Using Parallax to Measure Height

on a Stereo Photo Pair

heights can be measured using this method on a stereo pair if:

1. you know the flying height of the photo

2. you can find the principle point on both images

3. there is enough overlap in the images so that the feature you want to measure appears on both photos

4. the terrain is not excessively rough.

NOTE: Several instructions below talk about drawing on the photo. This is okay only on the paper photo copies. On an actual air photo please keep pens, markers and pencils off the photo surface as much as possible. Use clear plastic or tracing paper overlays if you need to make marks.

1. Find the principal point on each photo. (See single photo method).

2. Mark the principal point for each photo on the other. There is a very accurate technique using a stereoscope, but for now use the features on the photo. For example if the principle point on photo one is in the middle of a road, find the same spot in the road on photo two and mark it with a small dot.

3. Measure how far the plane moved on the photo between the two images, b. This is the distance between the two principle points, measured in inches or centimeters on the photo. We don’t need to know what this distance is on the ground. For the best accuracy, measure this distance on each photo and take the average. You only need to make this measurement once for a pair of photos. It will be the same for every height you measure on the pair. This is also called the photo base length.

4. Note the flight line on each photo. This is the line connecting the two principal points that you just measured.

5. Measure the displacement of the top of the feature parallel to the flight line, relative to the bottom (or other reference point) on each photo, p1 and p2. Draw a line perpendicular to the flight line that goes through the top and the bottom. The distance between these is the displacement for that photo.

6. Determine how much this displacement changed between the photos, Δp. If the top of the feature is on the same side of the principal point in each photo (leaning the same direction) you will subtract the smaller from the bigger. If they are on opposite sides of the principal point you will add them.

7. Use the formula to find the height h = H x Δp/(b+Δp). As with the single photo, b and Δp need to be in the same units, but because we are forming a ratio it doesn’t matter if you use inches, cm or mm. The answer, h, will be in the same units as the flying height H, usually feet or meters.

8. Repeat steps 5 – 7 for each object you are measuring on the photo pair.

Example: A pair of “pictures” of a tall pole in a field with low shrubs taken from an ultralight plane flying at 100 ft.

b1 = .90 cm b2 = 1.00 cm so b = .95 cm

p1 = .18 cm p2 = 1.57 cm Δp = p2 – p1 = 1.39 cm

h = 100 ft x 1.39 cm /(1.39 cm + .95 cm) = 100 ft x .59 = 59 ft