HOW TO USE TRANSITS & LEVELS ...A concise, easy-to-understand guide to the use of a basic construction tool. S urveying, if you check the dictionary, sounds like a discouragingly difficult procedure. In practical language, however, it simply means accurate measurement.* To the contractor, it’s a form of insurance against a foundation winding up two inches over somebody else’s property line. Surveying instruments, then, are not the formidable mechanisms they might seem at first glance. They are a means of locating a house properly on a lot, leveling a foundation, establishing grades, and all the many other functions involving accurate measurement which are so important to a contractor. There are two basic types of builders’ instruments—the optical level (sometimes called the dumpy level) which has its telescope fixed in a horizontal position, and the transit-level, which turns not only sideways, but also up and down. This enables you to determine whether a wall is perfectly plumb. The transit-level also allows you to run straight lines and to measure vertical angles. The optical level consists of a telescope, leveling screws, the vial or “bubble” and a 360-degree scale. The telescope lenses magnify so that a 20-power telescope makes an object seem 20 times closer than when viewed with the naked eye. The greater the magnification, natu- rally, the greater the distances over which the instrument can be used. Cross hairs within the telescope enable you to center your target. The clarity of the image within the telescope will probably surprise you the first time you examine a precision instrument of this sort. Before you line up the telescope, though, you must first level the instrument by means of the leveling vial and leveling screws. The vial located on the telescope works much the same as the ordinary carpenter’s level, except that it is a great deal more sensitive and accurate. By turning screws A and B, and watching the bubble carefully, the instrument can be leveled on the A/B axis. The telescope is then given a quarter-turn to the C/D axis and leveled again. After rechecking, the telescope can then be turned around the complete 360-degree circle and found to be perfectly level in every direction. The circle could be considered like two of the protractors you used in school, except that it makes a complete 360-degree circle rather than the 180-degree half-circle you had on your protractor. The circle on a level can be rotated independently so that it can be set at zero no matter which way the instrument is pointing. Starting at zero and rotating the telescope on it, we can now measure any horizontal angle. The 90-degree angle is the one most commonly used in construction work, so the circle is divided into four equal parts (or quadrants) to make it easy to “turn” the telescope over this angle quickly and accurately. Some instruments are also equipped with vernier scales, which break the measurement down into minutes (with 60 minutes in each degree). This sort of accuracy is especially important when sighting over long distances, or engaging in precision construction. The tripod, of course, is the allimportant foundation element. On some instruments, the tripod shoes also have spurs which allow the tripod to be seated firmly in the ground. Now let’s assume that you want to establish the foundation corners of a house: 1. You start with an empty lot. 2. You intend to put this rectangular shaped house on it, and you need to square off the four corners accurately. 3. Let’s assume we have established the front of the building line—the line from A to B. We must now complete the rectangle. We start by setting up our instrument at A, leveling it up and pointing toward B. Then we set the circle or pointer to 0 degrees. 4. Now, we swing our telescope exactly 90 degrees, which establishes the direction of point C. We measure off the correct footage from A to C * The original material from which this text was adapted is contained in a pocket handbook prepared by C. L. Berger & Sons, Inc., of Boston, Massachusetts. Much of the text was originally prepared by Herman J. Shea, former Associate Professor of Surveying, Massachusetts Institute of Technology. with a tape and drive in a stake at C. 5. Now, we move our instrument to point C. We sight back on A and set the horizontal circle at zero. 6. Next, we swing our level 90 degrees and establish point D. Tape the length and drive in a stake at D. 7. We have now established our foundation corners. A perfect rectangle has been established in a matter of minutes. The line of sight through the telescope is a perfectly straight line for as far as you can see. You can be assured that it is straight because it is a line without weight. It cannot sag, as a length of cord might. Any point along the line of sight is exactly level with any other point. To determine the difference in elevation between two points which can be observed from one position, set up the instrument about midway between these points. Each point should not be more than 150 to 200 feet away from the instrument. The height of the line of sight (horizontal cross hair) above or below each of the points is found by reading a measuring rod held by a co-worker. Figure 1-A shows a line of sight 69 inches above A and 40 inches above B. Simple arithmetic establishes that B is higher than A by 29 inches. To go a bit further, suppose you are working with one point below the line of sight, and the other above. In Figure 1-B, point C is 4feet 6 1/2 inches below the line of sight, and point D, the underside of a floor beam, is 7 feet 9 3/8 inches above the line of sight. This reading was determined by holding the rod upside down with the foot of the rod against the beam. Again, arithmetic tells us that point D is higher than point C by the total of the two measurements—4 feet 6 1/2 inches plus 7 feet 9 3/8 inches, or a total of 12 feet 3 7/8 inches. Differences in elevation requiring more than one set-up may be determined by the method shown in Figure 2. To find the difference in elevation between points A and D, we simplify it by using the terms plus (+) sight and minus (—) sight and carry the readings at each setup as shown. The difference of elevation between D and A is found by taking the difference between the sum of the plus sights and the sum of the minus sights. If the sum of the plus sights is larger, the final point is higher than the starting point. If the sum of the minus sights is larger, the final point is lower than the starting point. Many projects, especially buildings and roadways, are required to be built at specified grades or elevations. A point of known elevation is necessary to establish these grades, and this known point is usually called a benchmark. A benchmark should be a definite point, and it should be located outside the construction area. It could be a bolt on a water hydrant, the corner of a stone monument or building, or a spike in the root of a tree. Several benchmarks are helpful for a large job. The grades may then be computed by the “difference in elevation” method described in Figure 1 and Figure 2. Figure 3 shows how to measure horizontal angles such as EFG. The instrument is centered over point F, and rotated until point E is in line with the vertical cross hair and set the horizontal circle to read zero. Next, swing the telescope toward point G until the vertical cross hair is exactly on point G. The horizontal index pointer will have rotated about the horizontal circle by an amount equal to the angle EFG. If the instrument is equipped with a vernier scale you will be able to read the angle more closely than a single degree. When setting grades for a driveway, place the grade stakes on both sides of the driveway, usually at equal distances from the centerline, as in Figure 4. Assume that the rod reading for the driveway at this section is 4 feet 10 inches. Grade stakes A and B are set at equal distances from the centerline. In marking grades on A and B, mark an even number of feet of either grade, or a cut or fill. Any convenient reading, so that the bottom of the rod is somewhere along the stake, is used to note the grade. At point A, a rod reading of 2 feet 10 inches is obtained, and a line is placed on the grade stake. This line represents a cut of 2 feet and is marked as in Figure 5. In like manner, a rod reading of 7 feet 10 inches is found at point B. This is marked as a fill of 3 feet according to Figure 5. To use these grade stakes, a line is stretched from the marking at A to a point 5 feet above the marking at B. This line then indicates 2 feet above the driveway grade. The use of a transit level, of course, extends the versatility of the instrument because the telescope can be tilted up and down. This facilitates setting points in line, measuring vertical angles when the instrument is so equipped, plumbing and defining slopes and grades. Either the optical or the transit level may well prove to be one of the most useful of all the contractor’s tools, and well worth the bit of time it takes to master the techniques. PUBLICATION #C670337 Copyright © 1967, The Aberdeen Group All rights reserved