Elevation (ballistics)

In ballistics, the elevation is the angle between the horizontal plane and the axial direction of the barrel of a gun, mortar or heavy artillery. Originally, elevation was a "linear" measure of how high the gunners had to physically lift the muzzle of a gun up from the gun carriage to compensate for projectile drop and hit targets at a certain distance.
Pre-WWI and WWI.
Though early 20th-century firearms were relatively easy to fire, artillery was not. Before and during World War I, the only way to effectively fire artillery was plotting points on a plane.
Most artillery units seldom employed their cannons in small numbers. Instead of using pin-point artillery firing they used old means of "fire for effect" using artillery en masse. This tactic was employed successfully by past armies.
But changes have been made since past wars and in World War I, artillery was more accurate than before, although not as accurate as artillery one century newer. The tactics of artillery from previous wars were carried on, and still had similar success. Warships and battleships also carried large caliber guns that needed to be elevated to certain degrees to accurately hit targets, and they also had the similar drawbacks of land artillery.
WWII and beyond.
As time passed on, more accurate artillery guns were made, and they came in different varieties. Small artillery pieces were used as mortars, medium-sized artillery guns became tank guns, and the largest artillery guns became long range land batteries and battleship armaments.
With the introduction of better tanks in World War II, elevation was once again a problem for tank gunners, which had to aim through the Gunner's Auxiliary Sights (GAS) or even through iron sights. Though the problem was not that evident as tanks fired rounds at a higher velocity than normal artillery, making aiming less of a hassle.
As with World War I, World War II artillery was almost like its old counterpart. But in the war came the introduction of the FCS or the fire-control system, which made firing artillery accurately easier.
With the advancements in the 21st century, it has become easy to determine how much elevation a gun needed to hit a target. The laser rangefinder is a component of FCS, and can accurately determine the range of the target, thereby calculating how much elevation the gun needs, making today's guns highly accurate.