Satellite imagery

Satellite imagery are photographs of Earth or other planets made from artificial satellites. First satellite photographs of Earth were made in ???? by ?????. First satellite photographs of the Moon were made in ???? by ????.

In 1972 the United started the Landsat program, the largest program for acqusition of imagery of Earth from space. Landsat 7, the most recent Landsat satellite, was launched in 1999.

All satellite images produced by NASA are published by Earth Observatory and are freely available to the public. Other countries have their own satellite imaging programs. There are also private companies that provide commercial satellite imagery.

Satellite images have many applications in agriculture, geology, forestry, regional planning, education, intelligence and warfare.

Images can be in visible colours and in other spectras. There are also height maps, usually made with (??? radar? lasers?).

Blue Marble was a project for mapping the visible color of the Earth with 1km (?) resolution. The beautiful resulting images have become very popular in the media and among the public.

also SRTM, MODIS, GLOBE, etc.

A satellite can only see a relatively small area at the same time. For this reason mapping a large area (or the whole Earth) takes a lot of time and most satellite images are on average several years old. Because imaging satellites are in low Earth orbits, real-time imaging of a certain area is complicated. However, spy satellites can do this during wars and other military operations or to provide data for intelligence agencies. Still, the troops frequently find out that the ground truth is different from the images that were taken some time ago.

The resolution of satellite images varies. For most cities in the world photos with resolution of 10 meters or better are available. For areas outside cities the resolution can be 100-500 meters. Most of the Earth is imaged with resolution of 1 km or better. For many smaller areas images with resolution as high as 10 cm can be available. Satellite imagery is supplemented with aerial photography, which has higher resolution, but is more expensive.

Because the total area of the land on Earth is so large and because resolution is relatively high, satellite databases are huge. Storing and serving satellite imagery was a big technical challenge.

Satellite imagery can be combined with GIS data. For example, the EarthViewer can combine the images with maps and other information from various sources.

In early 21st century satellite imagery became widely available, when affordable easy to use software with access to satellite imagery databases was offered by several companies and organisation.

External links

• World Wind - an open source 3D Earth-viewing software developed by NASA that accesses NASA JPL database
• Keyhole EarthViewer - a commercial 3D Earth-viewing software with a cheap subscription
• Microsoft Terraserver - an electronic store for satellite images