Measuring Distances


Geography has been described as the discipline that asks "the why of where." Understanding, the why, however, first requires knowing the where. And, where is where? There!

Location, another term for where, can be defined in either absolute or relative terms. In an absolute sense, location is measured by some standard frame of reference--usually longitude and latitude, or some other grid. There are many advantages to knowing the absolute location of things, but determining precise locations, say to within a few meters (which is still rather crude and imprecise for some purposes, such as property surveys), is nearly impossible. Invariably, field workers rely on relative locations; how far and in what direction one thing is from another. A permanent point of the known, and government-documented, absolute location is called a "bench mark."  A temporary point of known, but non-governmentally-documented, absolute location is called a "datum."

The United States Geological Survey (USGS) and the U.S. Coast and Geodetic Survey have thousands of official bench marks around the country. These are brass rods with flared heads (like giant nails) set in a concrete. In some cases, the heads are flush with the ground. In others, the bench marks look like small pilars or kiosks. [sample] The head of each bench mark is stamped with a number, and relevant information about the location can be found in the directory of bench marks. U.S. Government bench marks are used mainly by land surveyors. For many projects, however, "official" bench marks may not be necessary, and almost any easily identifiable feature can be used as a bench mark or datum. For example, if you were part of a team charged with measuring the various stream channel characteristics of Waller Creek through campus, you could get by with making your measurements in reference to the northwest corner of the gauging station between 23rd and 24th streets. Anyone can establish a datum for their own purposes anytime and anyplace.

Three types of measurements can be, and often are, made in reference to ("tied-in to") bench marks--elevation, direction (or azimuth), and distance. Later we will discuss elevation and direction. First, we will deal with horizontal distance, which can be measured in one of four ways. Taken in increasing order of accuracy, these involve: 1) pacing, 2) stadia, 3) calibrated tapes, 4) laser instruments.

Pacing

Every field worker should know the length of his or her pace or step. Pacing is probably the oldest form of distance measurement. The Romans used this technique, and, in fact, some of our current measurements have their origins in Rome. The mile is approximately 1000 single-foot paces. The league is roughly the equivalent of 3 miles, or about the distance a person can walk in an hour.

There are two ways one can measure a pace, using one foot (as did the Romans), or two feet. Most field workers prefer the single-foot technique as it involves less counting. Our first outdoor exercise involves each student determining the length of her or his pace. In order to do so, however, we'll have to do some more precise measuring.

Stadia

At one time or another when we were children, each of us probably had some type of "range finder" which more than likely came out of a cereal box. These toys were made of either cardboard or plastic and had a series of parallel lines, oriented horizontally, along one side. As you held the toy at arm's length, with the bottommost calibration (the zero point) aligned at the base of an object of known height (usually a person approx. 6' tall) the distance you were from the object was read off the calibration aligned with the top of the object. To measure distances this way is to measure "stadia." A number of surveying instruments commonly used until the Age of Electronics had the ability to measure stadia. Such instruments include the hand level, dumpy level, alidade, telescopic alidade, transit, and theodolite. These instruments have a horizontal pair of cross-hairs built into their lenses. Distances are calculated by measuring off a rod ("stadia rod") held at a selected place, the number of calibrations that appear between the hairs.

Measuring stadia might seem to have many advantages. It certainly seems convenient. Unfortunately, it is not very accurate, and hence was rarely used even during the heyday of preelectronic devices.

Measuring Tapes

Everyone is probably, or should be, at least somewhat familiar with measuring tapes. Today, most scientific tapes are made of fiberglass and neither stretch too much nor conduct electricity (a real problem where electric fences are involved!). Steel tapes used to be standard, and were referred to as "chains" simply because before tapes became popular surveyors literally used chains that were designed for that purpose [example].

Measuring tapes are calibrated in either the English system, the metric system, or both (English on one side, metric on the other). They come in a variety of lengths up to 100 meters. Regardless of how long a tape may be, it is often not long enough. For example, what does a field worker do when she has a 30 meter tape and has to measure a distance which is longer than 30 meters? She "breaks chain." Breaking chain involves the use of what once were called "chaining pins," [example] but are now called survey pins or survey arrows. These are heavy gauge wire devices one foot long, pointed at one end, with a loop on the other, and painted in alternating red and white bands. When the end of the tape is reached, a survey pin is pushed into the ground at the end of the tape's calibration and then the person holding the zero end of the tape ("tail of the chain") repositions herself at the pin. This is done repeatedly until the entire distance is measured.

Field workers often find themselves needing to break chain before the end of the tape is reached. This is usually done on steep slopes. Distances are ALWAYS measured horizontally. Regardless of the slope of the terrain, the head and the tail of the chain are held at the same elevation. For precise surveys a bubble level is either fitted onto or built into the chain. When the slope is sufficiently steep that the person at the head is holding the tape uncomfortably high (approaching eye-level and higher), it is time to break chain. Finding the point on the ground which corresponds to any point on the tape can be accomplished with the use of a plumb bob, a solid brass cone (in times past they were made of lead, hence "plumb") with the flat end attached to a string which is draped over the tape at any calibration. A chaining pin can be placed where the point of the plumb bob touches the ground. The tape can then be repositioned.

Laser Instruments

Use of the Total Station, the latest state-of-the-art, high-tech, land measuring equipment will be discussed and used later in the semester.  Total Stations are about as accurate as measuring devices get.  They are also bulky, heavy, and expensive.  A cheaper, lighter, but not quite as accurate, electronic distance measuring device is the laser range finder.  One will be demonstrated in class.

Equipment and Tools

The amounts and diversity of materials available for field work are as abundant and as varied as the types of work that are carried out. Two good sources for tools and equipment are Forestry Suppliers, Inc., and the Ben Meadows Company. Their catalogs are online [Forestry Suppliers] [Ben Meadows].


Suggested Additional Readings


Job 1


Created by William E. Doolittle. Revised 19 January 2018, wed