The protractor also has a limited precision, so there is always some uncertainty
ID: 2161888 • Letter: T
Question
The protractor also has a limited precision, so there is always some uncertainty in the angle readings. This uncertainty will also propagate through the calculation of the weight, just like the uncertainty in the force readings does. The relationship between weight and the angles is not linear, though, so the propagation is not as straightforward.The easiest way to proceed is by looking at the highest and lowest values of the calculated quantity.Example: In a given experiment, a force is measured to be (483
Explanation / Answer
Basic field operations performed by a surveyor involve linear and angular measurements. Through application of mathematics (geometry and trigonometry) and spatial information knowledge,the surveyor converts these measurements to the horizontal and vertical relationships necessary to produce maps, plans of engineering projects, or Geographical Information System/ Land Information System (GIS/LIS). The highway surveyor must be adept at making the required measurements to the degree of accuracy required. Various types of engineering works require various tolerances in the precision of the measurements made and the accuracies achieved by these measurements. The use of common sense and development of good surveying practice in all phases of a survey cannot be overemphasized. All conditions that may be encountered in the "real world" during the actual field survey cannot be covered in any manual. A manual may specify certain techniques, such as a certain number of repeated operations, to achieve a required accuracy. The surveyor must then often use judgment based on the equipment being used and the field conditions encountered, to modify those techniques. Some field conditions (heat waves or wind for example) may make it impossible to perform some operations to a consistent degree of accuracy. 3.2 Accuracy and Precision Back to top 3.2.1 Accuracy Accuracy is the degree of conformity with a standard or accepted value. Accuracy relates to the quality of the result. It is distinguished from precision that relates to the quality of the operation used to obtain the result. The standard used to determine accuracy can be: An exact known value, such as the sum of the three interior angles of a plane triangle is 180°. A value of a conventional unit as defined by a physical representation thereof, such as the international meter. A survey or map value determined by superior methods and deemed sufficiently near the ideal or true value to be held constant for the control of dependent operations. Although they are known to be not exact, higher order NGS control points are deemed of sufficient accuracy to be the control for all other less exact surveys. 3.2.2 Precision Precision is the degree of refinement in the performance of an operation (procedures and instrumentation) or in the statement of a result. It is a measure of the uniformity or reproducibility of the result. 3.2.3 Accuracy Versus Precision The accuracy of a field survey depends directly upon the precision of the survey. Although through luck (compensating errors, for example) surveys with high order closures might be attained without high order precision, such accuracies are meaningless. Therefore, all measurements and results should be quoted in terms that are commensurate with the precision used to attain them. Similarly, all surveys must be performed with a precision that ensures that the desired accuracy is attained. However, surveys performed to a precision that excessively exceeds the requirements are costly and should be avoided. 3.3 Errors and Classification of Accuracy Back to top 3.3.1 General Statistically speaking, field observations and the resulting measurement are never exact. Any observation can contain various types of errors. Often some of these errors are known and can be eliminated by applying appropriate corrections. However, even after all known errors are eliminated, a measurement will still be in error by some unknown value. To minimize the effect of errors, the surveyor has to use utmost care in making the observations and utilizing only calibrated equipment. However, a measurement is never exact, regardless of the precision of the observations. Although this manual contains many guidelines and standards, the ultimate responsibility for providing surveys that meet desired accuracies remains with the field personnel. To fulfill this responsibility, the crew chief and his or her assistants must understand errors, including but not limited to: The various sources of errors. The effect of possible errors upon each observation, each measurement, and the entire survey. Economical procedures which will eliminate or minimize errors and result in surveys of the desired accuracies. 3.3.2 Blunders Many textbooks on surveying refer to a blunder as a gross error. One can easily make a case for a blunder to be considered an error. However, a blunder is really an unpredictable gross mistake made by the surveying team. It is not a hidden error that will go unnoticed, but usually it becomes apparent that something is wrong with the measurements. Examples of blunders are: Transposing two numbers (in field notes or computer input.) Misplacing decimal point. Incorrect reading (i.e. the foot value on a leveling rod.) Inadvertently altering set instrument constants in the middle of a project. Placing sighting device or the instrument at a wrong point. Misunderstanding verbal instructions or reading announcements (call out). Neglecting to level an instrument. Using the incorrect coordinates or benchmark values. Blunders are caused by carelessness, misunderstanding, confusion, or poor judgment. They are, for the most part, avoided by alertness, common sense, and good judgment. Blunders are detected and eliminated by using proper procedures, such as: Checking each recorded and calculated value. Making independent and redundant measure check observations and measurements. Making redundant measurements that allow closure computation of sections of the entire survey. Small blunders are more difficult to detect and correct especially if the number of redundant measurements is too small. Therefore, surveys must be carried out with sufficient redundancy to prevent a blunder from going undetected. All blunders must be eliminated prior to correcting and adjusting a survey for errors.
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