zero error—The amount measured by an instrument when the actual amount should be zero. Also called “index error” for some instruments. Assumed to be constant and to affect all measurements by the instrument, regardless of size, and therefore can be determined by comparing the true (known) value of a measured quantity to the measured value and does not have to be determined actually at zero.

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Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

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