Field studies that involve mapping almost always have the need to delimit the outlines of the study area. The most common technique for doing this is to conduct a traverse. In a traverse a series of points are surveyed by measuring the distance and direction from one point to the second, and from the second to the third, and so on to the last point. Some field workers, especially geologists, call each of the points of the traverse a "station." However, to avoid confusion with stations as determined by engineers, other field workers call the points of a traverse the "hubs," "corners" or just "points." The lines between the points are referred to by everyone as as the "legs."
The method by which a traverse should be conducted is determined by the accuracy required and by the time and equipment available. For many projects, a Brunton compass is adequate for determining the bearings of the legs, and the legs can be measured by taping or by pacing.
Traversing is actually quite simple. The field worker (and his or her crew, if there is one) begin by choosing a starting point and designating it Point A. Not infrequently, this is an easily accessible point. For example, if one is traversing a dry stream bed, the field worker may elect to begin at a bridge where a road crosses the stream. From here, she or he walks along the feature being traversed stopping wherever the feature makes a sharp turn, or begins to make a gradual curve. This point is marked with a stake, chaining pin, or flagging and designated as Point B. The field worker than moves on determining Point C, etc. Gradual curves are typically delimited by a series of points and short legs. When all the points are determined, the field worker returns to Point A. From here, the azimuth or bearing is taken on Point B. The distance between points A and B is measured and, along with the azimuth, is recorded in the field note book. As usual, textual notes are recorded on the left hand page, while a sketch of the traverse with relevant data is recorded on the right.
Often, closed traverses will not "close." That is, when the fieldwork gets back to Point A, the sum of the angles may not add up to 360, or when a detailed map is drawn from the field notes, the ending point and the starting point might not coincide. This happens more frequently than field workers like. Failure to close is in large part a function of the accuracy of equipment. Field workers using total stations with lasers have fewer problems with closure than do field workers sighting azimuths with a Brunton compass. Regardless of the equipment used, getting perfect closure is very unusual. Having a closure gap is nothing to be embarrassed about or ashamed of. The question is: How much error is acceptable. The degree of tolerance is at the discretion of the field worker or his supervisor or employer. Generally speaking, it is a proportion of the traverse length.
| First Order Accuracy | = | 1/25,000 | |
| Second Order Accuracy | = | 1/10,000 | |
| Third Order Accuracy | = | 1/5,000 | |
| Military Accuracy | = | 3% |
Another way to assist in improving the accuracy of a traverse is to determine the azimuths and measure the distances of a few legs between two points that are not in sequence or next to each other. For example, let's assume a meandering foot trail with points, A, B, C, D, E, F, and G. In addition to distances and azimuths recorded between points, the field worker should probably take a few others, such as between A and C, D and F, etc. In effect, this is triangulation as triangles are being completed. The closure, or lack of it as the case may be, can be checked. If the gap or error is too great, all the legs should be remeasured and the bearings reread. In closed traverses, it is also helpful to minimize closure errors by employing a similar triangulation procedure. In such cases, however, the new legs used to insure accuracy are drawn across the traversed area. The exact points to be used in checking the accuracy of traverses are left to the discretion of the field worker and are usually based on local conditions and factors. For instance, in the hypothetical trail traverse mentioned above, Point D might not be visible from Point B by virtue of a dense grove of trees. However, because Points A and C could be seen from each other, a leg was triangulated between them.
Created by William E. Doolittle. Last revised 14 December 2017