Satellite photographs

Introduction

Satellite photographs are a type of photograph used by geographers. This chapter explains what a satellite photograph is and its advantages and disadvantages. It specifically focuses on the most common type of satellite image, the Landsat image, and how to read a Landsat image.

What are satellite photographs?

Satellite photographs are images of the Earth's surface. The information for these images is captured and transmitted using specialised cameras, scanners or sensors which are fitted to an artificial satellite (ie not moons or stars). These man-made satellites orbit the Earth from a distance of up to 1000 kilometres above the surface.

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In 1960, the first satellite photographs of the Earth were created by a weather satellite. This was less than a year after the first satellite images of the moon were captured. Since then, satellite images have been used for a wide variety of purposes. Owing to satellite photographs being able to show cloud patterns with great clarity, they are often seen on television as a part of the weather forecast. Popular action movies and television programs have also made satellite imagery synonymous with military and government intelligence and security agencies. Satellite imagery is also used in the fields of cartography (creating maps of the Earth's surface) geology, engineering, oceanography (scientific study of the ocean) and agriculture.

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Advantages and disadvantages

One main advantage of satellite imagery is that it can show a large area. While oblique and aerial photographs may have a scale in which one centimetre on the page represents a few hundred metres on the ground, satellite images can represent tens of kilometres in a single centimetre.

Satellite images can also provide more information than conventional photographs. Scanners and sensors not only show vegetation and settlement patterns, but also make measurements of the Earth's surface and detect different frequencies of electromagnetic radiation. Collected information ranges from the amount of water vapour in the atmosphere, to the temperature of the land and sea.

Although satellite imagery is very useful, it does have some downfalls. Satellite imagery is time consuming, since it takes a long time to process the large pictures using such a high resolution. To create an adequate image from space, optimum conditions are also required. Weather patterns can be unpredictable and the sun, which is the major source of light, needs to be in an ideal position. If the conditions are not optimum, it may be days or even weeks until the satellite will return to the area which is required to be photographed.

Landsat imagery

One of the most common types of satellite images is the Landsat image. It was in 1972 that the first Landsat satellite was launched by NASA (National Aeronautical and Space Administration). Since then, Landsat satellites have been responsible for the production of millions of images. In 1999, the seventh and most recent Landsat satellite was launched. Landsat 7 has an altitude of around 705 kilometres and circles the Earth every 98.9 minutes.

Landsat satellites collect and transmit images which show both physical and cultural features on the Earth's surface using various colours. These colours, however, do not represent the actual or conventional colours of these features. For this reason, the colours used on Landsat images are called false colours.

How to read Landsat images

The altitude at which satellite photographs are taken sometimes makes it difficult to distinguish ground features. When studying a geographical image of an area, people often recognise features by their colour, shape, location and associated features. Since Landsat images use false colours, the task of identifying and interpreting particular features is made even more difficult. To make Landsat images easier to understand, a basic table of colours and their possible corresponding features is listed below.

Table 1. A colour guide for Landsat false colour images.

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In more recent times, there has been a growing trend towards replacing the false colours of Landsat images with more conventional colours. These colours are known as natural colours.

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