Estimating distance with the thumb jump
How far is the hut over on the slope? With the thumb jump you estimate distances using only your arm: thumb up, close one eye and then the other, and the thumb seems to jump sideways. How far it jumps reveals the distance – times ten. Here you learn the method step by step, which known sizes to work with and how accurate the result is.
- 1–2 minutes
Explained simply
Hold your thumb on your outstretched arm in front of a house in the distance. Close one eye and then the other – the thumb seems to jump back and forth.
Estimate how far it jumps at the house, for example three house widths. That is about one tenth of the distance.
So you multiply by ten: three house widths are about 30 metres, the house is about 300 metres away.
How it works
Your two eyes see the world from two slightly different points. Near things jump a lot when you switch eyes, distant things hardly at all.
Because your arm is about ten times as long as the distance between your eyes, the jump in the distance is about one tenth of the distance. In geometry this is called similar triangles.
What matters
Arm fully straight and kept still while switching eyes.
Use known sizes at the target: car, house, power poles.
Cross-check with the map or by eye.
Safety: what to watch out for
The straight line is not the route – terrain can turn a short distance into a long walk.
Never look at the sun over your thumb.
Plan with enough time before dark.
Step by step
Stretch out your arm
Stretch one arm straight out in front of you and hold your thumb pointing straight up.
- 1
Stretch out your arm
Stretch one arm straight out in front of you and hold your thumb pointing straight up.
- 2
Put your thumb on the target
Close your left eye and move your thumb so it sits exactly in front of the target with your right eye, for example the corner of a house.
- 3
Switch eyes
Open your left eye and close your right, without moving your arm. The thumb seems to jump sideways.
- 4
Estimate the jump
Estimate how wide the jump is at the target. Use known sizes: a car is about 4 to 5 metres long, a family house about 10 metres wide, the gap between power poles about 100 metres.
- 5
Multiply by ten
The distance is roughly the jump width times ten. If the thumb jumps about three house widths, so 30 metres, the target is around 300 metres away.
- 6
Cross-check
Also estimate by eye or with the map. If the values are close, the estimate is usable. Expect 20 to 30 percent error.
Common mistakes
- Arm bent instead of straight
- Moving the arm when switching eyes
- Estimating with unknown sizes
- Confusing straight-line distance with walking distance
Quick view
Suitable for
- Distances from about 100 metres to a few kilometres
- Estimating time to a target
- Practice on hikes
Not suitable for
- Precise measurements
- Targets without a known size beside them
- Fog and darkness
Prerequisites
- A target with known sizes next to it
- A clear view
Safety
- The straight-line distance says nothing about the route: gorges, streams and steep slopes quickly turn one kilometre into hours. Plan with a map and daylight.
- Never look at the sun over your thumb.
How it works
Your eyes are about 6 to 7 centimetres apart, and your outstretched arm is about ten times as long. If you look over your thumb with your left and right eye in turn, you see it from two slightly different angles. In the distance it therefore jumps by a span that is about one tenth of the distance. Estimate the width of the jump at the target and multiply by ten.
Advantages
- No equipment
- Fast
- Good for practice on any hike
Disadvantages
- Only an estimate
- Needs known sizes at the target
Deep knowledge
Background
The thumb jump has long been taught in scouting and military training. It is based on the same principle astronomers use to measure the distance to nearby stars: looking from two different points.
Variants
- Find your own factor
- At home measure the length from eye to thumb and the distance between your eyes. Divide one by the other to get your personal factor – usually between 9 and 11.
- Measure a known width
- If you know the size of the target itself, such as a house, compare how much of it your thumbnail covers. The less it covers, the further away it is.
Common mistakes
- Arm bent instead of straight
- Moving the arm when switching eyes
- Estimating with unknown sizes
- Confusing straight-line distance with walking distance
Limits and risks
- Accuracy about 20 to 30 percent
- No information on walking time
Sources (2)
- Headquarters, Department of the Army (USA) (2001): FM 3-25.26 Map Reading and Land Navigation · Estimating distance in the field
- Harold Gatty – Dover Publications (1999): Finding Your Way Without Map or Compass (first published as 'Nature Is Your Guide', 1958) · Estimating distances without aids