1. Estimating distance from a feature using your thumb

2. Hold out your arm with thumb raised
Close your left eye
Align the right side of your thumb with the left edge of an object
Now open your left eye and close your right eye
The thumb will have “moved” across the object to the right
If we know the size of a feature (eg from a map) we can make use of the fact that the ratio of the distance between the eyes to the distance from the eyes to the thumb is constant (1 to 10) to estimate distance from an object

3. Left eye
thumb
Right eye
View from above

4. Interpupillary distance
Left eye
thumb
Interpupillary distance
Eyes to thumb = 10 x interpupillary distance
Right eye

5. Range (distance from feature)
Feature of known size
(eg from map)
Range (distance from feature)

6. Proportion of feature between thumbs when switch eye
Range (distance from feature)

7. Range (distance from feature)
Proportion of feature between thumbs when switch eye = ratio of green line:purple line
Range (distance from feature)

8. Ratio of green line to the range is the same as interpupillary distance: eye to thumb distance.
This is 0.1

9. Ratio of green line to the range is the same as interpupillary distance: eye to thumb distance.
This is 0.1
The range therefore = 10 x length of purple line x ratio of green:purple line

10. Ratio of green line to the range is the same as interpupillary distance: eye to thumb distance.
This is 0.1
The range therefore = 10 x length of purple line x ratio of green:purple line
Range therefore = 10 x size of feature x proportion of feature when thumb “moves”

11. Ratio of green line to the range is the same as interpupillary distance: eye to thumb distance.
This is 0.1
The range therefore = 10 x length of purple line x ratio of green:purple line
Range therefore = 10 x size of feature x proportion of feature when thumb “moves”
e.g. if feature is 200m big and the proportion of feature when thumb moves is 0.6, the range is 10 x 200 x 0.6 which is 1200m