Training Module 4:

Determining Scale on a
Weather Satellite Image

Adapted from An Introduction to Satellite Image Interpretation, Eric D. Conway and the Maryland Space Grant Consortium, ©1997, Johns Hopkins University Press, Baltimore, 255 pp with Interactive CD-ROM.

For more information about this book and how to order copies go to the JHU Press On-line Catalog

Introduction

Often it is useful to make linear measurements on a weather satellite image. In order to do so, a scale must be developed for the image. Once a scale has been developed, approximate distances can be measured. This activity can be used with any satellite image that has recognizable, measurable geographical features.

Procedure

1. First, locate a feature on the map that is easy to recognize. Examples would be a lake that is clearly visible, a straight portion of a state boundary, a peninsula, etc. Measure the length of this object on the image with a ruler and record the distance.

size of object on image

2. Now find the same feature you measured on your image on a map printed in an atlas. Use the scale on the map to determine the actual length of the object.

actual size of object

3. Use a proportion to convert your scale to a 1 cm scale.

Note

A satellite image is most accurate directly below the satellite subpoint (the point directly below the satellite). This is usually the center of an image in polar orbiting satellite imagery. In geostationary satellite images, the subpoint is usually on the equator at the longitude over which the satellite is centered. In a satellite image, the further away from the subpoint, 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.