Laying off an angle of a given size

Custom Search

Solving Surveying Problems by Tape

Basic Engineering

Caring for and Maintaining a Survey Tape

LAYING OFF AN ANGLE OF A GIVEN SIZE. An angle of a given size can be laid off by tape by applying the tangent right triangle solution. Suppose that in figure 12-21, you want to lay off a line AC from A, 25 from line AB, Again measure off a convenient 80.0 ft from A to establish point B. Erect a perpendicular from B as shown by the dotted line. You want to measure off along this perpendicular side a (opposite side), the distance that, when divided by the adjacent side, will give the value of the natural tangent of 25. Use the following formula:

Measure off 37.3 ft from B to establish point C. A line from A through C will form an angle of 25 from AB.

Identifying Chaining Mistakes and Errors

In surveying, distinctions are made between ERRORS and MISTAKES. Errors are caused by factors such as the effects of nature, the physical condition of the personnel performing the survey, and the condition of your instruments. Mistakes, however, are simply human blunders. While errors may be compensated for, mistakes can be detected, correct, and better yet, prevented only by the exercise of care.

COMMON MISTAKES. Mistakes may result from poor work habits, lack of judgment, or confusion. They are often costly, time consuming, and difficult to detect. The easiest way to avoid them is to establish a definite procedure and follow it, being constantly alert during the

Figure 12-21.-Laying off an angle of a given size.

operations in which mistakes are possible. Some of the more common mistakes are as follows:

. Failing to hold graduations plumb over points

. Involuntarily transposing figures, such as recording 48.26 for 48.62

. Misreading figures that are viewed upside down, such as recording an upside-down 9 as a 6

. Reading a subdivided end-foot from the wrong end, as, for example, 0.28 ft instead of 0.22 ft

. Associating subdivided end-foot reading with wrong whole-foot mark, as 38.21 ft instead of 37.21 ft

. Subtracting incorrectly when using a minus tape

. Omitting an entire tape length

RECOGNIZING COMMON ERRORS. There are two types of errors: accidental and systematic.

An accidental error is, generally speaking, one that may have a varying value. Examples are as follows: variation of the tension applied to the tape, inaccurate sticking of pins or other markings, and inaccurate determination of slope. Accidental errors can be minimized by careful-ness, but not entirely eliminated.

A systematic error has a constant value. The standard error in a tape, for example, is a systematic error. Temperature and sag corrections are applied to correct systematic errors. Systematic errors can be compensated for or otherwise eliminated by the application of corrections.