Surveying For the Homestead

(Page 3 of 6)

September/October 1975
By Aimee Gelwick  
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When you've successfully sighted the target on the cross hairs, have the rodman carry the indicator down to the next corner . . . and while he's trucking, record the elevation you read from the rod. This figure is your backsight or B.S. (see the sample field notes in Fig. 11). Assume, for convenience, that the corner stake's elevation is exactly 100 feet and add the B.S. reading to 100 to get the height of your leveling instrument (H.I.). Note this in the next column.

When the rodman is ready, again sight the target (HOW'S THE BUBBLE DOING?) and read the elevation. Record this figure as the foresight (F.S.), which is subtracted from the instrument height to give the elevation of the second corner stake. (As a supplement to these calculations, draw and describe the corner points on the second page of your notes to aid your memory later.)

Move the tripod, set the level up between the second and third corners, and repeat the same procedure . . . measuring the elevations of all turning points until you wind up betweenNo. 3 and the spot where you started.

And here's how to check the accuracy of your work: The foresight you read on Corner No. 1 plus the height of the instrument (H.I.) should add up to a figure that matches the first elevation ... in this case, 100 feet. For general purposes, an error of .1 foot isn't too far off. Differences over .1 foot, however, should be remeasured by one of two methods: [1] Start over completely. [2] Return to a doubtful area, sight on a corner with an elevation you're sure of, and rework from there back to the original starting point. Be careful to place the instrument in locations that allow good visibility of both target spots.

If the points you wish to compare are too far apart for the method I've described, place a stake at some intermediate point close enough to be sighted and take the reading from that spot as if it were a corner. Record this station as a turning point (see the sample field notes in Fig. 11). If an obstacle stands between points for which you want the relative elevations, sight on a stake placed to one side of the barrier and record this too as a turning point (see Fig. 12).

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