# HORIZONTAL GEODETIC DATUM

The basis for computations of horizontal control surveys in which the curvature of the earth is considered It consists of the astronomical and geodetic latitude and the astronom- ical and geodetic longitude of an initial point (origin); an azimuth of a line from this point; the parameters (radius and flattening) of the reference ellipsoid; and the geoidal separation at the origin. A change in any of these quantities affects every point on the datum. For this reason, while positions within a system are directly and accurately relatable, those points from different datums must be transformed to a common datum for consistency. The horizontal geodetic datum may extend over a continent or be limited to a small area.

## Related Terms

1. The time interval between a change in the input signal to a process control system and the response to the signal.
2. The time interval, after a response to one signal or event, during which a system is unable to respond to another. Also known as insensitive time.

### POPPET VALVE

A cam-operated or spring-loaded reciprocating-engine mushroomtype valve used for control of admission and exhaust of working fluid; the direction of movement is at right angles to the plane of its seat.

### POLE-POSITIONING

A design technique used in linear control theory in which many or all of a system's closed-loop poles are positioned as required, by proper choice of a linear state feedback law; if the system is controllable, all of the closed-loop poles can be arbitrarily positioned by this technique.

### PNEUMATIC CONTROL VALVE

A valve in which the force of compressed air against a diaphragm is opposed by the force of a spring to control the area of the opening for a fluid stream.

### PLANT DECOMPOSITION

The partitioning of a large-scale control system into subsystems along lines of weak interaction.

### PIPE TAP

A small threaded hole or entry made into the wall of a pipe; used for sampling of pipe contents, or connection of control devices or pressure-drop-measurement devices.

### PIPELINE

A line of pipe connected to valves and other control devices, for conducting fluids, gases, or finely divided solids.

### PILOT WIRE REGULATOR

Automatic device for controlling adjustable gains or losses associated with transmission circuits to compensate for transmission changes caused by temperature variations, the control usually depending upon the resistance of a conductor or pilot wire having substantially the same temperature conditions as the conductors of the circuits being regulated.

### PHOTOELECTRICLOOPCONTROL

[CONTSYS] Aphotoelectric control system used as a position regulator for a loop of material passing from one stripprocessing line to another that may travel at a different speed. Also known as loop control.

### PHOTOELECTRIC LIQUID-LEVEL INDICATOR

A level indicator in which rising liquid interrupts the light beam of a photoelectric control system; used in a tank or process vessel.

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