Surveying For the Homestead

(Page 5 of 6)

September/October 1975
By Aimee Gelwick  
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Find the center point of the dome (by measurement with a steel tape) and mark it with a tack driven into a stake. Then set up a borrowed or rented transit by screwing it to the tripod head. Remove the lens cap and replace it with a sunshade.

The tripod is equipped with a plumb bob on a string (which can be taken up with a slip knot so that the weight will neither hang too high nor drag on the ground). Your aim is to adjust the stand so that the bob falls within half an inch of the dome's center point. It's easiest to position the tripod by planting two legs in the ground and moving the third. Fine adjustments can then be made with the leveling screws.

To prepare the transit for use, level the bubble tubes by the procedure described under "Elevations". The operation is complete when both bubbles on the transit remain centered as the instrument is turned.

Place one range pole at a starting point where a support beam is to be located . . . and insert a chaining pin into the ground just in front of the pole (a slender indicator gives more accurate readings). Focus on the marker, making sure it's vertical. Next, with all clamps slightly loosened, turn the transit's lower plate until the vernier index and the zero end of the circle are opposite one another (see Fig. 15). Tighten the upper clamp and make your final adjustments with the upper tangent screw. Focus accurately on the pin and repeat the procedure with the lower clamp and tangent screw. You'll then have a straight sighting from the center of the dome to the first marker pole.

By loosening the upper clamp, turning the telescope exactly 10 degrees, and sighting a pin held at the circle's perimeter, you can determine the site of the second beam (we've assumed, remember, that you intend to have 36 of the uprights), Tighten the upper clamp and pinpoint the marker at 10 degrees 00 minutes . . . and continue to rotate the telescope in the same way around the entire circumference.

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