Understanding and Using Remotely Sensed Image Scales

Aug 5, 2011
James Gibson

Historical Vertical Aerial Photograph of Boston Naval Shipyard - Naval Historical Centre

Scale can be used on remotely sensed aerial photographs and salellite images. The techniques are explained.

The Cataloguers Reference Shelf defines a remote-sensing image as “an image produced by a recording device that is not in physical or intimate contact with the object under study. This may be a map or other image that is obtained through various remote sensing devices such as cameras, computers, lasers, radio frequency receivers, radar systems, sonar, seismographs, gravimeters, magnetometers, and scintillation counters. When the image has cartographic or bibliographic information added, it is referred to as a remote-sensing map.”

Aerial Photographs:

The scale of an aerial photograph varies with the height of the camera above the ground and the camera focal length. The formula for calculating the scale is: focal length of camera lens / flying height of aircraft. If either change, then the scale changes.

To use an example:

An aerial photograph taken at a height of 4 500 m with a camera focal length of 5 cm gives a scale of 1: 90 000, or 1 cm to 0.9 km.

The calculation:

(0.05 m / 4 500 m) = 1 / x
0.05x = 4 500
x = 4 500 / 0.05
= 90 000

If changes occur in the focal length or height, the scale changes, for example:

If the height is increased to 6 000 meters the scale becomes 1: 120 000 or 1 cm to 1.2 km
If the focal length is increased to 20 centimeters the scale becomes 1: 22 500 or 1 cm to 0.23 km

Test your understanding using the sample questions and answers (see images below)

Satellite Images:

As Eric D. Conway explains in the NASA Introduction to Satellite Image Interpretation, “it is (often) useful to make linear measurements on a weather satellite image (see sample image below). In order to do so, a scale must be developed for the image. Once a scale has been developed, approximate distances can be measured. He outlines the procedure- the size of an object on the image and on a map- as follows:

Locate an easily recognizable feature on a map and measure the length of the same object on the satellite image. e.g. a lake measures 2 cm
Use the map scale (e.g. 1: 250 000) to calculate the length.
Use a proportion to convert scale to a ratio

The calculation:

The map scale is 1: 250 000
The length of a lake shore on a satellite image is 2 cm
The length of the lake shore on the map is 3 cm; 3 X 2.5 = 7.5 km
The Image proportion statement is 2 cm to 7.5 km
The scale is 1 cm to 3.75 km or 1: 375 000

Conway gives the following precautionary note: “A satellite image is most accurate directly below the satellite sub-point (the point directly below the satellite).

This is usually the center of an image in polar orbiting satellite imagery. In geostationary satellite images, the sub-point is usually on the equator at the longitude over which the satellite is centered.

In a satellite image the further away from the sub-point, generally the greater the distortion. Therefore, this method of determining a scale is not going to be entirely accurate, it is instead a method for making approximate measurements from imagery.

For measurements made along the perimeter of an image, an entirely new scale can be created for improved accuracy.”