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This chapter describes how to set a scale and measure distances and areas on satellite images. Using ImageJ, a freely available image analysis program that runs on most operating systems, users set the spatial calibration of an image, then select and measure distances and areas on it. The measurement results are reported in real-world units. The technique is most useful and accurate for nadir view (straight down) images. In this chapter, users examine satellite images of the Aral Sea, which has shrunk dramatically since 1960 because the rivers that flow into it have been tapped for irrigation. Users access satellite images of the region, then set a scale and measure the width of the sea each year. On another set of images, they highlight areas that represent water and measure them to see how these areas of the sea changed. This chapter is part of the Earth Exploration Toolbook, which provides teachers and/or students with direct practice for using scientific tools to analyze Earth science data. Students should begin on the Case Study page.