Satellite

Satellite imagery from within the visible electromagnetic spectrum (RGB), but further exciting images result from accessing data from beyond this narrow range. The history of satellite-based Earth Observation extends back to 1972, with the launch of the first Landsat system (Landsat 1), with an effective ground resolution of 80m and a revisit interval of 16 days.

This has improved dramatically with modern satellites (e.g. GeoEye, WorldView-1 and 2) offering sub-1m panchromatic imagery. Geoeye-2 which is currently in production is expected to offer panchromatic resolutions of 25cm. Remotely sensed satellite data is not only available as panchromatic images, or even as RGB; multispectral satellite imagery is also available, varying in spatial resolution, spectral resolution and number of bands. This has been the case since Landsat 1, with the numbers of available bands becoming increasingly numerous. Many systems (e.g. Ikonos; GeoEye) operate a four band (plus panchromatic) system, incorporating Red, Green, Blue and Near Infrared. These multispectral bands are available at lower resolution than the panchromatic image but can be ‘pan-sharpened’ – have their resolution artificially increased using the panchromatic image. Currently, WorldView-2 offers the widest range of spectral bands, including a ‘coastal’ band, a ‘red edge’ band and two infra-red bands (a total of 8 plus panchromatic) with a spatial resolution of 1.8m multispectral; 0.46m panchromatic.

Worldview-3, due to launch in 2014 will also include a short-wave infra-red band at 3.7 m resolution and improved resolution across both multispectral and panchromatic bands.
 
Satellite imagery is inexpensive, at least compared with both lidar and hyperspectral data, and allows large quantities of data to be captured with very little notice (e.g. WorldView-3 is expected to have a revisit rate of less than one day) and in extremely remote areas where mobilisation of aircraft would be prohibitively expensive.