Friday, April 11, 2008

Chart Information

Organization of the chart

  • Authority: The publisher responsible for the information in the chart.
  • Title: The title gives a description of the area covered by the chart.
  • Number: Different chart types of the same area can be distinguished by the chart's number.
  • Projection: Most likely the Mercator projection.
  • Scale: For example: 1:80,000. meaning that 1 unit on the chart represents 80,000 units.
  • Horizontal geodetic datum: The definition of the relationship between the ellipsoid adopted as the model of the Earth's shape, and the Earth itself. Though there are hundreds of datums in use, most are only locally valid.
    Yet, the WGS-84 datum is global in scope and positions obtained by satellite navigation systems are usually referred to this datum. Therefore, a correction needs to be applied to a WGS-84 GPS position to agree with charts using other horizontal datums. For example to correct WGS-84 to the European datum, add 0.06'N, 0.04'E to the WGS-84 position indicated by the GPS. Fortunately, most GPS receivers may be set to display positions in several other datums besides WGS-84 and perform the calculations for you.
  • Chart sounding datum: The tidal datum to which soundings and drying heights on a chart are referred. Often shortened to “chart datum” when it is clear that reference is not being made to a horizontal datum. Chart sounding datums are also used as reference for heights (lighthouses, mountains, bridges). Multiple datums can be used in one chart: L.A.T. for soundings and M.L. for heights.
  • Soundings & height units: Soundings and heights can be stated in - for example - meters, feet or fathoms.
  • Horizontal scale: Natural scale at for example 40° 15'.3 latitude where the horizontal scale can be used for measuring distances and where the chart scale is true.
  • GPS compatibility: Most charts neither have the precision nor the resolution to fully use the (differential) GPS positioning potential. Still plenty of charts result from surveys done in the 19th century.
    Also, GPS data often requires a correction for a local horizontal chart datum before it can be used in the chart.
  • Corrections & edition: The chart is for example a 2004 edition but is - when properly corrected - still valid in 2006. Corrections are published continuously and the changes should be mentioned in the bottom left corner of the chart.

Information in the chart

  • Depths reduced to chart datum: A sounding like (35) indicates 3½ metres of water under Lowest Astronomical Tide (when depths are notated in “metres” and the chart datum is “L.A.T.”). An underlined sounding like (04) indicates a height of 40 cm above L.A.T..
  • Isobaths: Lines connecting positions with the same depth: depth contours.
  • Heights reduced to chart datum: Heights of for instance, lighthouses, mountains and cliffs are more often reduced to another datum such as Mean High Water (M.H.W.) or Mean High Water Spring.
  • Tidal information: Details of both the horizontal and the vertical movement of the water is often included in the chart.
  • Buoys & marks: Lightships, lateral and cardinal marks.
  • Seabed qualities: Pebbles, seaweed, rocks, wrecks, pipelines, sand (fine or coarse) and other seabed characteristics for anchoring.
  • Lighthouses: Their height, color, range, and other properties.
  • Magnetic variation: The angle between the true North and the magnetic North varies in place and time. The local variation is indicated in the compass card.
  • Conspicuous positions on the shore: Churches, radio masts, mountain tops, etc. that can be used for compass bearings and other means of navigation.

    Selection of chart symbols

    Large Navigational BuoySuper(light)buoy, Lanby.navigation buoyBuoyLight buoy.
    green buoyLateral green starboard hand buoy.marine navigationred buoyLateral red port hand buoy.
    safe water markSafe water mark (red/white).map and chartcardinal buoyCardinal buoy, West mark.
    stoneStone; drying height above chart datum.nautical chartfoul seabedFoul seabed. Avoid anchoring here.
    danger by soundingDanger, least depth by sounding.sailingissues marine navigationdanger draggedDanger, depth swept by wire drag.
    visible wreckWreck visible at chart datum.chart projectionvisible at low tideWreck showing Mast(s) above chart datum.
    dangerous wreckDangerous wreck, depth unknown.referencewreck, not dangerousWreck, not dangerous (10 m below chart datum).
    tidesPosition for which tidal levels are referencecurrentsPosition for which tidal stream data are tabulated.
    sector lightSector light, colour changes on different resourcedangerDanger line, in general.
    oil rigOil rig, prohibited zone of 500 m.safetyRig, lightedLighted platform, prohibited zone of 500 m.
    ObstnObstruction.compass roseWdSeaweed, describes seabed.
    PAPosition Approximate.coordinatesPPebbles, the seabed.
    StStones - Danger.latitude and longitudeWhisWhistle Buoy.
    FL 42m 29MFlashing light, 42 meters above datum, range 29'.online and freeLFl 10sLong flashing light, period 10 seconds.


    • Mercator projection: Most coastal nautical charts are constructed with this method. Angles are true and distances can be measured using the vertical scale.
    • Stereographic projection: Used for chart covering small areas. Like the Mercator projection use the vertical scale to measure distances.
    • Gnomeric projection: Used for large areas. Great circles appear as straight lines on the chart.
    • Great circle navigation: The shortest course on earth between two positions is a great circle; for circumnavigating and ocean crossings.
    • Loxodrome: A line which makes the same angle with all meridians. Theoretically not the shortest route, but a handy straight line on a Mercator chart.
    • Horizontal geodetic datum: Defines the relationship between the ellipsoid adopted as the model of the Earth's shape, and the Earth itself. Coordinates which refer to, for instance, should be corrected before plotting them in a chart based on another horizontal datum. If your GPS receiver consistently disagrees with known positions by a constant amount and direction, then check that you have set it to display the correct horizontal datum.
    • Chart sounding datum: The tidal datum (fictitious plane) to which soundings, heights, elevations and drying heights on a chart are referred.
    • Vertical scale: Distances in nautical miles or minutes (') should be measured at the same latitude on the vertical scale.
    • Corrections: Each chart is liable to corrections which are published by either a national body or the publisher of the nautical chart.