Remote sensing involves gathering information about an object without direct contact. It has been used since the 1850s for aerial photography from balloons and developed further for military purposes. Satellites now allow remote sensing on a global scale. Common applications include geology, environmental monitoring, forestry, agriculture, and mapping land use. Remote sensing uses sensors that detect reflected electromagnetic radiation to form images. Key concepts include spectral signatures, spatial and spectral resolution, and multispectral imaging. Active sensors like radar and lidar also measure distance using electromagnetic pulses.