A survey of areas large enough to require loops of control.


Related Terms

OFFSET LINE

A secondary line established close to and roughly parallel with the primary survey line to which it is referenced by measured offsets.

OFF-LINE

1. A condition existing when the drive rod of the drill swivel head is not centered and parallel with the borehole being drilled. 2. A borehole that has deviated from its intended course. 3. A condition existing wherein any linear excavation (shaft, drift, borehole) deviates from a previously determined or intended survey line or course. 4. State in which an equipment or subsystem is in standby, maintenance, or mode of operation other than online.

LASER TRANSIT

A transit in which a laser is mounted over the sighting telescope to project a clearly visible narrow beam onto a small target at the survey site.

HEIGHT OF INSTRUMENT

In survey leveling, the vertical height of the line of collimation of the instrument over the station above which it is centered, or above a specified datum level.

HYDROGRAPHIC SURVEY

The survey of a water area, with particular reference to submarine relief, and any adjacent land

IMPROVED CHANNELS

Dredged channels under the jurisdiction of the U.S Army Corps of Engineers, and maintained to provide an assigned CONTROLLING DEPTH. Symbolized on National Ocean Survey charts by black, broken lines to represent side limits, with the con- trolling depth and date of the survey given together with a tabulation of more detailed information

LEVELING

A survey operation in which heights of objects are determined relative to a specified datum

ASTROLABE

An instrument which measures altitudes of celestial bodies, used for determining an accurate astronomical position, usually while ashore in survey work. Originally, the astrolabe consisted of a disk with an arm pivoted at the center, the whole instrument being hung by a ring at the top to establish the vertical.

ASTRONOMICAL POSITION

A point on the earth whose coordinates have been determined as a result of observation of celestial bodies. The expression is usually used in connection with positions on land determined with great accuracy for survey purposes.

NATIONAL TIDAL DATUM EPOCH

The specific 19-year cycle adopted by the National Ocean Survey as the official time segment over which tide observations are taken and reduced to obtain mean values(e.g., mean lower low water, etc.) for tidal datums. It is necessary for standardization because of apparent periodic and apparent secular trends in sea level. The present National Tidal Datum Epoch is 1960 through 1978.

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