Observations Of The Earths Surface

The brightness of the earth's surface in VIS imageries depends on the time of the day, season of the year, the geographical location etc. It is easy to distinguish dry land from a water surface and to identify shorelines, rivers, lakes, islands etc. Dry land, depending on the type of relief, assumes various grey shades in VIS imageries.

The tone of the underlying surface in IR imageries depends on temperature and so its appearance is affected on the latitudinal, diurnal and seasonal variations. During day light hours and in summer dry land surfaces appear darker than the water surface but at night it has a lighter tone than a water surface. IR imageries reveal only large irregularities of relief of vegetation pattern which are associated with marked temperature gradients whereas VIS pictures can reveal even small terrain variations.

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