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Compass Deviation Analysis Calculator

Instructions for Running the Calculator:

By default, the calculator display is already loaded with a sample set of deviation observations in the white columns.

Click the Calculate button and observe. The ship figure will begin to rotate, and the Apparent Coefficients A through E will appear in the "Deviation Coeficients" table on the right.

pause button Click the Pause button and the ship figure will stop rotating.

slicer Move the slider left or right to manually rotate the ship to any desired heading. Click the pause button again to return to auto-rotation mode.

The "blue" ship figure indicates the ship's heading in degrees magnetic.
The "gold" ship figure indicates the ship's heading, according to the ship's magnetic compass
The deviation (in "red") shows the compass deviation (the difference between the ship's magnetic heading and the ship's compass heading) at the point of rotation indicated.

The maximum deviation for the entire data set is also displayed. In this example, the worst compass error would be 20.1 degrees easterly deviation, occurring on a compass heading of 65 degrees.

Entering Your Own Data into the Calculator:

Click Clear, and the tables for both the Deviation and the Deviation Coefficients will re-set to zeros.

Enter your own deviation data into the two white columns.
Make sure you indicate if the deviation is E'ly or W'ly, using the pull-down choice.
Press Calculate and see the results.

Note: The calculator works well in most cases for actual data. You can't simply "guess" and put random numbers in the data set, because actual compass data has a very characteristic mathematical signature. The calculator works by assuming the compass deviation on any heading will take the form:


In which A, B, C, D & E are the Approximate Coefficients of deviation, and theta is the compass heading. This tends to work very well most of the time, for deviations that do not exceed 30 degrees.