# NATIONAL GEODETIC VERTICAL DATUM

A fixed reference once adopted as a standard geodetic datum for heights in the United States. The geodetic datum now in use in the United States is the North American Vertical Datum of 1988. The geodetic datum is fixed and does not take into account the changing stands of sea level. Because there are many variables affecting sea level, and because the geodetic datum represents a best fit over a broad area, the relation- ship between the geodetic datum and local mean sea level is not consistent from one location to another in either time or space. For this reason the National Geodetic Vertical Datum should not be confused with MEAN SEA LEVEL.

## Related Terms

### TWA

Time Weighted Average

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.

### POINCARE SURFACE OF SECTION

A method of displaying the character of a particular trajectory without examining its complete time development, in which the trajectory is sampled periodically, and the rate of change of a quantity under study is plotted against the value of that quantity at the beginning of each period. Also known as surface of section.

### PLANNING HORIZON

In a materials-requirements planning system, the time from the present to some future date for which plans are being generated for acquisition of materials.

### PHASE PORTRAIT

A graph showing the time derivative of a system's position (or some other quantity characterizing the system) as a function of position for various values of initial conditions.

### PHASE PLANE ANALYSIS

A method of analyzing systems in which one plots the time derivative of the system's position (or some other quantity characterizing the system) as a function of position for various values of initial conditions.

### PERS SUNSHINE RECORDER

A type of sunshine recorder in which the time scale is supplied by the motion of the sun.

### PERMANENT BENCHMARK

A readily identifiable, relatively permanent, recoverable benchmark that is intended to maintain its elevation without change over a long period of time with reference to an adopted datum, and is located where disturbing influences are believed to be negligible.

### PEEL-OFF TIME

In seismic prospecting, the time correction applied to observed data to adjust them to a depressed reference datum.

### PAYOUT TIME

A measurement of profitability or liquidity of an investment, being the time required to recover the original investment in depreciable facilities from profit and depreciation; usually, but not always, calculated after income taxes.

### Related questions

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