Surveying

Surveying is a means of relatively large-scale, accurate measurement on the Earth's surface. Surveys were undertaken for practical reasons: road-building, estate evaluation, and boundary-control.

Surveying theory is based on the principles of geometry and trigonometry, and the methods used are based on the principles of physics.

It is probable that surveying had its origin in ancient Egypt. The Great Pyramid of Khufu at Giza, built about 2700 BC, is 755 feet long and 480 feet high. Its nearly perfect squareness and north-south orientation affirm the ancient Egyptians' command of surveying.

Egyptians apparently used rope with knots or marks at uniform intervals for land measurements, wooden rods for distance measurements, levels, and the wooden groma to establish right angles. These simple instruments helped the Egyptians reestablish boundary markers obliterated by the Nile's flood waters.

The Greeks used a form of log line for recording the distances run from point to point along the coast while making their slow voyages from the Indus to the Persian Gulf about 325 BC.

Plane tables were used in Europe in the 16th century, and the principle of graphic triangulation and intersection was practiced by surveyors. In 1620 the English mathematician Edmund Gunter developed a surveying chain, which was superseded only by the steel tape in the beginning of the 20th century.

By the late 18th century modern surveying can be said to have begun. One of the most notable early feats of surveying was the measurement in the 1790s of the meridian from Barcelona, Spain, to Dunkirk, France to establish the basic unit for the metric system of measurement.

In the 20th century, two revolutionary surveying changes have been introduced: photogrammetry, or mapping from aerial photographs (about 1920), and electronic distance measurement, including the adoption of the laser for this purpose as well as for alignment (in the 1960s). Important technological developments benefiting surveying in the 1970s included the use of satellites.

Is a Transit the Same as a Theodolite?

Well, almost. A transit, used for the fundamental determination of star positions, is one of the most important of the astronomical instruments. Invented by the Danish astronomer Ole Roemer (1644-1710) in 1690, it is a small refracting visual telescope that rotates only on a horizontal axis accurately set in the east-west direction. Unlike other telescopes, the transit is not clock-driven to follow the stars. It can point to any altitude, but can be trained on a star only when that star is due north or south of the observer; that is, on the celestial meridian. The transit is mounted on a tripod to insure stability for long periods of time, and is provided with levels to determine its tilt, and with fixed reference points to determine its orientation. A modern instrument may have an electrical drive, controlled by the observer.

A theodolite is an optical instrument used in surveying to measure horizontal and vertical angles. It is designed on the same principles as the transit; generally, the traditional or American-style instrument, with exposed controls and verniers, is called a transit, and the more modern European model a theodolite.

The theodolite is smaller than a transit and has a telescope that is less than six inches long, compared to about ten inches for the transit. The theodolite telescope uses prisms in order to enlarge the image and has three leveling screws; the transit, on the other hand, uses lenses and has four leveling screws.

Because adjusting and measuring devices are enclosed in a theodolite, its appearance is smooth and streamlined compared to the transit. It has optical micrometers that allow angles to be read more accurately with a single measurement than can be read by repetition with a transit.