Using reticle binoculars to estimate range

An essential skill for all Marine Mammal Observers is the ability to estimate range to a sighting. Reticle binoculars and range finding sticks are the most common methods used, but it is also possible to use a sextant.

Demonstrated here, is everything you need to know about estimating range using reticle binoculars. For information on range sticks, please visit our website.

What you need to know:

Before you start, you will need to do some measuring.

Firstly, measure your eye height whilst wearing work boots, and then the height of your observation platform. If the information is not readily available, use a plumb line, which is essentially a weight on the end of a piece of string. Drop the weight over the side, until it reaches sea level, mark the string, and then measure the distance from the weight to your mark. Just remember, a small change in height translates to a large change in horizontal range estimation, so it is essential to measure heights accurately

In addition to these measurements, you will need to know the number of mils in each reticle of your binoculars; refer to your reticle binocular specifications.

Method for estimating range with reticle binoculars:

Firstly, add together your eye height and the height of the observation platform to find your height above sea level.

When a sighting occurs, place the top division in the reticle scale on the horizon and count down to the animal. Using the number of reticules, work out the number of mils, for example if an animal is sighted at three reticles and each reticule is 5 mil, then that’s 15 mils.

An example of how to estimate range using reticle binoculars. © OSC 2013.
An example of how to estimate distance using reticule binoculars. © OSC 2013.

Finally input all figures in to the following equation to estimate range:

Range = (eye height + height above sea level) x 1000 / no of mils

Putting it into practice:

It is unrealistic to assume that conversion for a sighting can be calculated instantly, and as an MMO, this is often essential as it will determine whether a marine mammal is in or out of a mitigation zone, and thus if operations can start.

To speed up the process, it is advisable to put the equation into an Excel™ spreadsheet and produce a table (see below). Keep it handy, and sightings can be translated into distance instantly.

An example of a table produced to determine distance estimation. Eye height above sea level = 15 m, so, to estimate the distance for one reticule: 15x100/5 = 3000 m. Source: OSC, 2013.
An example of a table produced to determine range with reticle binoculars. Eye height above sea level = 15 m, so, to estimate the range for one reticle: 15×100/5 = 3000 m. Source: OSC, 2013.

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