Error by amplitude is only one of a few methods of determining compass error on the high seas, but though it is simple, it
can only be done when the sun is on the celestial horizon, meaning only at sunrise or sunset.
Step 1: Observe the sun's bearing on the celestial horizon, noting time of observation. This is best accomplished with
an azimuth circle, or telescopic alidade for smaller bodies.
Step 2: Find time in GMT for use in NA, either converting from CT or ZT.
Step 3: Find Declination, make sure to note whether South or North and the trend in values, i.e. if the values are
increasing, the d correction is to be added; if the values are decreasing, the d correction is to be subtracted.
Step 4:Convert Latitude and Declination from minutes and seconds into minutes and tenths of minutes. The easiest way
to accomplish this is to use your calculator and divide the minutes by 60 and add the whole degrees. (ex. For 2806 N, 6/60=0.1,
28+0.1= 28.1 N.)
Step 5: Find Amplitude (A) by using the equation sin A= (sin Dec/cos Lat). By converting to minutes and tenths above,
we made it easier to enter these values into your calculator.
Step 6: Finding Zn from A.
Your calculator will simply spit out a number when you press enter, but as you notice at right, that number is now preceded
by a W and succeeded by an N. Where did those come from? The W designates that the body observed is in the western sky; it
is setting. You know this by one of two methods, either by noting the time at which the solar observation was made, though
be careful using this method as not all bodies rise in the morning and set at night, such as the moon. The second method is
simply to note the bearing of the observation. Bearings between 0 and 180 are designated as Easterly. Bearings between 180
and 360 are designated as Westerly.
The N is simply the designation of Declination. i.e. If the Dec is N, the A is designated N.
What do these values indicate? They indicate which direction the Amplitude is to be applied to find Zn. An A value of W 16
N can be read as 16° North of West, and is equivalent to a Zn of 286°.
Step 7: Solve for Compass Error
Using either TVMDC or GET, solve for compass error or deviation, and you're done. TVMDC is for correction magnetic compasses,
GET is for gyros. GET stands for Gyro + East error = True, if you know two parts, you can find the third.
