SIY10.TXT Error and Blunder 47 Chapter 10 Error and Blunder In this chapter you will learn about your mistakes. You will need: a) To think. Mistakes have been made before. If you haven't made any mistakes while learning to survey, then it can only be because you are not learning. You were smart to skim the entire book first. Just wait until you have actually tried some of this! The way to keep from making a fool of yourself is to keep track of your mistakes. It does you much less than no good to have the answer, but then not know how good that answer is. No fool like a confident fool. Error is part of all numerical measurement. In most surveying situations, you can never know what the correct answer is. The best which you can do is to come close. And to know about how close. Mistakes come in two kinds. The kind with which you are most familiar, the BIG ones, are blunders. Blunders are big enuf so that they are obvious once you have found them. They occur only occasionally. You hope. Blunders can be eliminated. Once caught, the blunder is corrected. Error is the sum of all the little mistakes. The ones which are so small that you can't even see the little buggers, let alone catch 'em. The best you can do is to keep your errors small. In terms which even a Neanderthal could understand, error is like being eaten by worms. Blunder is like being eaten by a cave bear. The compass is the most important source of error when surveying with this method. The one best thing which you can do to improve the quality of your surveying is to take backsights. This is little extra trouble, especially if you have an assistant with an extra compass. I usually accept a one degree difference between the compass and the backcompass. Anything more is blunder, and the compasses are read again. If you don't take backsights, at least take two compass readings. The worst problem with the Silva Ranger is that you can sight it while tilted and not know it. This binds the needle and causes blunder. Always check that you have not made the blunder of binding the compass needle. Eliminate the blunder by holding the compass level. When the blunder is gone, it's gone. SIY10.TXT Error and Blunder 48 Another cause of compass error is to have the needle perturbed by nearby iron or steel. A backsight usually catches this blunder. Compared with compass error, the error contributed by the tape and clinometer are minor. Table 4 shows the calculated theoretical errors for several combinations of instrument errors. SIY10.TXT Error and Blunder 49 Table 4 A Table of Errors COMPASS TAPE INCLINOMETER TOTAL COMMENTS error error error error degrees % degrees % 1 0.1 2 1.75 standard quality 0.5 0.1 2 0.88 excellent compass 2 0.1 2 3.49 poor compass 1 0.1 2 1.75 standard quality 1 0.2 2 1.76 tape to 2 tenths 1 1 2 2.01 tape to one foot 1 5 2 5.30 tape to five feet 1 0.1 2 1.75 standard quality 1 0.1 4 1.76 poor clinometer 1 0.1 10 2.32 guess clinometer 1 0.1 2 1.75 standard quality 1 5 5 6.11 compass & pace 2 10 10 10.7 compass & guess 5 10 10 13.4 quick compass & guess 1 0.1 2 1.75 standard quality 0 0.1 2 0.12 tape & incl error only 1 0 2 1.75 compass & incl error only 1 0.1 0 1.75 compass & tape error only SIY10.TXT Error and Blunder 50 For those of you who prefer English to numbers, BE CAREFUL WITH THE COMPASS! You can measure the distance to only the nearest foot, guess at the inclination, and still nearly all of the error comes from the compass. How many more ways can I say it? Take a good compass reading, then repeat your compass reading. Then take a backsight. Or two. These theoretical errors can be considered to be the expected closure error. In practice, you should reject any survey with a percent closure error of more than the theoretical error for your instruments. You have blundered. Go do it again. If you don't survey in loops so that you can check the closure, you won't catch those blunders. If you are in a situation where you can't do it again, consult. There are some mathematical tricks which might save a bad job. Usually the easiest way is to just resurvey it. The error in the location of a station is the percent theoretical error times the distance from where you knew you were right. The error in the acreage of a parcel is approximately 1.5 times the percent theoretical error, times the acreage. The second commonest source of gross blunder, after faulty instrument technique, is faulty recording technique. I always have my recorder repeat each number after recording. I make an office copy of the notes while they are still fresh in my mind. Be sure that you know which station goes with what numbers. This is especially important if you took multiple compass readings or are branching. The recorder should also be sure that all data are recorded before allowing the instrument reader to move on to the next station. The third commonest gross blunder is reading the wrong end of the compass. This is all too easy to do, even with the Silva Ranger. Keeping track of what you are doing with a sketch map helps. The fourth gross blunder is lost survey notes. I offer no easy solution. By taking backsights and rejecting loops with bad closure, it is possible to achieve an error of about 0.5% using the Silva Ranger compass. If you need more than 99.5% accuracy, you need better instruments. You probably should hire a professional surveyor for the job. Do the preliminary survey yourself so that the surveyor doesn't waste any of his expensive time. It will also keep him honest. SIY10.TXT Error and Blunder 51 If you have determined an error in surveying a line, you may distribute the error according any of a number of schemes. All of them require more arithmetic than I would do by hand. Dave Beiter CAVE Inc 1/2 Fast Road Ritner, KY 42639 606/376-3137