The important distinction between accuracy and precision in surveying is explained as follows: 

l Accuracy denotes the degree of conformity with a standard. It relates to the quality of a result and is distinguished from precision, which relates to the quality of the operation by which the result is obtained.

The accuracy attained by field surveys is the product of the instructions or specifications to be followed in doing the work and the precision in following those instructions.

For example, the "accuracy of a surveyors tape" means the degree to which an interval of 100 ft, as measured on the tape, actually agrees with the exact interval of a standard 100-ft tape.

If a tape indicates 100 ft when the interval it measures is only 99.97 ft, the tape contains an inaccuracy of 0.03 ft for every 100 ft measured.

The accuracy of this particular tape, expressed as a fraction, is 0.03/100, or approximately 1/3,300.

l Precision denotes degree of refinement in the performance of an operation or in the statement of a result. It relates to the quality of execution and is distinguished from accuracy that relates to the quality of the result. The term precision not only applies to the fidelity of performing the necessary operations but, by custom, has been applied to methods and instruments used in obtaining results of a high order of accuracy. Precision is exemplified by the number of decimal places to which a computation is carried and a result stated. In a general way, the accuracy of a result should determine the precision of its expression. Precision will not have significance unless accuracy is also obtained.

If you measure a linear distance with a tape graduated in feet that are subdivided into tenths, you can read (without estimation) only to the nearest tenth (0.1) of a foot. But with a tape graduated to hundredths of a foot, you can directly read distances measured to the nearest hundredth (0.01) of a foot. The apparent nearness of the second tape will be greater; that is, the second tape will have a higher precision.

Completely precise measurement is impossible in the nature of things. There is always a built-in or inherent error, amounting to the size of the smallest graduation. Precision for the first tape above, expressed as a fraction, is 0.1/100 or 1/1,000 and for the second tape, 1/10,000. Precision in measurements is usually expressed in a fractional form with unity as the numerator, indicating the allowable error within a certain limit as indicated by the denominator, such as 1/500.

In this case, you are allowed a maximum error of 1 unit per 500 units measured. If your unit of measure is in feet, you are allowed 1 ft for every 500 ft.

In general, any survey has to be carried out accurately, meaning that errors and mistakes have to be avoided. The precision of a survey, however, depends upon the order of precision that is either specified or is implied from the nature of the survey.

The various orders of precision are absolute, not relative, in meaning. Federal agencies control surveys. They are generally classified into four orders of precision; namely, FIRST ORDER, SECOND ORDER, THIRD ORDER, and

Table 13-1.-Control Traverse Order of Precision

FOURTH ORDER control surveys. The FIRST ORDER is the highest and the FOURTH ORDER, the lowest standard of accuracy. Because of the type of instruments available in the SEABEEs, most of your surveys may not require a precision higher than a third order survey. When the order of precision is not specified, you may use table 13-1 in this training manual (TM) as a standard for a horizontal

control survey when using the traverse control method. For surveys that call for a higher order of precision, you will have to use theodolites to obtain the required precision.

The triangulation control method is discussed fully in Engineering Aid 1 & C, NAVEDTRA 10635-C. At present, however, you may have survey problems that require the use of the triangulation method. In such a case, you may use table 13-2 in this TM as a guide for the order of precision if it is not specified in the survey.

The practical significance of a prescribed or implied order of precision lies in the fact that the instruments and methods used must be capable of attaining the required precision. The precision of an instrument is indicated by a fraction in which the numerator is the inherent error. (In a 1-min transit, the inherent error is 1 min.)

The denominator is the total number of units in which the error occurs. For a transit, this last is 90, or 5,400. The precision of a 1-min transit. then, is 1/5,400, adequate for a third order survey.

Precision of a tape is given in terms of the inherent error per 100 ft. A tape that can be read to the nearest 0.01 ft has a precision of 0.01/100, or 1/10,000adequate for second order work.