Thus far we have focused exclusively on mapping in one and two directions (lines and areas). The third dimension, height, was mentioned only in regard to how it must be taken into consideration when measuring horizontal distances. Height, or elevation, is, however, even more than simply affecting horizontal measurements. Physical geographers often need to measure elevations in order to understand topography and geomorphic processes. Human geographers, especially those concerned with topics such as agricultural landscapes, have a similar need to know how to assess relief. For instance, the details of a hill slope must be known in order to make urban improvements [example]. Fieldworkers need how to know how to use a hand level, and how to measure with a clinometer. Here we will discuss the former. The next set of notes will discuss the latter.
A hand level is a small instrument involving a metal tube, with lenses on each end, a cross hair on which items are sighted, and a leveling bubble. It is a precision instrument, but one that is really only good for what might be called "close approximations." There are two ways in which vertical measurements can be made with a hand level. The first can be done by one person working alone, the second requires at least two people and a stadia rod.
Working Alone. Differences in elevation between two points can be measured by going through a series of steps. (1) The fieldworker needs to now the height of his or her eye above ground level. This can be done by standing against a wall, facing it, sighting horizontally, and then marking the wall at the cross hair. The distance from the mark to the floor is one's "eye height." (2) The fieldworker stands at a low point of known elevation (a benchmark or datum) and sights a level line through the hand level. (3) The point on the slope at eye height is then noted, usually in reference to a rock, twig, leaf, etc. The fieldworker walks to that point, stands on it and repeats the process. (4) This is done until the fieldworker's eye height is greater than the point whose elevation is being calculated. (5) At this point, the vertical distance between the point where the fieldworker is standing and the point whose elevation is desired is then estimated. (6) The sum of all the eye heights sighted and the last estimate are added together and then added to the elevation where the process began. Obviously, this procedure can only be used in determining elevations higher than a known point. Work must progress up-hill.[example]
Hand Leveling with a Stadia Rod. This procedure can progress up-hill or down-hill, but requires a second person. [example] It is also the same process professional surveyors use in determining elevational differences [example]. Several steps are involved and detailed notes are recorded in the field notebook. For purposes of illustration, we'll progress from a point of known elevation (a benchmark, BM, or datum) on top of a hill and progress to a lower elevation. One strategy [example, scroll down] involves: (1) One fieldworker, the rodperson, holding the stadia rod vertically on the benchmark, while the person with the hand level stands at a point further downslope, but at a point where eye level is higher than the benchmark. A sighting is taken on the stadia rod and recorded in the notebook as the "+ sight." This figure is then added to the elevation of the benchmark and recorded in the fieldbook as the "HI" for Height of the Instrument. If the workers are concerned with horizontal distances, a measurement is recorded from the BM to the instrument person. (2) The rodperson then moves the stadia rod to some point further downslope. This is called a "turning point," or simply a "turn." The person with the hand level turns around 180 degrees, but continues to stand on the same spot, and sights on the rod. A reading is taken and recorded in the fieldbook as the "- sight." This figure is then subtracted from HI and recorded in the fieldbook under the column "Elev". The distance from the instument person to the stadia rod is measured and recorded, if the job calls for it. (3) The process is repeated however many times as necessary until (4) the stadia rod is placed on the location for which the elevation is unknown, but desired.
A second strategy is similar, but involves the person with the instrument knowing hers or his eye height. This strategy involves (1) The person with the instrument standing on the benchmark at the top of the hill. [example, scroll down] He or she records the eye height in the fieldbook as the first "+sight. This is then added to the known elevation and recorded in the field book as the "HI." The rodperson then sets the rod at some point down the slope. A reading is taken on the rod and recorded in the fieldbook as the "-sight." This figure is then subtracted from the HI and recorded in the fieldbook under the column "Elev." If called for, the horizontal distance from the BM to the stadia rod is measured and recorded. (2) The process is then repeated, (3) as many times as necessary, until (4) the stadia rod is placed on the location for which the elevation is unknown, but desired.
Created by William E. Doolittle. Last revised 26 June 2013, wed