METHODS USED IN SURVEYING

    TRADITIONAL METHODS

Compass and chain:

From colonial times, through the 1800s, most boundary surveys were performed with a compass and Gunters Chain, usually 66 feet long and divided into 100 links. The compass was either mounted on a tripod or a single pole, called a Jacobs Staff. These early surveying tools were not very precise, but were sufficient in the days when land values were 50 cents per acre, or less.

 

Transit and tape:

Eventually, the use of the compass gave way to the transit, and the chain to the steel tape. While the compass was generally only able to measure the magnetic bearing of a line to the nearest one-quarter degree, a transit is able to measure the angles between lines to less than one minute of arc. The steel tape, usually 100 or 200 feet long and graduated in hundredths of a foot, provided an accuracy significantly greater than the Gunters Chain. The transit and tape permitted the more precise measurements necessary in land subdivision planning, construction surveying, and nearly all boundary or land title surveys. Until recently, this method was used for most surveying work.

 

Transit and stadia:

Angles were measured with a transit and distances measured by optical methods. A rod or Stadia Board was graduated in hundredths of a foot, and horizontal crosshairs in the telescope of a transit, called Stadia Hairs, were positioned so that, based on trigonometry, at a distance of 100 feet the stadia hairs subtended exactly one foot on the rod. Thus, within about 500 feet, a distance could be read directly from the rod. Due to its speed and efficiency, this method was most common for topographic mapping. For the most part, stadia has also given way to electronic instruments.

 

    MODERN METHODS

 

Theodolite and electronic distance measurement:

There are no exact standards differentiating an instrument referred to as a Transit from one that is referred to as a Theodolite. Generally, a theodolite is a much more precise instrument. Some can measure an angle to within 1/10 of one second of arc (one thousandth of a foot in one mile), but 1-second or 3-second theodolites are typical. Also, the angles on a transit were read off of a circular metal plate, graduated in degrees and minutes, while the theodolite replaced the metal plate with an internal etched glass plate and the ability to read an angle through the eyepiece via a series of mirrors and lenses.

By the 1970s, relatively small, lightweight and easy-to-use electronic distance measuring devices, called EDMs were in use. They were mounted on the theodolite, and operated on the principle of transmitting a narrow beam of infrared light to a reflector and measuring the time it takes to return.

Before long, the advance of technology and miniaturization of electronic components enabled the building of theodolites that measure angles electronically, measure distances with their own internal EDM, and display a variety of data on an LCD screen. These super-theodolites are referred to as Electronic Total Stations. In addition to enhanced speed and accuracy, the digital data can be automatically downloaded to an electronic data collector for transfer directly to computers for calculations or CAD drafting. In addition to the speed and accuracy that they provide, the decreasing cost of the electronic total stations has allowed them to virtually totally replace all previous methods and instruments for most survey work.

 

Photogrammetric mapping:

Topographic mapping may be done from aerial photographs and is particularly useful for large areas. Usually, the photography is made specifically for the project involved. Accurate ground survey work must be used to establish measurements, both horizontally and vertically, to photo-identifiable points to insure scale accuracy and proper orientation of the photo model. Supplemental field surveys are usually required for locating features that are not identifiable on aerial photographs, such as underground utilities, wetlands, culverts, and any feature too small to be seen.

 

GPS:

The GPS, or Global Positioning System, is the newest method available to surveyors. The system is based on a constellation of 24 satellites in precise orbits around the Earth. Todays GPS receivers can directly calculate the position of any place on the Earths surface from signals broadcast from the satellites. While inexpensive, handheld receivers can provide a position to within a hundred feet, or less, more sophisticated receivers can provide a position to within a few inches. If two or more of these receivers are used, and one is placed on a known position, the directions and distances between the receivers over very large areas can be determined with a precision never before obtainable. However, due to technical limitations, GPS technology is not suitable for precise determination of elevations. At present, the high cost of these receivers is prohibitive for use in all but the larger control surveys or aerial mapping projects.