Quantcast Identifying Leveling Mistakes

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Identifying Leveling Mistakes

The leveling mistakes discussed here are not intended to include all possibilities but will give an idea of the more common ones. The survey party personnel should be aware of these possibilities and should be careful to avoid these mistakes. Some of the common mistakes are as follows:

1. Not setting the rod on the same point for an FS and the following BS. Using a turning pin, pedestal, stake, or marking the location with chalk on hard surfaces will help you to recover the identical point.

2. Neglecting to clamp the target or the rod when extended. Any slippage can pass unnoticed and result in a wrong reading that may require an entire rerun of the line to discover the mistake. The rodman should watch the rod or target for any movement as the clamp is tightened. The rod extension or target should be read again after the clamp has been set.

3. Reading the wrong mark. This is a common mistake. The figures on a rod may be obscured by brush or may fall in a position in the field of view so that the instrumentman cannot see two consecutive numbers. Under these conditions, he may read the wrong mark or even read in the wrong direction. This is a great possibility when an inverting eyepiece is being used. For example, if the figure 2 is the only number visible, the instrumentman might read "up" the rodó2.1, 2.2, 2.3 when actually he should be reading 1.9, 1.8, 1.7. Another possibility is miscounting the number of divisions. There is no way to check or discover these mistakes except to be aware of their possibility and to read carefully.

4. Recording a reading in the wrong column. In leveling, readings are not entered into the notebook in a normal sequence, such as left to right across the page. There is always a chance that one or more values may be recorded in the wrong column. The recorder must be alert to avoid making this mistake.

5. Reading the wrong angle sign in trigono-metric leveling. The instrumentman can accidentally call out a wrong sign in reading the angle. This type of mistake can be eliminated by the recorder watching the telescope as a pointing is made on the rod. If the wrong one is called out, both the recorder and the instrumentman can resolve it immediately.

6. Recording the wrong sign. The sign varies depending on whether the rod reading is a BS or an FS, and whether the angle is a depression or an elevation. Also, the difference in elevation computation requires a sign reversal if the angle is read for the BS, but not for the FS. These variations can be confusing; the recorder has to be careful to avoid mistakes. This can be done by recording the angle and rod reading signs as read. The sign conversion, if needed, shows up when you compute the DE. Examining the computations to see if all BS DEs have a sign opposite to the angle sign is simple.

7. Subtracting the BS or adding the FS in differential leveling. If the BS or FS is recorded properly (see Number 4 above), you can discover the mistake when you add the BS column and the FS column for a computation check. 

8. Using the wrong horizontal cross hairs. This occurs on an instrument provided with stadia hairs.



 


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