Cartographers are in a constant battle to ensure that their maps are accurate and up to date. They therefore need to gather information on as wide a scale, in as much detail, and as regularly as possible. Traditional means of collecting and recording information – ground survey, scientific measurements, terrestrial photography, publications and maps – are still as valid today as they have been for centuries. However, the ability to observe and photograph the Earth’s surface from above provides perhaps the greatest insight into how our planet looks, and how it is changing. From the earliest days of flight, the ability to capture aerial views has been an invaluable tool in the compilation and revision of maps.

One of the most significant developments in terms of looking at the Earth from above has been the emergence of Earth-observing satellites. Launched with the specific purpose of capturing images of the Earth’s surface and data on environmental and atmospheric conditions, these satellites have been providing detailed views of the Earth for over thirty-five years. The first satellite specifically for observing the Earth’s surface was Landsat 1 (originally called Earth Resources Technology Satellite ERTS-1), which was launched in 1972. It provided dramatic new views of the Earth and its images found numerous applications in, for example, agriculture, regional planning, geology and environmental monitoring. The latest in the Landsat series, Landsat 7 was launched in April 1999 and it is now accompanied by numerous other satellites generating images and data of many different kinds.

 

© NASA Landsat 7

 

Satellite imagery and the related science of satellite remote sensing – the acquisition, processing and interpretation of images captured by satellites – is now used to observe, monitor and map the Earth. Sensors carried by satellites can capture electromagnetic radiation in a variety of wavelengths, including those visible to the eye (colours), infrared wavelengths and microwave and radio radiation detected by radar sensors. These data can be processed to allow the detailed interpretation of landscapes, vegetation (including the identification of specific types of vegetation and relative health of crops), and environmental and atmospheric conditions – crucial today in monitoring the processes involved in climate change. Many types of map can be created depending on the nature of the sensor and the data collected. Some missions now even collect height data, allowing the creation of detailed relief maps of the Earth’s physical features.

The level of detail discernible on satellite images – effectively the size of the smallest feature which can be detected – is known as image resolution. The early Landsat satellites had resolutions of 80 m (262 feet), while the latest sensors, such as those on the commercial IKONOS and Quickbird satellites now capture highly detailed images with resolutions of less than 1 m (3.3 feet). Such high resolution images provide great detail for the compilation and revision of general topographic maps and town plans.

One important aspect of satellite remote sensing is that satellites regularly revisit the same point above the Earth and so can gather time-sequence images of exactly the same area. Earth-observing satellites follow one of two types of orbit: geostationary or polar. Geostationary satellites (most commonly meteorological satellites observing climatic conditions and used extensively for weather forecasting) effectively sit above the same point of the Earth’s surface, allowing constant collection of images of the same area. Satellites in polar orbits travel around the Earth in a north-south-north direction. As the Earth rotates they progressively capture images of adjacent (or partially overlapping) areas. Such satellites typically revisit the same point every 16–26 days, but this period can be decreased dramatically if a satellite is able to capture oblique images – the French SPOT satellite can in fact revisit the same point every 1–3 days by looking sideways at the target area. Such revisit capabilities allow enormous amounts of change data to be gathered and provide cartographers with sequences of images from which details of changes can be identified and mapped.

 

The Aral Sea

    

 

    

Times Comprehensive Atlas, 1st Edition, 1967      Times Comprehensive Atlas, 12th Edition, 2007

 

Satellites and new sensors are continuing to be developed and the resolution of the images they capture continues to improve. Sensors recording very detailed topographic, environmental and atmospheric data will remain a critical tool in observing and mapping the Earth. The vast and growing archive of images will also continue to be a great resource for cartographers in keeping their maps and atlases up to date.