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the Sprayer Station. From these measurements, it calculates the resul-
tant magnetic heading angle, thereby providing a built-in electronic
compass.
Like all magnetic compasses, the Sprayer Station compass will be
affected by any ferrous or magnetic materials in the vicinity, such as
metal structures, motors, speakers, etc. It will also be affected by
nearby electric fields, such as the wiring for lights. These nearby
sources of magnetic interference will distort the magnetic field and
produce errors in the compass heading. These errors are known as
magnetic deviation.
Although the Sprayer Station compass is a 2-axis device, the earth's
magnetic field occurs in three dimensions. That is, part of the earth's
magnetic field is oriented in the vertical direction. The closer one's
location is to the north or south pole, the stronger this vertical compo-
nent becomes in comparison to the horizontal components. The effect
this has on the Sprayer Station is to introduce an error in the compass
reading if the Sprayer Station is tilted from the horizontal plane.
Therefore, it is important when installing the Sprayer Station to en-
sure the support pole is mounted vertically, and not tilted to one side.
Also, keep in mind that when your vehicle experiences pitch and roll,
the compass heading will be affected accordingly.
Because the compass heading is used in the calculations for true wind,
any errors in the compass heading will also produce errors in the re-
ported true wind speed and direction. This is adjusted for in the
Sprayer Station by using the GPS-sourced course over ground when
the vehicle is moving.
About Magnetic Variation and True Heading
The earth acts like a giant magnet, with a magnetic north pole and a
magnetic south pole. The axis of the magnetic poles is offset approxi-
mately 11.5° from the axis of the earth's rotation. Therefore, the
earth's magnetic north and south poles are in different locations than
the earth's geographic north and south poles. In addition, the earth's
magnetic field is non-uniform, and changes over time. Magnetic
variation, also known as magnetic declination, is the angle between
magnetic north and true (or geographic) north, at the observer's cur-
rent location.
A magnetic compass measures heading with respect to magnetic
