In some cases field workers may find it more convenient to tie features and objects, that is map them in reference, to a specific point known as a benchmark or datum. Benchmarks are permanent and established by a governmental agency. They are usually low concrete monuments capped with an encoded metal plate. The National Geodetic Survey publishes on CD-ROM a list of all benchmarks and their locations in the United States. It also maintains a website (NGS-Benchmarks) from which one can find the locations of, and details about, benchmarks. One official benchmark can be found on the southeast corner of 24th and Whitis streets on the UT campus [click here]. Its Permanent Identification Number or PID is BM0626, and its approximate latitude, longitude, and elevation are: N 30 deg. 17 min. 15 sec., W 097 deg. 44 min. 26 sec., 182.250 m or 597.93 ft. A number of other benchmarks established by various agencies can be found nearby [click here].
Similar to a benchmark, but established arbitrarily and temporarily for a specific project is a point known as a datum. It can be something as inconspicuous as an "X" chisled into the corner of a concrete driveway, or something more visible, such as a monument not unlike the official government ones. To illustrate, arbitrary data (data is plural for datum) are typically used in the mapping of features on archaeological sites, and usually involve an iron pin set vertically in a concrete footing. Permanent data facilitate replication.
Tying into a datum or benchmark is easy. All that is involved is measuring the distance from each object to be mapped back to the benchmark or datum. This information is recorded in the field note book in both tabular and graphic forms.
Of course, distance alone doesn't tell us much without knowing direction, does it? How can we determine location if all we have is knowledge of one dimension? By what is known as "triangulation." As the name implies, this process is derived from trigonometry and involves knowing at least three vectors and two points. For example, if you know the lengths of each side of a triangle, but do not know any of the angles, the triangle can still be drawn by striking arcs of appropriate lengths from the end points of one vector. The point where the arcs cross marks the third point of the triangle and the other two vectors can be drawn in by connecting the points.
For mapping a locale by tying features or objects to a datum, all that is needed are several vectors, and two data (datums) one of which can actually be one of the mapped objects, say a prominent tree. Accuracy is insured by having a number of vectors which cross each other.
Created by William E. Doolittle. Last revised 26 June 2013, wed