Aerial Photography and Remote Sensing – Study Outline

You should be able to:

1. list three different presentations of a scale
2. find the scale of an image three different ways
3. describe three reasons why the scale of an image might not be the same for the entire image

A. Scale

1. The scale tells you the relationship between the size of an image and the size of the real feature,.It answers the question “what is the distance on the ground between point A and point B in the photograph?”

2. The scale is often expressed in three ways

a. the verbal or equivalent scale gives the relationship between useful sized units on the image and the real world. 1” = 2 mi. or 3 cm = 25 ft

b. a graphic scale is a line or bar on the image that shows the scale, especially handy when making measurements with a straight edge or when the image must be enlarged or reduced

c. ratio or relative fraction (RF) tells the relationship if you use the same units on the photo and the real world.

i. usually expressed as 1:PSR or 1/PSR

ii. where PSR photo scale reciprocal

B. Resolution

1. tells you the size of the smallest object detectable in an image

2. resolution is determined by

a. the nature of the image system

i. pixel size in a digital photo (max resolution is 2 X pixel size)

ii. grain size on a piece of film

iii. (and the quality of the lens)

b. the clarity of the air between camera and subject

c. the scale of the photo

3. calculation

a. the maximum ground resolution can be calculated by using the scale to calculate what the smallest unit visible on the photo represents on the ground e.g. two .01mm pixel on the photo would represent what size square or dot on the ground?

C. The scale of an image can be determined several ways

1. if the type of camera and the height of the camera above the ground are know, the scale is f/H=RF

a. f is the focal length of the camera (a property of the lens)

b. H is the height of the camera above the ground

c. f and H must be in the same units in order for the answer to be the relative fraction

2. compare with map of known scale

a. features on the image are located on a map and the distance between then is measured on both

b. distance on the ground = length on map * reciprocal of map scale (as a relative fraction)

c. image scale = distance on image/distance on ground

3. measure features with know actual size on the photo

a. scale = measure of feature on image/actual size

b. using this method with surveyed control points will get the most accurate scale, and is necessary for precision mapping measurements.

4. often the scale is given by the company that produced the photo

D. Scale problems

1. The given scale may only be an approximate or average value for a group of photos.

2. any change in elevation will change the scale. if H changes…

a. …the scale (RF) gets bigger as the surface elevation increases (H gets smaller) and smaller as surface elevation decreases

b. in hilly terrain with a large scale photo the change can be significant!

3. distortion by the lens may affect the scale near the edge of a photograph

4. If the camera is tilted the scale will decrease from one edge of the image to the other