Resolutions and data products

Four kinds of resolution:

• Temporal resolution (revisit time): how often a sun-synchronous satellite passes over the same area. Repeated images show change: the book compares IRS images of the Himalayas and northern plains in May and November (more deciduous trees and crops show in November), and images of Banda Aceh, Indonesia, before and after the December 2004 tsunami.
• Spatial resolution: the sensor's ability to tell two close objects apart, like spectacles helping you separate two close letters. The finer the resolution, the smaller the objects it can show: from Landsat MSS (80 m) through IRS LISS-III (23 m) and SPOT (20 m and 10 m) to IRS PAN (5.8 m).
• Spectral resolution: how many bands of EMR the sensor records, and how narrow. It works like a prism or a rainbow splitting white light into colours. Water absorbs strongly in the infrared band; dry surfaces reflect strongly in the green band.
• Radiometric resolution: how finely the sensor tells apart small differences in brightness, shown as the number of grey levels (for example 0-63, 0-127 or 0-255).

Data products:

• An image is any picture of a scene, whatever energy recorded it; a photograph is an image recorded on photographic film. So all photographs are images, but not all images are photographs.
• Photographic images use the optical range (0.3-0.9 µm), on black-and-white, colour, black-and-white infrared or colour infrared film; aerial photography normally uses black-and-white. Photographs can be enlarged a lot without losing contrast.
• Digital images are made of pixels, each with an address and an intensity value, its Digital Number (DN). Smaller pixels keep more detail.

From the video: the teacher shows a patch of DN values from 0 to 255: low numbers are dark, absorbing surfaces like deep water; high numbers are bright, reflecting surfaces like fresh snow.

Example (new): a car about 4 m long might just show up as one or two pixels on a 5.8 m image, but would be invisible on an 80 m image, where a whole football field fills only about one pixel.