Locating distant, isolated pest plants in the bush with great accuracy

A guest post by Chris Cosslett, Blackened Billy Rewilding Services

August 2021

Described below is a very reliable method for finding your way to isolated pest plants that can be seen from a distance but must be approached blind through the bush. Using it, we have found our way straight to individual old man’s beard plants up to 700 metres away from where we could see them.

You will need a:

  • Laser rangefinder – a hunting one with a range of about 1 km

  • Compass – a proper orienteering one with a good long baseplate. A sighting compass with a flip-up mirror would be best.

  • Handheld GPS device – set to show magnetic bearing rather than true bearing

  • Notebook and pen/pencil.

compass showing direction of travel

To begin you need to calculate the length and orientation of the straight line between the plant and your viewing point. To establish this line:

  1. Select a viewpoint with a clear, unobstructed view of the pest plant. Set a waypoint with your GPS device. Write down the waypoint number in your notebook. Stay on this spot while you make the following measurements.

  2. Use the rangefinder to measure the distance from you to the plant in metres. Write the distance down.

  3. Use the compass (see diagram above) to calculate the magnetic bearing from you to the plant. To do this, point the Direction of Travel Arrow on the compass baseplate at the plant. Holding the baseplate in this position, rotate the round Compass Housing until the red Orienting Arrow on the base of the Compass Housing aligns with the red end of the Magnetic Needle. It is very important that this is done accurately. Check it several times. When you are confident that the magnetic needle is perfectly aligned with the compass-housing Orienting Arrow WHILE the Direction of Travel arrow is pointing at the plant, read the bearing on the Compass Housing Degree Dial at the Index Line directly below the Direction of Travel Arrow. This number is the magnetic compass bearing from your waypoint to the plant. Write it in your notebook. (Note: keep the compass away from metals and magnets while you are using it. They can pull on the magnetic needle and distort the reading.)

  4. Next, calculate the magnetic compass bearing from the plant back to your waypoint by adding or subtracting 180 degrees (use the degree dial as a guide if you need it), to or from the compass bearing you calculated in step (3). This new number is the direction back along your straight line from the plant to your viewing point. For example, if your bearing direction at step (3) was 127 degrees, adding 180 degrees will give you 307 degrees, the opposite direction. Write this new bearing in your notebook.

Now you are ready to head into the bush and find your plant. You have: a starting point (step 1), the distance you have to cover (step 2) and a compass bearing to ensure you are on the right track (step 4).

To find the plant now that you are moving towards it from your waypoint:

  1. Tell your GPS device to find the waypoint from which you made your survey (1). Your GPS device will tell you the magnetic bearing from your new position back to your original waypoint, along with the distance in metres.

  2. Move away from the waypoint until the distance and compass bearing on your GPS device match the ones you recorded in your notebook at steps (2) and (4).

  3. If the going is easy then all you need to do is stay on the compass bearing you calculated at step (4) and keep walking away from the waypoint until you reach the distance you measured at step (2). (Note that you will probably find the plant a little closer – see step (9) below.)

  4. If the going is difficult, follow your nose towards the plant until you are in the general vicinity, and then fine-tune your location with the GPS device until you reach the right distance and the right bearing. One way to do this is to walk away from the waypoint until you reach the correct distance, and then move left or right, keeping the distance the same, until you reach the correct bearing. Sometimes it will be easier to walk beyond the plant and navigate back onto it, e.g. if there is a track on a ridge above it. In this case, walk along the track until your GPS device gives the correct magnetic compass bearing (calculated at step 4), and then follow the bearing until you reach the correct distance.

  5. Once your GPS device displays the distance and direction recorded in your notebook (steps 2 and 4, above) you will be close to the plant. But there is a complication: slope. If the plant is higher or lower than your original waypoint (step 1) then the distance you measured with your rangefinder will be greater than the distance between the waypoint and the plant according to your GPS device. In the diagram below, your waypoint position (step 1) is point A and the plant is point B. The line of sight you measured with your rangefinder (step 2) is distance c but because your GPS device works in plan view, the distance it measures from the plant to the waypoint is the horizontal distance b, which is shorter than c. The steeper the slope, the greater the discrepancy between distance c and distance b. For this reason, according to the distance shown by your GPS device, the plant will appear to be closer to the waypoint than the distance you calculated at step 2. Unless the slope is very steep the discrepancy won’t be great. Just move back towards the waypoint (1) along the bearing line (4) until you find the plant. Bear in mind that the steeper the slope, the further the plant will be back towards the waypoint. On very steep slopes the discrepancy can be significant – see below. (Note that the effect of slope, and the means to correct for it, are the same whether the plant is higher or lower than your waypoint position.)

One way to mitigate the effect of slope is to reduce the angle by making your initial survey from several hundred metres away. Or you might be able to climb or descend a hill to do your survey from a point on about the same level as the plant

Advanced techniques for correcting for the effect of slope

If you are working on a slope and want to calculate the exact distance that your GPS device will display when you are at the plant, you can use trigonometry to calculate the horizontal distance b (distance according to the GPS device) from distance c (line of sight) and angle A (slope). I have made it easy for you here. There are two steps to correcting for the effect of slope: calculate slope, and then correct the distance.

Calculate slope

Slope method 1: Clinometer

To calculate slope exactly you can use a clinometer. Manual and electronic models are available.

Slope method 2: Extended arm

You can use your extended arm to gauge approximate slope as follows. Your arm straight up above your head is 90 degrees. Straight out flat ahead is zero. Half-way between these two is 45 degrees. Divide the distance between 45 and zero into thirds and you have 15 degrees and 30 degrees.

Slope method 3: Fist

Another approach to estimating slope is the fist method.

Stretch your arm out straight and make a fist, and angle it so that you can see the back of your hand. The distance across four knuckles (one fist) translates to 10 degrees of angle. Turn your hand on its side and calculate the angle from the horizontal to the target plant as multiples of fists, e.g. 2½ fists gives an angle of 25 degrees.

The fist method works for all sizes and ages of people because people with big hands have long arms, and conversely.

Correcting the distance using a slope multiplier

Once you have calculated the slope in degrees between your waypoint (step 1) and the plant, refer to the table below for the relevant multiplier. Multiplying the distance you measured with your rangefinder (step 2) by the relevant multiplier will give you the horizontal distance as shown on your GPS device.

For example, if the target plant is 200m away according to the rangefinder and the target plant is at 15 degrees above the horizontal, the correct multiplier is 0.966, so the horizontal distance from the plant to the waypoint (the distance according to your GPS device) will be 200m x 0.966 = 193m.

As you can see from the table, the effect of slope under about 25 degrees and at moderate distances is pretty minor. If you are working on a steep slope, however, the effect can be significant, particularly in view of the difficulty of moving around on very steep slopes. For example, if the target plant is 100m away from your waypoint according to the rangefinder, and 45 degrees above the horizontal, then the actual distance according to your GPS device is going to be 100m x 0.707 = 71m. Scrambling for an unnecessary 30m over rough ground on a 45 degree slope means a lot of wasted effort. Therefore using multipliers to calculate horizontal distances before you set out is well worth it when working on steep slopes.