If that last installment about trig levels made your head hurt, then this should be a relief. Differential or "conventional" leveling is a surveying technique to carry elevations from a known point to various unknown points. In other words, they help us to determine the elevations of unknown points if we know the elevation of one point. A conventional level does not have a telescope that rotates up and down. (Or, "transits" as we call an instrument that does just that.) Today's conventional levels are precisely set to maintain a level line of sight around a 360° viewing area. The horizontal position of the instrument is not usually so crucial. It is placed in a position of convenience which allows a view of the backsight (known elevation point) and foresight (unknown elevation point). In the old days, levels were much more temperamental to set up. They had four-foot screws that had to be adjusted against each other to maintain an accurate level. Today's automatic levels can be set up in moments. They use a delicately suspended sensor to maintain the level with the help of three-foot screws. With the advent of digital levels, barcode-reading beams have replaced the telescope itself. The principles of leveling, however, have not changed one iota. The real limitation of differential leveling is that the line of sight at eye height must be maintained from backsight to foresight. So, in undulating terrain, the set-ups will be greatly increased in number as the team steps its way up or down the slope, limited by the practical height of the rod. The rod may be any extendable graduated pole or stick and there are many variations of that simple concept on the market. For greatest flexibility in varying terrain, a telescoping fiberglass rod of up to 25' is best. In Phoenix you probably don't need more than 13', although the uses these rods are put to often go far beyond mere leveling. In any event you should keep field notes of your work, recording readings as you go and maintain a close similarity to this illustration.
The abbreviations in the above illustration stand for:
All readings associated with a given point are on the same line. The sight on the benchmark is a BS and the value is always added to a known elevation to produce the HI. The FS value is always subtracted from the HI to produce the next known elevation. An IFS is a sideshot which is not subsequently used as a BS and therefore has an unverified value. All the turning points and monuments here have an associated BS and FS and are therefore part of loop that, in this case closes back on the point of beginning (POB). The misclosure is 0.02' and is probably acceptable with no required adjustment to the sighted elevations. There are formulae for setting standards for loops but suffice it to say that you can expect some misclosure that may be distributed to every point as long as it is not excessive. If you have 15 turning points and a misclosure of +0.05' you can give 0.01' to every third point remembering that the correction is cumulative and that the last corrected point will have a new elevation 0.05' above (or below if the error was negative) the raw data reading. |
