Unit 5 Flashcards
What is photogrammetry?
Science of making measurements from photographs
How is photo scale determined?
PS = photo distance / ground distance
Relationship between the distance on an image/map and the actual distance on the ground?
Photo scale
scale = f/h
focal length / flying altitude of aircraft will give you photo scale
what happens as you move away from the principle point?
more height displacement will occur
what must be true for there to be no height displacement at the principle point?
Must be a true vertical image; no tilt or angle so that the optical axis is exactly 90 degrees from the ground surface (NADIR)
How can you find the principle point?
Using fiducial marks
how can shadow length be used for height estimates?
Using a photo scale and the angular elevation of the sun (a) + trig
height = length*tan(a)
What is the issue with using relative shadow length?
It is within a ball park of the actual height, the shape of the object, time of day, or terrain can alter the shadow drastically
What is the best approach to measure heights from images?
Stereoscopic parllax
what is the disadvantage of using the relief displacement method?
can only be done if there is enough height displacement
What 3 things must be true to use stereoscopic parallax?
- Must have a stereoscopic photo pair
- image tilt must be less than 3 degrees
- images ust be taken from the same height
define apparent displacement
change in the apparent location of an object from a change in the point of observation
Why is less than 3 a degree tilt important for stereoscopic parallax?
to ensure that the displacement in the photo is due to the height not tilt
the apparent displacement of an object when viewed from two vantage points?
stereoscopic parallax
