Instrument Correction Factor
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No cave surveying instruments are perfectly accurate and most instruments have small but consistent systematic errors. Systematic errors are errors that are the same for every measurement. For example, your compass might read slightly east or the inclinometer might read a one-degree high. Compass allows you to compensate for instrument errors by entering correction factors for compass, tape and inclinometer. These correction factors can improve the accuracy of your maps and help to make your loops close better.
Compass has five correction factors: Compass Correction, Inclinometer Correction, Tape Correction, Compass Correction-2, and Inclinometer Correction-2. These values are entered into the Survey Header and they effect all the shots in a particular survey. These values are added to the azimuth, inclination and length values for each shot. This compensates for any problems with the instruments. For example, if your tape was one foot short, you would measure each shot one foot longer than it really was. As a result, you would set the Tape Correction to minus one foot. This would subtract one foot from the length, thereby compensating for the defective tape.
The Compass and Inclination Correction factors are always specified in degrees. The Tape Correction factor's units are based on the Length Units except that feet and inches are not allowed. For example, if the length measurement is set to use meters then the Tape Correction factor would be displayed and entered in meters.
Compass Correction-2, and Inclinometer Correction-2 are applied to the back sight values if the survey has back sight values.