Observation and Fire Direction
The following section describes observation and fire control in the US army. Other nationalities in WW2 had different procedures or limitations.
The US observer can pass the initial target by a number of means. For the "azimuth" methods, the observer's position must be known and plotted by the Fire Direction Center (FDC):
Here are some examples:
The observer also passes or requests:
The FDC responds with:
Initial shots will be off by about 400 yards for estimates and 100-200 for map data.
While in the US army, almost any platoon commander could call for supporting fire (though he wouldn't be guaranteed of getting it), in most other armies only the observers, HQ's, and liaison officers would call. 
The relative position of the observer-target (OT) line with respect to the gun-target (GT) line does not affect the observer procedure in the adjustment of observed fires. Errors are determined in yards, and corrections in yards are sent to the battery or battalion fire-direction center (FDC). The FDC converts these coordinates into appropriate fire commands for the pieces. This is accomplished by plotting the corrections on a target grid and measuring data from the resulting plot from the pieces in order to place the next burst(s) at the point designated by the observer.
In area fire, the observer must select a well-defined point from which to adjust, which may be a well-defined terrain feature or some portion of the target, such as a truck or a tank. For surprise fire, he may select some nearby point, adjust on it, and then shift the fire to the area of his target.
Observers estimate distance by experience or by using a "yardstick" - two shells fired at what should be 400 yards apart. Lateral distance is easy to do using field glasses or a map. All angles are done in "mils" (1/6400 of a circle), for which there is a very easy relation: lateral distance = mils*range in thousands, which is good up to about 400 mils.
Typically only the portion of the mission directions that have changed are sent in the correction orders -- the rest is assumed to be the same.
If two or more observers at different angles to the target can coordinate, fire can be more accurate and better sensed, since triangulation and error averaging can be used. This seems uncommon and would probably have to be set up before the mission starts. However, combined observation seems to be about the only way to get good sensing at night -- it's easy to tell angles but hard to tell distances without the surrounding scenery.
The officer in charge of the US Fire Direction Center (FDC) arranges all barrages and artillery plans, and he approves or disapproves of fire missions requested by his observers. Computers1 assist the calculations and communication. Fire missions ordered by higher HQ are executed promptly.
Due to differences in how the fire calculations were performed, there were vast differences in the response time to a call for artillery. US and British artillery could respond in 2-3 minutes, while other nations averaged about 12 minutes. 
1 In WW2, a "computer" was a GI with a slide rule, not an electronic device!
Division and corps level artillery commanders coordinate general high-level assets and also the fire of battalion artillery to support units other than their directly-supported units. Targets can be reported by troops, artillery liaison personnel, ground or air observers, personnel of the artillery observation battalion (sound, flash, or radar ranging), adjacent or higher headquarters or recon, photo analysis, or intelligence. Plans are made in accordance with the nature of the target, the desired results (destruction, neutralization, harrassing, and interdiction), observation, ammo, and other considerations.
|Targets||Mortars, Rockets, Naval|