Seacoast artillery position finding was defined as the system used to determine the range and direction of a target from a battery or directing point. Seacoast guns were not aimed at a target, they were aimed at a point called the Set Forward Point. The Set Forward Point being defined as a point on the course of a target at which it is predicted that the target will arrive at the end of the predicting interval plus the time of flight (of the projectile) for the range (distance). The Coast Artillery utilized three basic systems of position finding.

1. The Horizontal Base System
   In the horizontal base system base-end observing stations were placed at each end of a known base line laid out along the coast line. This line constituted the known leg of a triangle. The base line did not necessarily run back in a direct line to the gun position. The displacement of the guns and other factors were calculated by the plotting room. The stations were designated as B' (Prime) and B" (Secondary). The station closest to the directing point was designated as B'. There were several groupments of base end stations to allow for combat casualties and target tracking from different points of view.
    
2. The Vertical Base System
   In the vertical base system, the target was located by the offset method used in surveying, in which the direction and distance of the target from a known point are determined. The direction was determined by reading the azimuth as in the horizontal base system. The distance was determined by the depression angle method which involves the solution of a vertical right triangle of which one leg is the desired range, the other leg is the effective height of the observation instrument above the target, and the hypotenuse is the line of sight from the observer to the target.
   
   An instrument called a Depression Position Finder (DPF) was used to determine vertical angles. The vertical base system requires only one observing station as the depression angle is solved mechanically by the observation instrument.
    
3. The Self Contained System
   Self-contained range finders were designed to determine both range and azimuth at the same time. The stereoscopic height finder M1 originally designed for antiaircraft artillery was adopted as the standard self-contained range finder for seacoast artillery.
   
   Base-end stations were manned by an Observing Detail consisting of an observer and a reader. The reader's telephone in a horizontal base system was connected to the earphone set of the appropriate arm setter (B' or B") in the battery plotting room at the gun emplacement.
   
   In a vertical base system, the telephone system is connected to the arm setter in the plotting room who repeats the range information to the the plotter who marks the range by means of range graduations along the edge of the arm. In a Battery Commander's station, additional personnel were assigned to maintain the Battery Commander's chart.
    

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