Surveying, says Aimee Gelwick, isn’t by any means as complex and mysterious as many lay persons believe . . . and it can be a very useful and enjoyable skill for the homesteader to learn how to survey your property. In Part 1 of this article, How to Survey Land, the author described how to measure distances and record the findings in field notes. She now turns to the two other basics of the surveyor’s art: the determination of comparative elevations and the measurement of angles on the land.
How to Survey Your Property: Elevations
Problems to do with leveling and relative elevations require the following equipment:
[1] A hand level, or surveying level with tripod. The tripod mounted instrument resembles a telescope and can be rotated on its base to indicate where a single elevation will fall anywhere on a 360-degree circle around the level’s location.
[2] A leveling rod with target (see Figure 9 in the image gallery).
[3] A set of chaining pins (see Figure 1 in Part 1, How to Survey Land).
[4] A 100 foot steel or metallic woven tape (optional).
[5] Notebook and pencil.
Let’s suppose, for example, that you’ve checked your area’s building codes and are about to pour a concrete foundation for that homestead cabin. Here’s how you can use surveying techniques to be sure of a level underpinning for your house.
First, place stakes at the corners of the foundation site and run a string around the tops of the markers. You’ll then be able to sight the string through the level and adjust the stakes by pounding them into the ground until the cord is horizontal (see Figure 10 in the image gallery). (Or, as is sometimes done, the elevations of the stakes can be compared by setting a target rod on top of each in turn and shooting a reading of the rod as it rests on each stake.)
Set up the level as follows: Screw the instrument to the tripod head, remove the lens cap, and put on the sunshade. Loosen the thumbscrews of the stand and spread the legs, which should be pushed into the ground to make the top of the tripod as nearly horizontal as possible. Then tighten the thumbscrews again. (It’s a good idea at this point to take a practice sight on the string or one of the stakes to be sure you I’ve placed the stand in a favorable working position.)
The next step is to level the sighting instrument on its tripod. (This is a fine operation, and must be rechecked frequently as you work.) Loosen the telescope’s locking clamp and swing the instrument to a position directly over one pair of leveling screws. Adjust the pair simultaneously — by turning them in opposite directions — to level the instrument’s bubble tube. Then tighten the screws firmly (without jamming them), turn the telescope 90 degrees so that it lines up with the other pair of leveling screws, and center the bubble again. The process is complete when the telltale bead remains in exactly the same position as you rotate the instrument through 360 degrees.
Now sight a stake and focus in the telescope’s cross hairs by adjusting the eyepiece. Then use the objective focus knob to bring the marker’s top into clear view. RECHECK THE BUBBLE at that time and — if it gives a satisfactory reading — drive in the corner stake until the string is in line with the horizontal cross hair.
Next turn the telescope and follow the string to about the center of its length on one side of the foundation . . . and pound in the next corner marker until the cord lines up with the level’s cross hair. INSPECT THE BUBBLE ONCE MORE, continue sighting along the string to the stake and check its level with the instrument. Refocusing should allow you to adjust all the markers in the same way without moving the tripod. (If the foundation is very large, you’ll find it helpful to place an extra stake at the midpoint of each side.)
The level can also be used to solve problems of relative elevation. You might, for instance, want to know the comparative heights of your property’s corners in order to avoid building that new cabin in a low area where flooding might later be a hazard. The principle is the same as before, but this time instead of the stakes alone — you’ll need a target rod and a friend to hold it for you.
Set up the sighting instrument as I’ve described, above and between the first two corners you want to compare. Then, while your helper holds the rod on a stake driven into the ground at one corner, focus on the target and read the height of the marking which is directly in line with the level’s cross hair. Have the radioman tilt the indicator backward and forward slightly to make sure you’re recording the lowest reading.
If the rod is too low to be seen, signal your assistant to raise it. This is done by setting the target on point 7.00 and loosening the rod’s side screw so that you can extend the measure. Then the top half of the device can be raised and lowered until you sight the target on the cross hair. Read the vernier scale located to the side of the target face if you want to get your accuracy down to hundredths of a foot.
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 Figure 11 of the image gallery). 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 between No. 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 Figure 11 in the image gallery). 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 Figure 12 in the image gallery).
When you’re working with a long distance which requires several turning points between corners, a check for accuracy can be made as you go along. Just add up the elevations for backsights between, say, Corner No. 1 and Corner No. 2, do the same with the foresights, and compare the totals. The difference between the total backsight elevation and the total foresight elevation should equal the difference between the elevations of the first and second corners. (For example, total B.S. = 15.60 feet, total F.S. = 20.15 feet, difference = 4.55 feet . . . which should also be the difference between the elevation at Corner No. 1 and the elevation at Corner No. 2.)
Laying Out Angles
The following equipment is needed to lay out angles on the land:
[1] Engineer’s transit and tripod (see Figure 13 in the image gallery). The transit resembles a level in having a telescope which is rotated on one plane . . . but instead of sighting elevations on target rods, it records the angle traveled as the telescope is turned between two points. This instrument comes in handy for making square corners or (more creative) acute or obtuse angles.
[2] Two range poles.
[3] A set of 11 chaining pins.
[4] A 100-foot steel or metallic woven tape (optional).
[5] Notebook and pencil.
The method of turning angles with a transit will be of special interest to any reader of MOTHER EARTH NEWS who wants to construct a dome . . . particularly a large dome. A structure such as a barn, for instance, would require support beams placed around the sides at equal intervals, and the strength of the shelter would depend partly on the accuracy of their location. That accuracy is no problem if you have access to — and can use — one of these instruments. A transit set up in the dome’s center can give you the desired spacing by dividing the 360 degrees of the circle into equal angles (see Figure 14 in the image gallery).
Let’s assume for convenience that you plan to place 36 beams . . . in which case the angles must be 10 degrees each.
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 Figure 15 in the image gallery). 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.
Transits are also used to locate land boundaries. If you’re trying to find a property line from old surveying records, for instance, set the instrument up over the points described (stationary locations called bench marks) and turn the prescribed angles in the manner you’ve already learned. The procedure in this case is to focus on one landmark and turn the telescope the required number of degrees. (The compass in the transit’s top plate will help you get oriented.) Then place a range pole and chaining pin at the starting point and pace off the required distance to the next station. See Figure 16 in the image gallery for a field notebook record of transit work.
For factual data on surveying and bench marks in your area, write the U.S. Coast and Geodetic Survey, Department of Commerce, Washington, D.C. Your nearest Government Land Office should also be able to give helpful suggestions. In addition, state senators receive pamphlets and other information from time to time describing lands in their area. Contact the Bureau of Information at your state capital for particulars.
Good luck, and don’t let the technical side of surveying scare you off. There’s really nothing mysterious about land measurement . . . and you’ll find that a command of its basics is a very useful and enjoyable homestead skill.
