Absolute and Relative Wind
3 August, 2022 by
Absolute and Relative Wind
Gunwerks LLC, Aaron Davidson

This is a very easy subject. Easy to understand, but without the reference, easy to mistake. Let's start with a few definitions.

Let’s define Azimuth for our purposes as long range shooters: The horizontal angle or direction of a compass bearing. Basically, just the compass direction where North = 0 or 360 degrees, East is 90 degrees, South 180 and West 270, etc.

There are two places we use Azimuth when we are calculating long range ballistics. The first is the direction to the target. This is important if you have enabled the advanced calculation of Earth based effects (Coriolis and Eötvös). Usually, we’ll use our laser rangefinder to capture the target azimuth when we capture the incline angle to the target and the range. We call this set of three data points the shot vector.

If we are using Azimuth to define the wind conditions, we consider this a global reference frame. Essentially, North is always North. We always use the direction the wind is coming from. For example, if my target is East, my target azimuth is 90 degrees. If the wind is coming from my left directly, or North, my wind azimuth would be 0 degrees. If the wind direction is slightly behind and left, the wind azimuth would be more like 330 degrees.

If we want to simplify, perhaps for shorter shots, or improved communications, we may want to shift our reference frame from global, to a shooter-relative system. For a shooter relative system, we use a clock position nomenclature and define the intended target as 12 o’clock. From the previous example, if the wind is from the left, it would be a 9 o’clock wind. From behind a 6 o’clock, and quartering would be perhaps 7 or 8 o’clock.

With this reference system, a compass based measurement tool is not needed to measure wind vectors. However, in the absence of that measurement capability for the target azimuth determination, we will need to limit our maximum distance of shot due to our inability to calculate earth based effects.

If the calculation uses target azimuth, the shooter relative method is still valid. It’s easy to communicate, and the solver tools will automatically account for all of the needed information for all of the advanced calculations.

See also Base Wind vs Vector Wind