Aerial Photography and Remote Sensing – Study Outline

Measuring Distance and Area

You should be able to:

1. measure the distance between two points along a specified path on an image

2. measure the area of an irregularly shaped feature

A. Measuring distances

1. a curved path is usually divided into a number of straight segment and the sum of the lengths Is determined. The number of segments to use is a tradeoff between matching the curve exactly and introducing errors by making many small measurements or moving the measuring device.

2. especially for large scale images and short distances, defining the endpoints can be important, e.g. do we measure road distances to the center of the intersection? Do we measure to the center or wall of a building?

3. care in recording all units and conversion factors used will reduce errors

B. Measuring area

1. always remember that the scale tells the relationship for distances not area. Area is distance squared.

a. a scale of 1:1000 means a 1 in. line on photo equals a1000 in. line on the ground

b. a 1 in. square on the same image equals a 1000 in X 1000 in square on the ground or 1,000,000 sq. in.

c. similarly 12 in. = 1 ft. but 144 sq. in. = 1 sq. ft.

d. when calculating areas all the scales and conversion factors must be in area units

2. geometry -- some features are regular enough that we can use the formulae for area of regular shapes such as a rectangle or a circle to find the area of a feature on an image

3. transects – drawing regularly spaced lines on an image and adding up the total length of the line segments on the features of interest

a. relative area (% cover) can be calculated without the scale as length of lines on feature / total length of lines

b. total area on the photo is length of lines on feature X line spacing

c. an appropriate area scale must be used to convert to area on the ground

4. dot grid – a transparent grid of dots with know spacing can be laid over an image and the dots inside features of interest can be counted.

a. relative area (% cover) can be calculated without the scale as number of dots on feature / total number of dots

b. since each dot represents a fixed area on the photo (may be given as sq. in. / dot, or dots/sq. in. or dots/cm^2…) the area on the image under the dots can be calculated as number of dots on feature X area/dot

c. note that some dots grids will indicate sq. ft./dot or acres/dot. This only works for the scale indicated on the grid!