Compass Deviation - an insidious problem for navigators

All compasses are subject to deviation and for reliable navigation a routine calibration check is essential. If errors of more than a few degrees are found, the compass sensitivity can be severely affected. On some headings it becomes sluggish and responds slowly to changes in the boat's heading while on others the slightest movement causes it to swing erratically. Steering a reliable course becomes impossible and although the compass may still appear to read correctly, large errors could be present.

In most cases, these difficulties can be overcome by 'adjusting' the compass. This is the process of placing small magnets around the compass site or of altering the positions of adjustment magnets within the compass body. Traditionally, it has always been considered a task for specialist compass adjusters and beyond the means of the average boat owner. This is no longer true. With C-Swing, preparing a deviation chart is now simply a question of selecting a suitable shoreside object, entering its true bearing from the chart, then entering its compass bearing as the boat is sailed on a range of headings. With the built-in maths package, headings do not need to be exactly on compass points and, if suitably spaced, as few as 5 are sufficient to prepare a full chart showing deviations at 10° intervals. Furthermore, C-Swing also computes a set of magnetic coefficients and gives guidance on placing corrector magnets and making adjustments.

C-Swing's Capabilities:

  • Tutorial help. 13000 words with over 35 colour illustrations. Gives full instructions and help with using C-Swing. As an added bonus, the help file includes 6 chapters explaining basic compass theory, magnetic variation, deviation, isogonic and isallogonic charts, magnetic coefficients, corrector magnets, compass maintenance and installation.
  • Deviation charts and tables are prepared automatically and can be printed out in a form suitable for use at sea.
  • Uses the 'least squares' and Gauss-Jordan methods to statistically smooth input data and compute magnetic coefficients from a minimum number of swing observations.
  • Handles reference bearings from shoreside objects measured by direct observation across the compass, azimuth ring, pelorus or shadow pin.
  • Sun reference bearings are in real time for the moment the observation is entered into the computer.
  • Almanac printer. Prepares a table of the sun's position and rate of motion at ten minute intervals. This is useful if you are planning a swing and need to know if the if the sun is likely to be obscured by hills or shoreside objects. Also, if you are not able to take the computer to sea, you can prepare a table ashore then use it to look up the position of the sun during the swing.

Deviation chart produced with C-Swing

Computer requirements:

  Windows 95, 98, NT, 2000, Me, XP, Vista or 7.
  4 Mbytes RAM + 2 Mbytes hard drive space.

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