Guest Editorial: Double-Rodded Level Circuits Explained
Professional Surveyor Magazine - April 2008
The double-rodded level circuit is a time-saving leveling technique that Carl Parsons, a party chief from West Virginia, described to me many years ago. Since that time, I have been in many suitable situations to use the technique and am now happy to describe it to other surveyors for their benefit.
In doing research for this article, I have found the best text book reference to this technique in a surveying book entitled Surveying Theory and Practice, by John Clayton Tracy Ph.D., C.E. (1947 by John Wiley & Sons, Inc.). In his description of the procedure Mr. Tracy provides an alternate name, "Double-Line Leveling," which is a more accurate description of the technique because the procedure does not employ two rods as the more common name, the one that I have heard, implies. Mr. Tracey also provides two note forms for taking notes with the technique that are different than what I am showing in this article; the note form in this article appears to be the simplest to use and understand.
Basically, this leveling procedure involves running two separate level circuits between two points and keeping notes for both simultaneously. This involves setting two sets of turning points in pairs. There must be a minimum vertical distance of one foot (or meter) between each pair. In Figure A the pairs of turning points are shown horizontally separated for clarity, when in reality in the field they would be set very close together horizontally while maintaining a minimum vertical separation.
Like anything, the technique has its advantages and disadvantages. The advantages to a double- rodded leveling are:
- Twice as much work can be accomplished in approximately the same amount of time with little extra effort because two lines of levels are being run at the same time.
- Most common "busts" in leveling can be immediately detected and corrected as the work progresses because two separate instrument elevations are kept in the field book and they should be approximately the same value all the time.
The disadvantages are:
- The procedure can be confusing because at the beginning and ending of the two level circuits some foresights have to be taken before the backsights, contrary to normal procedure. Even experienced crews will have problems learning it. Mr. Parsons had to explain it to me several times before I came to understand it.
- The technique is not suitable for all types of terrain because you have to be able to set one turning point higher than the other within each pair of turning points. There is a potential for an undetected "bust" if the points are not set apart enough vertically and the foot (or meter) mark on the rod is misread by the instrument operator. The crew members have to be aware of this potential and take steps to eliminate it.
- Software is not commercially available to aid in the reduction or adjustment of the work. I wrote a program for the HP-48 that I use, but it is very rudimentary and its publishing here would not enhance this article. I suggest setting up a spreadsheet on a palm-top computer for reduction in the field.
In our example (Figure A), the level is set up at Instrument Point 1, and a foresight reading of 4.25 is taken at Turning Point 1 High and is recorded in the field book as shown in Figure B. The rod man then goes to the known point "A," and a backsight rod reading of 4.10 is taken and recorded as shown in Figure B.
The instrument man then changes the position of the level vertically. Another backsight reading of 4.02 is then taken on the rod at the known point and recorded. The rod man then moves to Turning Point 1 Low, and a foresight reading of 5.60 is taken and recorded (Figure B).
The level is then moved to Instrument Point 2, a backsight reading of 3.75 is taken and recorded on Turning Point 1 High, and a backsight reading of 5.18 is taken on Turning Point 1 Low. This gives us an instrument elevation on 103.60 on both the high and low sets of turning points.
The rod man then goes to the second set of turning points. A foresight reading of 3.89 is taken and recorded on Turning Point 2 High, then a foresight reading of 5.03 is taken and recorded on Turning Point 2 Low. This gives us elevations of 99.71 and 98.57 respectively (Figure B).
Step Two (Repeated)
The level is then moved to Instrument Point 3, a backsight reading of 3.36 is taken and recorded on Turning Point 2 High, and a backsight reading of 4.51 is taken on Turning Point 2 Low. This gives us an instrument elevation of 103.07 on the high set of turning points and an instrument elevation of 103.08 on the low set of turning points.
The rod man then goes to the third set of turning points. A foresight reading of 3.89 is taken and recorded on Turning Point 3 High, and a foresight reading of 5.03 is taken and recorded on Turning Point 3 Low. This gives us elevations of 99.18 and 98.05 on Turning Points 3 High and Low respectively (Figure B).
Step Two is repeated through as many pairs of turning points as required through the entire length of the level circuit.
Step Three (Final Step)
The level is then moved to Instrument Point 4, and a backsight reading of 2.27 is taken and recorded on Turning Point 3 High. The rod man then moves to Unknown Point "B," and a foresight reading of 4.33 is taken on Unknown Point "B," giving us an elevation of 97.12 for Point "B" from the high set of turning points.
The instrument man then changes the position of the level vertically. A foresight reading is then taken and recorded on Unknown Point "B." The rod man then moves to Turning Point 3 Low, and a backsight reading of 3.22 is recorded. This gives us a level elevation of 101.27 from the low set of turning points and an elevation of 97.14 on Unknown Point "B" (Figure B).
To achieve an adjusted elevation for Point "B," the elevation from the high set of turning point is averaged with the elevation from the low set of turning points, giving us an average elevation of 97.13 for Point "B." It would be possible to run a double-rodded level circuit using three wire leveling, but I have never had cause to do so. To be successfully done, a three-wire level circuit by the double-rodded method would require software that would process the data in the field as the work progresses. I am absolutely certain that the new digital laser scanning levels could very successfully do double-rodded level circuits, but their data collection and processing capabilities would have to be programmed by the manufacturers to do so. The reader is invited to test and experiment with this method.
About the Author
Terrance Mish received a Bachelor of Science Degree in Surveying from Ferris State College in 1981. He is a licensed land surveyor in Virginia and Texas.
» Back to our April 2008 Issue