All distance and depth measurements have errors. This section explains these errors, why some survey techniques are more accurate than others and why we take check measurements. The ideas in this section are based on statistics so if it gets a bit complicated then skip to the next section and come back here later. Typical mistakes include reading the wrong numbers from a tape measure, making a measurement with the tape snagged around some ship's structure or reading the wrong values from a form when processing the measurements. Mistakes are sometimes called gross errors or blunders.
Systematic errors are ones that can be repeated and can be accounted for in processing.
If you calibrate a tape measure against a known standard and find that it always measures distances that are too long, the difference is a systematic error and can be removed when the measurements are processed. Tape measurement made several times under the same conditions is unlikely to give exactly the same value for each measurement. Judgement of the tape reading will vary as will tension on the tape depending on how hard you pull. If you remove the mistakes and the systematic errors then some variation in the repeated measurements will still be seen, this is called random error. The procedures we use for making measurements and then processing them to form a site plan should aim to remove mistakes and systematic errors.