental checklist: what datum, what units, how recent, and which overlays you turned off by accident.

Chart Corrections

Charts go stale. New wrecks, relocated buoys, dredged channels, and extinguished lights appear first in Notice to Mariners (NtM) and Local Notice to Mariners (LNM) issued by the Coast Guard (and related NGA/NOAA correction systems for the chart series you use). Professional practice:

• Correct paper charts by hand from notices, or replace with a new edition when corrections pile up.
• Keep electronic charts on a current update cycle from the authorized source.
• After a storm or construction season, expect temporary changes and verify critical aids visually and by Light List / LNM before night passenger runs.

Chart literacy pocket card

1. Distance → latitude scale (1′ lat ≈ 1 NM); never longitude for distance.
2. Large scale = detail; small scale = overview.
3. Chart No. 1 = symbol dictionary.
4. Soundings = below datum (often MLLW); add tide for real depth.
5. Uncorrected chart = navigational negligence.

Own the chart language and Q162 becomes measurement and judgment instead of guesswork — the same standard you owe every passenger when the inlet gets skinny at low water.