Eighteen months ago I wrote about completing some fundamental field measurements when you find yourself in the field and you don’t have the gear to do so1. In that blog I described various ways to determine (i) direction without a compass, (ii) length and distance without a tape, (iii) height without a clinometer, and (iv) level without a level. Here I would like to introduce a further three ways to measure heights in the field.
Word of note: All of the following techniques work only on level ground, as being upslope or downslope of the feature of interest complicates the geometry. Even on a slope though you can do this by conducting the measures while standing on the slope at an equal level to the base of the object; this cancels the slope and ensure you are reading from “level” ground.
Protractor on a stick
When caught out without a clinometer, and feeling the clino in your compass is insufficient (they are low accuracy due to the short sight baseline along the compass plate edge), we can extend the sight baseline by replacing the compass with a long, straight object. Think of a metre stick for this. If we screw a plastic protractor to the metre stick, with the protractor aligned along the edge of the stick, and hang something for a plumb bob from the centre of the protractor, we have created an inclinometer. We can sight along the metre length of straight wood, which improves the accuracy of our angle measurement, then with our free hand pinch the plumb line against the protractor to hold in place as we remove the stick from our face to read the angle. Note, though, the angle is read from the 90⁰ mark in this case. When the stick is held absolutely level, the 90⁰ is straight down to the ground. On raising the stick that remains our reference, rather than the horizontal. Thanks to the beauty of geometry, our measure done this way will give us the same value as if we measured from the usual horizontal reference.
A modification of this is to align your compass edge along the edge of the stick and use the clinometer in your compass to measure the angle. In effect, you are extending the sighting baseline of your compass from twenty centimetres to one hundred; this will improve your accuracy of angle measurement.
To calculate height then, all we need is our angle, and our distance to the tree containing the nest. We use:
Height = (angle / 100 ) * run
For example, if I measure an angle to the nest of 36⁰ and distance to the tree is 16.5 m, I can calculate the height as
= (36% / 100) * 16.5 m
= 5.9 m
Piece of paper (My thanks to NRTG forestry instructor Morgan Brown for showing me this)
Okay, so you don’t have a clinometer or your home-made inclinometer. Do you have a piece of paper? Fold the paper to create a diagonal. If done as shown in the figure, that folded angle is 45⁰. Now, a 45⁰, by definition, has the run (distance from you to the tree) equal to the rise (height of nest above ground). So, if we can walk to a point where we can line the nest up perfectly along the angle of the paper, we know we are standing at 45⁰ to the nest. Here is the cool part. By measuring the distance from where we stand to the tree, that is the same height as the nest above the ground. So, if I am 5.9 m from the tree, the nest is 5.9 m high. Simple, elegant, accurate.
To improve our accuracy, we could hang a plumb bob along the vertical edge of the page to ensure it is vertical. We could even hold the folded paper against our metre stick to extend the sightline as we showed above.
Marks on trees
Assume that you are wanting to measure that height of nest and you have no tools whatsoever. Perhaps you are a naturist wandering around stark naked and the nest has taken your fancy. It can still be done.
Make a mark (e.g., snap a branch, gouge the bark) on a nearby tree right at your eye level; this will establish the horizontal reference. Now step back a few paces. Line up the sightline of the nest on the tree trunk and note that location. Measure or estimate your distance to the tree you are using to mark (this is the run for the calculation of angle). Then measure or estimate the distance between your eyelevel mark on the tree and where your sightline crossed the tree (this is the rise for calculation of angle). From these simple measures you can calculate angle as rise / run and from that, and estimated or paced measure to the nest tree, determine nest height just as done previously.
Continuing the examples from previous, if I measure:
- Distance from where I am standing to tree I am using for marking = 112 cm
- Height of mark where sightline crosses tree above my eye level on marking tree = 40 cm
- Distance to nest tree from where I am making observations = 16.5 m
Angle (in percent) is (rise / run) * 100
= (40 cm / 112 cm) * 100 = 36%
Height = (angle / 100 ) * run
= (36% / 100) * 16.5 m
= 5.9 m
For estimating when lacking measuring tools it is useful to know certain lengths of features always with you. Such as length and width of hand; length of arm; height of hip; and, of course, pace length. With these body parts as measures we can then easily calculate lengths and distances.
There is a reason that I chose to revisit this topic a year and a half after originally bringing it to you, my attentive readers. There is a powerful message here about many ways of accomplishing the same goal. I have shown you four ways to determine height of any feature above what you can reach: the cross staff (from the July 2021 blog), a 99 cent protractor, a piece of a paper, or simply placing marks on a tree. Combined with regular tools for this – the clinometer, Abney level, compass – this provides seven different ways of accomplishing the same task. We need to always keep in mind and appreciate that there are multiple ways of doing any activity; this is how we avoid being at a loss when our preferred tool fails us (and tools always fail at some point). As long as we understand the principles we can modify and improvise. This frees us from stagnation of relying on only one way of doing things, for that is the highway to close-mindedness.
Sean Mitchell
1 This was a blog from July 2021 titled “So You Broke or Lost Your Instrument…”
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