Blunder Signatures

In spite of the fact that blunders are unpredictable, they do have one property that enables us locate them: Blunders leave a signature on the error value. For example, let’s say that you have made a blunder on a length measurement so that the shot is ten feet too long. This will add ten feet to the loop error. More important, the ten-foot error will be in a direction that matches the azimuth and inclination of the shot. If the shot had an inclination of 10 degrees and an azimuth of 173, the error would be 10 feet in the direction of 173 degrees azimuth and 10 degrees inclination. This creates a signature that is unique. This means that only shots with the same azimuth and inclination could cause the error that we see. As a result, it is often possible to zero in on the exact measurement that caused the blunder.

The signatures are often unique because there are two measurements that leave their mark on the signature. In our example, the combination of azimuth and inclination controls the signature. The odds of having these two measurements match exactly in a different shot are fairly low.

Even when you have an error signature that matches two or more shots, all is not lost. When two or more shots produce a signature that matches the error, you cannot be sure which shot has caused the error. However, you still have narrowed the number of likely candidates.