Tying-into a Baseline

The simplest way to map a small area is by tying features on the landscape to a baseline. In some respects this is similar to a coordinate system, but uses only one axis rather than two. That one axis is called a "baseline." Essentially, all that is involved in mapping an area by means of a baseline is to layout a straight line through the area to be mapped (the baseline), then note the location of features in terms of their distances from the baseline, measured at a right angle, and the distances of these points from one end of the baseline.

The location of a baseline can be determined in one of three ways. First, it can be established on the basis of a known line. For example, if you needed to map a clearly defined parcel of land, say a city block, you might choose to use one edge of the sidewalk as a baseline. Or, if you were mapping some rural area you might choose to use a fence line. The second way to establish a baseline is on the basis of two known points. Often an immediately identifiable line does not exist, but two prominent features on the landscape can be used as points between which a baseline can be laid. For example, if you are needing to map an area in which there is an intersection of two roads and a historical marker some distance away, you might choose to layout a baseline between these points. Finally, baselines can be established arbitarily. That is, whatever her or his task, the field worker can simply pick any two points and layout the baseline between them. In the final analysis it really makes no difference which method is used, although the first two do have the advantage of being replicable; later fieldworkers could come in and replicate or check your notes. Regardless of the method used, the azimuth or direction of the baseline should be measured with a compass and recorded.

Baselines can be "staked" with either standard wooden stakes which can be written on if quasi-permanent and accurate mapping is needed (such as is done with land surveys), or with chaining pins if quick-and-dirty, and less accurate mapping is all that is needed. The tail of a tape measure is then held at one end of the baseline, Point A, which can be considered a datum, and the head of the tape is taken to the point perpendicular to the first object to be tied-in. This latter point can be determined with some accuracy by means of a devise known as a right-angle prism, or less accurately by a method called "shooting a whammy." Here the distance from Point A is recorded as is the distance from the baseline to the object or feature. The process is then repeated until each item is tied-in.

Distances along the baseline are recorded in two ways; relatively, from one point to the next, and in terms of stations. With very long baselines, one cannot afford to go back to Point A and start over every time a feature is tied-in. Standard procedure in these cases is to place a stake (or a chaining pin) every 100 meters or feet. These points are the stations. Point A is station 0+00. A point 100 meters (or feet) from Point A, is given the station designation 1+00; a point 200 meters down the baseline is stationed 2+00; a point 852.83 meters from Point A is designated 8+52.83.  Double-checking distances from one point to another in reference to the stations not only provides accuracy, but it also reduces time that would otherwise be spent going back to point A for each measurement. It also allows for one person to work with one tape. 

Created by William E. Doolittle. Last revised 13 September 2000, wed