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LOCATING DETAILS BY
TRANSIT AND TAPE In the EA3 TRAMAN you studied the procedures used to tie in and locate points, using a transit and tape. These same procedures are used for tying in and locating topographic details. Determine the vertical location (or elevation) of the detail points, using direct or trigonometric leveling procedures. Horizontally locate the details either by directions or distances or a combination of both. Use a method, or a combination of methods, that requires the least time in a particular situation. Directly measure the dimensions of structures, such as buildings, with tapes. When details are numerous, assign each one a number in the sketch and key the detail to a legend of some kind to avoid overcrowding. For directions, use azimuths instead of deflection angles to minimize confusion. Locate details as follows:
Figure 8-2.-Stadia hairs. 1. measure the angle and distance from transit stations 2. measure angles from two transit stations 3. measure distances from two known points 4. measure an angle from one station and distance from another station 5. measure swing offsets and range ties As you can well imagine, detailing by transit and tape is a time-consuming process that requires chaining many distances and taking many level shots. This is necessary when a high degree of accuracy is required. However, for lower-precision (third and fourth order) surveys, a less time-consuming method is to locate the details by transit and stadia. LOCATING DETAILS BY TRANSIT AND STADIA As an EA, most of the topographic surveying that you will do is of a lower degree of accuracy that is well suited to the transit and stadia method. When you are using this method, horizontal distances and differences in elevation are indirectly determined by using subtended intervals and angles observed with a transit on a leveling rod or stadia board To explain the meaning of this, we will first discuss the principles of stadia and then look at field procedures that are used in stadia work. Stadia Equipment Terms, and Principles The following discussion will familiarize you with the equipment, terminology, and principles used in stadia surveying. Although this discussion of stadia surveying is included in this chapter on topography, you should be aware that stadia can be used in any situation in which it is desired to obtain horizontal distances and differences in elevation indirectly. The results, though, are of a lower order of precision than is obtainable by taping, EDM, or differential leveling. However, the results are adequate for many purposes, such as lower-order trigonometric leveling. A thorough understanding of stadia is highly important to any surveyor. You should supplement the knowledge that you gain from the following discussion by reading other books, such as Surveying Theory and Practice, by Davis, Foote, Anderson, and Mikhail.STADIA RODS. Where sight distances do not exceed 200 feet, a conventional rod, such as a Philadelphia rod, is adequate for stadia work. For longer distances, however, you should use a stadia rod. Stadia rods usually have large geometric designs on them so that they may be read at distances of 1,000 to 1,500 feet or even farther. Some rods do not have any numerals on them. From the geometric pattern on the rod, you can observe intervals of a tenth of a foot and sometimes a hundredth of a foot.Stadia rods generally are 10 to 15 feet long, 3 to 5 inches wide, and about 3/4 inch thick. They may be made in one piece or in sections for ease in carrying them. Some stadia rods are flexible and maybe rolled up when not in use. Flexible rods are merely graduated oilcloth ribbons, tacked to a board. Some examples of stadia rods are shown in chapter 11 of the EA3 TRAMAN. STADIA HAIRS. The telescope of transits (as stadia hairs, that are in addition to the regular vertical and horizontal cross hairs. Figure 8-2 shows two types of stadia hairs as viewed through a telescope. As shown in this figure, one stadia hair is located above and the other an equal distance below the horizontal (or middle) cross hair. On most equipment, the stadia hairs are not adjustable and remain equally spaced.STADIA INTERVAL. As you look at a stadia rod through a transit telescope, the stadia hairs seem to intercept an interval on the rod. The distance on the rod between the apparent positions of the two stadia hairs is the stadia interval or stadia reading.Usually, you determine stadia intervals by sighting the lower stadia hair at a convenient foot mark and then observing the position of the upper stadia hair; for example, the lower hair might be sighted on the 2.00 foot mark and the upper hair might be in line with 6.37. By subtracting, we have the stadia reading (6.37 -2.00 = 4.37). It may happen that the stadia reading is more than the length of the rod. By using the middle hair, you may observe a half-interval and multiply it by 2 to get the stadia reading. STADIA CONSTANT. Light rays that pass through the lens (objective) of a telescope come together at a point called the principal focus of the lens. Then these light rays continue in straight-line paths, as shown in figure 8-3.The distance between the principal focus and the focal length(f) of the lens. (i), you get a number known as the stadia constant (k). Sometimes the stadia constant is called the stadia factor or stadia interval factor. A convenient value to use for the stadia constant is 100. Stadia hairs usually are spaced so that the interval between them will make the stadia constant equal to 100. STADIA DISTANCE. The distance from the stadia As shown in figure 8-3, this distance (d) is
Figure 8-3.Light rays converge at principal focus of a lens. equal to the stadia constant (k) times the stadia reading (s). |
 
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