Suppose you want to determine the elevation of a point at the summit of a long slope, and the nearest BM is at the foot of the slope some 30 ft or so below the summit. Obviously, you cannot sight a rod held on the BM and another held on the summit from the same instrument setup point. You must work up the slope in a series of steps, as shown in figure 14-12, by establishing as many intermediate TPs as you need to solve the problem. A "turning point" is defined as a point on which both a minus sight (FS) and a plus sight (BS) are taken on a line of direct levels.

As shown in figure 14-12, if we assume that the elevation of the BM is correct, the accuracy of the elevation you determine for the summit depends upon how accurately you determine the elevation of each intermediate TP. This accuracy depends upon a number of things, the most important of which are the following:

1. If you are doing leveling of ordinary precision, FS and BS distances should not exceed 300 ft. Therefore, the first setup point for the instrument should be not more than 300 ft from the BM, and the first TP should be not more than 300 ft from the instrument. To balance shots, you should place the instrument about the same distance from the BM as the distance to the TP.

2. Obviously, the first setup point must be one you can observe with a rod held on the BM and also a rod held on the first TP.

3. Generally, setup points should be used that make rod readings as small as possible. The reason small rod readings are desirable is that, for a rod held out of plumb, each reading on the rod will be in error. The larger the rod reading, the greater the error. Suppose, for example, a rod is so far out of plumb that it indicates 12.01 ft for a reading that should be 12.00 ft if the rod were plumb. For a 12.00-ft reading on the rod, the error is 0.01 ft. For a 2.00-ft reading on the same rod held in the same manner, however, the error would be only about 0.002 ft.

4. A TP must have not only visibility and accessibility, but also stability; that is, it must furnish a firm, nonsettling support for the base of the rod. Suppose you select a point in soft, yielding ground as your first TP. Assume the elevation of the BM is 312.42 ft. You take a BS on the BM and read 3.42 ft. Then, HI is 312.42 + 3.42 = 315.84 ft.

The rodman shifts the rod to the TP. You take an FS and read 5.61 ft. The elevation of the TP is, therefore,

Now, you shift the instrument ahead and take a BS to carry on the line of levels to a new TP. But suppose that before you take the BS on the rod, the TP has settled 0.02 ft in the ground. Then you take a BS and read 4.74 ft. There is now an error of 0.02 ft in the new HI, and every subsequent HI and elevation of TP will be off by the same amount.

So BE SURE that each TP is stable. When the use of a point in yielding ground is unavoidable, you need to base the rod on a turning point pin or turning point plate. A pin is driven in the ground; if you dont have a regular pin, a marlinspike or a railroad spike makes a good substitute. You should use a plate on soil too soft to support a driven pin.

Leveling methods are subdivided into two major categories: DIRECT and INDIRECT. Direct leveling describes the method of measuring vertical distance (difference in elevation) directly with the use of precise or semi-precise leveling instruments. Indirect leveling methods, on the other hand, apply to measuring vertical distances indirectly or by computation. Unlike direct leveling operations, indirect leveling operations do not depend on lines of sight or intervisibility of points or stations. Some of the surveying instruments commonly used for indirect leveling methods are the transit and theodolite.

This method of leveling uses the measured vertical distance to carry elevation from a known point to an unknown point. Direct leveling is the most precise method of determining elevation and yields accuracies of third or higher orders. When this method is specified for lower accuracy surveys, direct leveling is sometimes referred to as "spirit" or "fly" levels. Fly levels are leveling operations used to rerun original levels to make sure that no mistake has been made. Fly levels use a shorter route and smaller number of turning points than the original survey. Lets take a look at some of the processes involving direct leveling.