Indirect fire is the use of artillery to fire at targets out of the crew's line of sight, by firing in a high arc out to long distances and/or over blocking terrain. Usually an observer in closer proximity to the target reports the target's estimated position back to the crew or their headquarters, and the weapon is then fired at that point on the basis of mathematical calculations for the necessary horizontal and vertical angles and shell velocity. The observer then takes note of any error in the resulting fire, and if necessary requests adjustments relative to the impact point of the initial rounds, e.g. "right 400 add 2000", in units of distance understood by the army's conventions.