Originally Posted by Frogwatch
Once spent an entire day laying face down in the mud of a particularly horrible cave passage slowly mapping it with compass, tape and clinometer. The shots (point to point) were short and involved crawling over awful stuff. Got home and began drawing on paper before we entered it in the computer (old punch card entry) and one series of shots that was supposed to go around in a circle didn't even come close to closing no matter how much we manipulated the data. Suddenly had this really awful realization that it had been the first time I had gone cave mapping with my glasses as I normally wore contact lenses. I placed my glasses adjacent to the compass and saw it move. We threw the data away. To this day, over 30 years later, nobody has mapped that passage.
There is still no way to do cave mapping but compass and clinometer although the tape has often been replaced with a infra-red gadget and there are apps that plot data as it is entered on a waterproof and shock resistant keyboard. This helps see such errors as they occur.
On that trip, theyguy who was doing sketch and data recording in the "map book" (waterproof and tear resistant paper) fell asleep it took so long on one shot. He awoke when the flame from his carbide lamp burned hole in the map book cover.
couldn't you have simply measured the deviation from the glasses then have corrected the data after you discovered the error?