Remote sensing is any form of measurement where the measurement device is not in direct contact with the target. For example, the temperature of a water body can be measured directly by placing a thermometer in the water, or indirectly using a satellite sensor that measures emitted thermal energy from the water, which can be converted to temperature.
In terrestrial environments remote sensing techniques are usually applied to map: the type of material or feature(s) present on the ground (e.g. sand, grass, trees, forest) and their biophysical properties, including physical structure (height of trees, biomass) and bio-chemical properties (chlorophyll concentration, water content, amount of light absorbed for photosynthesis). A range of remote sensing instruments can be used in terrestrial environments, including passive sensors which rely on reflected sunlight or emitted thermal energy and active systems, such as radar or airborne laser, which generate their own source of illumination.