ASA 105: Skill 9, Measurements
9.Measurements: Convert courses and bearings between true, magnetic, and compass. Determine Latitude and Longitude. Measure distance.
Nautical measurements on a chart are either Angular or Linear. Courses are angles and distances are linear. The other vital nautical measurement is Time given in hours and minutes.
Leading lights, such as this pair in Nantucket, are a guide for staying in the channel. They may be used to check you compass deviation. In that instance we use a line, the range, to correct an angle, compass deviation.
The first measurement for navigators is True North. The longitude lines on the chart are North/South lines. The North Pole is an absolute. Compasses, however, don't point at the North Pole, they point at the Magnetic Pole which shifts ever so slightly each year. The inner rose of the compass rose printed o a chart represents the Magnetic Pole and shows the Variation from True North.
Compass Rose from a chart of the North Carolina coast showing 10 degrees West Variation. Coastal charts have a compass rose which indicates the local variation. A vessel may have a magnetic field of its own that further influences the ship's compass.
The arithmetic for converting angular directions is:
TVMDC AW
True - Variation - Magnetic - Deviation - Compass Add Westerly Variation or Deviation (therefore subtract Easterly V or D)
True North is the direction to the North Pole. On a chart it is the direction of the Longitude lines meeting at the North Pole.
Variation is the angle between the North Pole and the Magnetic Pole at a specific place. It is expressed as X degrees E or W and shown on the inner compass rose on nautical charts. New England in the Chart 1210Tr region has a Variation of about 15 degrees West. The coast of South Florida has a Variation of about 4 degrees West. Pilot charts of oceans show a map of variations as isometric lines.
Magnetic is the arithmetical solution of True and Variation.
Deviation is the deflection of the ship's compass by the magnetic field of the vessel. Modern fiberglass yachts have negligible deviation in most cases.
Compass (PSC) is the arithmetical solution of Deviation to Magnetic expressed as X degrees East or West.
Magnetic compass measurements are made in degrees between 0 and 360. We no longer use the older points of N, N1/4E, N 1/2E, etc. A cross reference to those points and the measurement by degree is found in Eldridge on page 225.
On the ASA 105 exam a sample deviation table is provided. It is used by entering the table with either the PSC heading or the Magnetic value plotted on a chart. There is some debate about interpolation of values. If you interpolate or round up or down always do so the same way.
A sample (fictitious) deviation table is presented below. If you plot a course on the chart of 232 degrees Magnetic the appropriate deviation to apply is 2 degrees East hence the compass should read 230 degrees PSC. Likewise if you take a bearing over the ship's compass of 260 degrees, apply a deviation of 4 degrees West when plotting the bearing on the chart as an observed bearing of 256 degrees Magnetic.
For example:
Latitude and Longitude:
Longitude is a line running from North Pole to South Pole. Longitude is measured as X degrees East or West of "the Prime Meridian" or 0 degrees Longitude which is a line passing through Greenwich, England. The training chart, 1210Tr shows Longitude between West 070d 36m and West 071d 35m.
Latitude is the angular measurement of lines parallel to the Equator above or below the Equator. Training chart 1210Tr shows Latitude range of N 41d 09m to N 41d 45m. Since one minute of arc Latitude is one nautical mile, the chart spans a 36 nm distance, North/South. The width of the chart is about 45 nm. A minute of Longitudinal arc is not a nautical mile (except at the Equator).
A scale of Nautical miles and yards is found elsewhere on the chart.
Depths and heights are measured as indicated on the chart. On USA charts. depths are in feet or fathoms. Heights are in feet.
Location given as Latitude and Longitude is done in this way: Latitude North or South of the Equator followed by Longitude East or West of the Prime Meridian in Greenwich, England.
On chart 1210 Tr find Buzzards Bay Light Tower about 4 nm WSW of the Cutty Hunk, Elizabeth Islands. Its position is represented as: N 41deg 23.5 min and W071deg 2 min. Longitude is always given with thee integers for the degrees.
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