The changes in the reflectivity/emis-sivity with time, is called :

ANS:C - temporal variation

Spectral variations are the changes in the reflectance or emittance of objects as a function of wave-length. Colour of objects is a manifestation of spectral variation in reflectance in the visible region.

Spatial arrangements of terrain features providing attributes, such as shape, size and texture of objects which lead to their identification are termed as spatial variations.

Temporal variations are the changes in the reflectivity or emissivity with time. They can be diurnal and/or seasonal. The variation in reflectivity during the growing cycle of a crop helps to distinguish crops which may have similar spectral reflectances, but whose growing cycles may not be the same. A plot of spectral reflectance vs. growth stages of a crop provides a phenologic pattern, which is characteristic of a crop, even at the species level. Therefore, remote sensing data acquired over the same area at different times can make use of the temporal characteristics to discriminate crops in a better way.

Polarisation variations relate to the changes in the polarisation of the radiation reflected or emitted by an object. The degree of polarisation is a characteristic of the object and hence can help in distinguishing the object. Such observations have been particularly useful in the microwave region.