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Try out this Deviation Linear Interpolation Calculator. It's more than a calculator, however. It's designed to show you how to do the calculations yourself. After all, you won't have this calculator available for the test!

It's pretty neat. Plug in the values for your particular problem, press the "Calc" button and the new page will give you the answer and show you how it came up with the answer, for the values you entered.

Linear interpolation is often used to convert a heading from Magnetic to Compass or vice versa. If you're using a table of your boat's deviations (see sample table below) and the heading you need isn't listed on the table, a linear interpolation from the two closest points listed is probably the best approximation of the deviation you need to complete your work.

You can enter the deviations with or without an East/West designation. (Not necessary if entries are both East or both West, but if they're different, you'd better be specific.)

Note: Remember, the calculator doesn't care whether you're interpolating a deviation for a Magnetic heading or a Compass heading, but it'll sure matter for your work so remember to pull your headings from the correct column.


Magnetic or
Compass
Heading
Deviation
(e.g. 3E or 5W)
Table entry 1
Unlisted
Heading for which
Deviation is needed

Leave this field blank
Table entry 2












What about Magnetic 050?
(Interpolate using 045 and 060)

Partial Sample Ship's Deviation Table
Magnetic
(Heading)
DeviationCompass
(Steer by)
0005 E355
0150015
0304 W034
0458 W053
06011 W071
07514 W089




What about Compass 020?
(Interpolate using 015 and 034)




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