Review article Synergy in remote sensing-what's in a pixel?

Abstract

We address various questions that arise from the fact that a pixel or the related instantaneous field-of-view (IFOV) on the ground is often larger than we would like it to be.The problems arise as a penalty imposed by technology for the fact that a spacecraft gives an overview of a very large area. We study the question of why the pixel size is important when one studies satellite imagery and also some questions related to the factors that contribute to the recorded signal in a remotely-sensed data set. This includes a discussion of the instantaneous field-of-view, both from the simple geometrical point of view and from a more physical point of view. It also involves a study of the point spread function and some discussion of the problems associated with the determination of the point spread function of a given scanner. The questions of the calibration of the detectors in an instrument and of the intercalibration of nominally identical members of a series of instruments are also considered. The integration of remotely-sensed data into a GIS almost inevitably involves resampling and this leads to further complications in understanding the origin of the signal (digital number, DN) associated with a given pixel. The effects of different methods of interpolation are considered as well as the consequences of resampling in relation to the classification of an image. Several other topics are also considered and these include (a) the achievement of geometrical rectification with an error substantially smaller than (e.g., only 20 per cent of) the length of the edge of the instantaneous field-of-view, (b) data compression or upscaling, (c) mixed pixels, and (d) the study of sub-IFOV size objects. The general conclusion is that it is important to realise that what contributes to producing the digital number on a computer tape or in a disk file of an image is not a simple thing. There is no simple answer to the question 'what exactly gives rise to the signal detected and recorded in a pixel in a remotely-sensed image?' The main point to be made is to try to ensure that it is realised that there is a problem and to give some indication of the nature of that problem.