Chart Projections, Scales and Datums

Mercator Projection

Read first: the charts and publications a ship must carry. This topic explains what is drawn on those charts and how far you can trust it.

In short: the Mercator chart is the navigation chart. Rhumb lines are straight, angles are true, and you measure distance on the latitude scale.

• A chart puts a round Earth on flat paper. Something is always distorted. The method used is the chart projection.
• Mercator = a cylindrical projection. The cylinder touches the Earth along the equator.
• Meridians and parallels are straight lines at 90° to each other.
• Latitude is stretched more and more towards the poles (by the secant of the latitude). The poles cannot be shown.
• It is conformal: angles and small shapes are correct. Areas far from the equator look too big.
• A rhumb line (constant course) is a straight line. Lay it off with a parallel ruler and steer it.
• Distance: measure on the latitude (side) scale, level with the mid-latitude of the leg. 1′ of latitude = 1 NM. Never use the longitude scale (top and bottom): it is not in miles.
• Transverse Mercator: the cylinder touches along a meridian instead of the equator. It suits large-scale harbour plans and areas that are long north–south. A common mistake is to say you take distances from the bottom (longitude) scale on it: you do not. Use the latitude scale or the chart’s own scale bar.
• Source: The American Practical Navigator (Bowditch), Ch. 4 §405–407.

Gnomonic Projection

In short: the gnomonic chart is a planning chart. Every great circle is a straight line, but you cannot navigate on it.

• The Earth is projected from its centre onto a flat plane touching it at one point (an azimuthal, perspective projection).
• Every great circle is a straight line. That is its only job.
• Rhumb lines are curved. Angles and distances are distorted. You cannot lay off a course, measure a distance or plot a fix on it.
• It carries few of the usual chart features: it is for planning only.
• Use: ocean passage planning. Gnomonic routeing charts are about 1:17,500,000 to 1:32,000,000.
• Exam answer in one line: Mercator = navigation, gnomonic = planning.

Rhumb Lines and Great Circles

In short: a rhumb line keeps one course but is longer; a great circle is the shortest way but the course keeps changing.

• Rhumb line: crosses every meridian at the same angle, so the course stays constant. Straight on a Mercator chart, curved on a gnomonic chart.
• You may read that a rhumb line “cuts the meridians at a right angle”. That is only true for a course of 090° or 270° (along a parallel). The general rule is the same angle.
• Great circle: any circle on the Earth whose plane passes through the centre (the equator, every meridian). It is the shortest distance between two points on a sphere.
• A great circle is a straight line on a gnomonic chart and a curve (arc) on a Mercator chart, bowing towards the pole.
• Its course changes all the time (except along a meridian or the equator). In practice you steer it as a series of rhumb lines.
• Use: ocean passages, where the distance saved is worth it.

Plotting a Great Circle on a Mercator Chart

In short: draw it straight on the gnomonic chart, pick off points, plot them on the Mercator chart and join them with rhumb lines.

1. On the gnomonic chart, draw a straight line from departure to destination. That is the great circle.
2. Read the latitude and longitude of points along the line at regular steps (for example, every few degrees of longitude).
3. Plot those points on the Mercator chart(s).
4. Join them with straight lines. Each leg is a rhumb line. Together they follow the great circle closely.
5. Steer each leg and alter course at each point. Check every leg is clear of land and dangers.

Chart Scales

In short: scale = distance on the chart : distance on the Earth. The larger the scale, the more detail. Always use the largest scale available.

• 1:2,000 is a larger scale than 1:2,000,000. The smaller the number after the colon, the larger the scale.
• Large scale = small area, lots of detail. Small scale = big area, little detail.
• Admiralty range: largest about 1:1,500, smallest 1:50,000,000 (the world).
• Rough guide: the purpose of the chart sets its scale.

• The Admiralty catalogue (NP131) lists all Admiralty charts, more than 3,500. The numbering starts at chart 2 (chart 1 was withdrawn).
• On the bridge: a small-scale chart can leave out a small danger. Move to the larger-scale chart as soon as you reach its limits.

Charts and Chart Books to Carry

In short: carry charts that are adequate and up to date for the voyage, at the largest scale available, plus the books to find, read and correct them.

• SOLAS V/27: nautical charts and publications must be adequate and up to date for the intended voyage.
• SOLAS V/19.2.1.4: charts and publications to plan and display the route and to plot and monitor positions. An ECDIS can meet this.
• The largest-scale chart available for the area.
• NP131, Catalogue of Admiralty Charts and Publications: to choose the right charts.
• NP5011, Symbols and Abbreviations used on Admiralty Paper Charts. NP5012 does the same for ENC symbols in ECDIS.
• NP294, How to Keep Your Admiralty Products Up-to-Date.
• NP133A, Chart Correction Log: recommended, not compulsory.

Reading an Admiralty Chart: Symbols, Edition, Source Data and Watermark

In short: before you use a chart, read its notes, check its edition and corrections, look at how good the survey is, and make sure it is genuine.

• Symbols: NP5011 shows every symbol and abbreviation on Admiralty paper charts.
• Title block and notes: projection, scale, depth units, horizontal datum, vertical datums, the satellite-derived positions note, cautions and warnings. Read them all.
• Edition ≠ publication: the date of first publication is printed in the bottom margin. The New Edition date sits beside it. A chart first published years ago can be a recent edition. A New Edition replaces every earlier copy: never use the old one.
• Corrections: Notices to Mariners corrections applied to the chart are recorded in its bottom-left margin.
• Source data diagram: shows which surveys were used to build the chart, with their date, scale and source (for example “leadline survey”). It tells you how old a survey is, not how good it is (IHO S-67). Old or sparse surveys: treat the depths with caution and keep extra under-keel clearance.
• Watermark: genuine Admiralty paper charts carry a watermark in the paper (the older “Flying A” or the newer “ADMIRALTY”). Hold the chart up to the light to see it. The back has a thumb label with logo, chart number, area, barcode and date. Fake charts exist: buy only from approved Admiralty chart agents.

Horizontal Datum: WGS 84 and Datum Shifts

In short: a datum is the reference from which positions are calculated. GPS uses WGS 84. Your chart and your GPS must agree, or the plotted fix is in the wrong place.

• Geodetic (horizontal) datum = a model of the Earth’s shape plus an origin for latitude and longitude.
• GPS uses WGS 84 (World Geodetic System 1984), a model of the average shape of the globe. Modern charts reference WGS 84.
• Older charts may use a local datum. The same latitude and longitude can then be a different spot on the chart.
• The chart note on satellite-derived positions gives the shift: move GPS positions a stated number of minutes north or south and east or west to agree with the chart. Apply it before plotting.
• Can you use a chart on a different datum from GPS? Yes, if you know the difference between the two datums.
• ECDIS warns you when a chart uses a different geodetic datum.
• Unknown datum — change your strategy:
  • make a risk assessment of the conditions and circumstances;
  • increase under-keel clearance and the distance off dangers;
  • fix by traditional methods: visual bearings, transits, radar ranges and bearings;
  • compare the manual fixes with GPS to find the offset.

Vertical Datums: Depths, Heights and Clearances

In short: depths are measured down from a low-water datum (LAT); heights are measured up from high-water datums (MHWS, HAT).

• Chart datum (CD): the level from which charted depths and tide heights are counted. The IHO standard is LAT (Lowest Astronomical Tide) where practical.
• Depths: below CD (LAT). Usually there is more water than charted. Weather (high pressure, offshore wind) can lower the sea below the prediction.
• Drying heights: above CD, shown as underlined figures.
• Heights of land features and lights: above MHWS (Mean High Water Springs), a high-water datum. A light’s elevation is above MHWS.
• Vertical clearances (bridges, overhead cables): above HAT (Highest Astronomical Tide) on recent Admiralty charts, as the IHO recommends. Older or foreign-origin charts may still use MHWS: check the title notes.
• You may find lists that put “heights from HAT” and “lights from MHWS” side by side. Both are partly right: features and lights from MHWS, clearances from HAT.
• Example: a drying height of 2 m with a tide of 3.5 m leaves 1.5 m of water over the bank.

Survey Quality: CATZOC

In short: Zones of Confidence (ZOC, shown as CATZOC in an ENC) grade how far you can trust the depths: A1 best, D worst, U = Unassessed.

• U = Unassessed, not “Uncharted”.
• ZOC covers depths only (soundings, contours, rocks). It says nothing about the coastline, buoys or pipelines.
• The most important factor is seafloor coverage. Only A1 and A2 had a full search. A significant feature = more than 2 m proud in depths under 40 m, or 10% of the depth in deeper water.
• ZOC does not tell you the survey date. Sandwaves and dredged channels can change after the survey.
• On the bridge: in C and D zones, stay well clear of shoals and add under-keel clearance.
• Source: IHO S-67 (2017), Annex A.

Raster and Vector Charts (RNC vs ENC)

In short: an RNC is a scanned picture of a paper chart; an ENC is a database of charted objects that ECDIS can query and alarm on.

• RNC (raster): a digital scan of an existing paper chart, made of a dense grid of dots.

• ENC (vector): every object (depth, buoy, light) is stored as data. It is scalable and seamless and can trigger safety-contour and grounding alarms in ECDIS. You can query CATZOC.
• An ENC cell name starts with the code of the hydrographic office that produced it, for example GB203804.
• Dangers of electronic charts: trusting the screen too much; zooming past the compiled scale (it looks precise but is not); details hidden at small display scales. A radar overlay helps you check the chart against the real coastline.