Rangefinder CalculatorRangefinder CalculatorFor arm length in cm =70Distance to Object (Metres)'Distance'Mark from Top of Rangefinder (Centimetres) 'Calliper interval'40000.2730000.3925000.4820000.6215000.8510001.327001.925002.723004.582006.9210013.92Height above sea level in metres =20Distance to horizon in metres =17164.06

Distances have been marked onto this stick using the formulae2000m500mCalliper interval (measured from top of the stick)HorizonObservers handInsert Arms length. Hold range finding stick in your hand and get somebody else to measure from the stick to your shoulder in line with your eyeInsert height above sea level: this includes height of viewing platform and height of observer (to eye level only)

Alternative distance generatorCalculation for making a simple rangefinder to measure a distanceModify these figuresDistance to object = d450metersEye Height above Water : h20metersArm's Length : b70cmCalliper interval: Ci=bh(v-di)=3.0cm(h2 + vdi)Above formula was modified from : Heinemann, D. (1981) A range finder for pelagic bird censusing. J. Wildl. Manage. 45(2) 489 493. Original formula shown below.Calliper interval Ci =bh(v -di)orDistance =h(bv-hc)h2 + vdi)bh +vcb= distance of observers eye to top of calliperh= observers eyes above waterv= visual horizon 3838visual horizon= 3838 (h 1/2)d= distance to object

For MMOs it is good practice to be able to be able to judge the extent of the mitigation zone so that you are able to assess if any marine mammals observations are detected within this area. You may also be required to verify your methods to the party chief/ client representative.

In this example Distance is 500m (which is the typical mitigation zone for seismic work), it is good practice to include several distances on the range finding stick such as 1000m and 3000m 5000m as this will improve range determination of distant sightings of marine mammals.This is calculated by adding the height of viewing platform and the height of the observer measured from the feet to the eye (this is not the total height of the person).

For example: Viewing platform = 25.2m MMO was measured as having an eye height of (180cm) =1.8m Eye height above water H= 25.2 + 1.8H= 27mThis is measured to calculate the distance you hold the stick away from your eye. It is best measured with assistance from somebody else.

Hold the range finding stick up in your prefered hand, grasp the stick with your thumb and the tip of the index finger. Measure from the stick to the end of shoulder (in line with the eye).Calliper interval is the distance from the top of the stick

Guide to using the rangefinder

For calculating distances at sea a common method is the use of a rangefinder. Essentially distances can be calculated using trigonometry principles if we know the height of the observation platform and have a measure of how far away in relation to the horizon objects appear. Although commercial hand held range finder can be bought, they can also be made quite easily from a bit of plywood (or similar). Each rangefinder has to made specifically for the observation platform and observer, as the height of the observation platform (which is platform + observer) needs to be calculated for the range finder to be provide the most accurate estimate of range. Rangefinders are a useful tool to validate Marine Mammal Observers (MMOs) judgement that animals have been detected within the mitigation zone. Being able to provide evidence to validate any decisions made to other parties may be required, this may be especially important if a delay to the start of the activity is advised (e.g. the clients representative may want to understand how you decided that a marine mammal was within the mitigation zone). Materials required: Piece of wood or similar (measuring ~15cm), measuring tape, permanent markers/pencils/ The rangefinder calculator is embedded in the Excel worksheet.1.Ask someone to measure the horizontal distance from your eye to the base of the stick, this should be held vertically in your outstretched arm, and the measurement should be in centimetres. Your shoulders should be held square for this measurement. Enter the measurement into the calculator. 2.Determine the height of your viewing platform and add this value to your own height (measured to eye level only). The units used for this measurement should be metres. Enter these figures. Press return and get your own rangefinder figures in the table for construction of the stick. 3.Now mark your stick from the top as the Excel table shows. Due to the calliper intervals for some of the distances, not all need to be marked on the range finder. It is best to mark key distances such as 500m (which is usually the mitigation zone) plus a distance closer than this e.g. 300m and a few reference distance perhaps 1000m and 1500m.4.Add your name and arm length to the handle part of the stick.Operation of the range finder: In order to operate the range finder stick, line the top of the rangefinder with the horizon and read the distance to the object from the scale. Note that this only works when you have an unobstructed view to the horizon. Test the accuracy of your estimates and practice using the finder stick, by comparing your distances estimates in relation to that of any known objects (e.g. navigation buoys etc) whose range can be derived from the ships navigation instruments. If there are any significant discrepancies, keep practising until you refine your technique, or revisit the calculations used and make a new range finder.