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Cave Survey – Basic Tape & Instrument Usage
by David Ochel and Bev Shade
Thanks for helpful feedback goes to Dale Barnard.

Last Updated: February, 2009
Feedback or questions? Contact David Ochel:

General:
  • You always measure distance, azimuth (compass) and inclination (clinometer) between two established (and somehow marked/flagged) survey stations. This is a survey shot. Typically, one person does the “tape”, choosing stations and having one end of the tape, while a second person does “instruments” and measures azimuth and inclination for each shot. The third person is the sketcher (the person with “the book”). The sketcher will write down the results of tape and instruments, and make a scaled drawing of the passage. It is possible to do this with only two people, in which case one person will be both book and either instruments (typical) or tape.
  • A free line of sight between two stations is always preferable. Measuring “through” obstacles should be a VERY rare exception. And if you do it more than once a year, dark clouds will follow you around.
  • Sketchers should read your numbers back to you – listen and make sure it matches what you measured. Do NOT take the next measurement until the sketcher has confirmed your last set of numbers.
Tape:
  • Metric: Read meters and centimeters. Right: 2.21 m is “two point two one” meters. Wrong: 2.01 is not “two point one” (it’s “two point zero one”) and 2.20 is not “two point two” (it’s “two point two zero”).
  • Check where the tape begins every time. Usually, the end of the metallic tip, not the beginning of the actual tape, is zero. This is not always the case, especially with cheap tapes from China.
  • Be careful about tapes that have metric readings on one and English ones on the other side of the tape – don’t confuse them during the survey.
  • If the tape is not long enough, be creative in extending it. Or set an intermediary station.
  • Many cavers have started using “distos” (laser-based distance measuring devices) instead of tapes for in-cave surveys. Be aware that their distance is limited, often about 15 m. Before taking a disto caving, verify a couple of readings with an actual tape before relying on a disto’s accuracy. Note that “zero” typically starts at the end of the disto, not at the top where the laser sits. Once you are convinced of the accuracy of the disto, remember to measure distances twice when using a disto in the cave survey. It is very fast, and prevents errors from shaky hands. Brace the disto against the wall for stability, then take a deep breath and hold it to be less shaky. Do NOT point the disto at another person without warning, the laser can cause permanent eye damage.
  • Choosing stations: long shots are not always the best. Put stations at intersections, so that the survey can continue down all passages. If you shoot past intersections, don’t be surprised if the sketcher calls you back to re-do the shot. Put stations in places that the instrument person can reach.
Instruments (compass and (in)clinometer):
  • Suunto or Brunton precision instruments can/should be read to the half (.5) degree.
  • If the scale appears out of focus, instruments sometimes have an adjustable “eye piece” that can be screwed out or in a little to adjust focus.
  • Although the manufacturer instructions tell you to use both eyes when reading an instrument (one looking through the eyepiece to read the scale, the other focusing on the target), many cavers use one eye (other eye closed) in order to avoid parallax issues, which can introduce errors especially over shorter distances.  Here is what the Suunto manuals say about this:
    • “The axes of the eyes of some people are not parallel, a condition called heterophoria. This can even vary in time and be dependent on different factors too. Therefore, in order to be sure that said phenomenon does not affect the accuracy of readings, it is suggested that the operator checks this possibility before taking the actual readings as follows: Take a reading with both eyes open and then close the free eye. If the reading does not change appreciably there is no disalignment of the eye axes, and both eyes can be kept open. Should there be a difference in the readings, one has to keep the other eye closed and to sight partly past the instrument body making use of the optical illusions whereby the hairline continues past the instrument body and is seen against the target.”
  • In dark environments (caves ;) ), the person leading the tape will typically put a “station light” on the target station to allow the instrument person to focus on the target. The instrument person will need a light to illuminate the scale of the instrument.
  • Backward shots are a possibility. If it is easier to read from the “new” station to the “old” station, do it! Make sure to tell your sketcher that you are reading backwards, and/or ask him/her whether they want you to give them the corrected numbers (in case of inclination, that’s a plus instead of a minus and vice versa, in case of the azimuth that’s the small instead of the large numbers on the scale).
  • Reading instruments both forward and backward in the same survey is sometimes used to increase the survey accuracy (for example, if you want to win a survey contest). Either write down both results if you have software that will compute the median for you later (survey contest participants don’t have such software), or compute the median yourself (ideally there is no deviation between both readings anyway) before writing it down.
  • If you buy any instruments, make sure you buy them with the correct scales. Not all are the same! Compass should be 0 to 360 degrees, not in 90 degree quadrants. Clino should have a scale that reads from -90 to +90 degrees.
  • If you have difficulties getting a free line of sight from your station to the target station, you can move the instrument above or below, or to the right or left of, the station and ask the person holding the station light to move the station light for the same distance in the same direction. Doing this should be an exception and not the rule, though.
  • Any instrument can get damaged. Water leaks inside can make them too foggy to read, or they might begin to stick with age. If it is obvious that an instrument is not functioning correctly, stop the survey.
Compass:
  • The azimuth is ideally read from somewhere exactly above or below the station.  If that’s not possible, think a virtual line going through the two stations and read the compass either in front of or behind your stations on (or exactly above or below) this virtual line. Or consider a backwards shot (see above).
  • Magnetic pitfall #1: Your headlamp, when turned on, may create a magnetic field that causes the compass needle to deviate from the magnetic North. Check this before starting the survey!
  • Magnetic pitfall #2: Metal near your station may deflect the compass needle.
  • Human pitfall #1: The compass dial MUST rotate freely. If the compass body is held at an angle, the compass dial will jam and give bogus numbers. How to avoid this: measure twice, and wiggle the compass in between measurements. Turn your head back and forth while sighting and see that the compass dial moves freely back and forth.
  • Brunton/Suunto precision instruments typically have a “forward” and “backward” scale – the larger numbers are “forward” (from where you are to what you are targeting) and the smaller ones are “backward” (the other way around).
Clinometer:
  • Inclination is ideally read exactly left or right to the station at the same height as the station. If that’s not possible, think a virtual line going through the two stations and read the clino either in front or behind of your station on (or exactly to the left or right of) this virtual line. Or consider a backwards shot (see above).
  • Inclination is always positive or negative. An inclination of +32° is “plus thirty two”, not “thirty two”.
  • Brunton/Suunto precision instruments sometimes have one of various “other” scales in addition to the 0-90 degree scale that you want. The degree scale is usually on the left, check before you start the survey.
  • When having a small degree of inclination, always double-check whether you are reading a positive or negative inclination.
  • It’s in the nature of inclination that when you are reading positive inclination (target station is higher than your station) and you are close to (like three “clicks” above) the 20° mark, the inclination is +23°, and when you are reading negative inclination and you are three “clicks” above the 20° mark, it’s -17°. Or is it the other way around? ;-) Always be aware of this.
“Minus ninety”
  • Reading a compass on a steep inclination (think a pit in a cave) is a pain and the results are often inaccurate. It is always favorable to find two stations that are exactly on top of each other. The inclination from the higher to the lower station in this case is always, who would have thought, -90°, and the azimuth is always, surprisingly, somewhere between 0° and 360° (recorded as 0°). No need to read instruments, just measure the distance! Use the tape (possible weighting the end with a carabiner as a plummet) to ensure that you have a straight vertical line between the two points.
  • In this situation, we have a problem with distos! You can’t be sure of your plumb line without a guide line such as a tape or rope. Of course, if you are carrying a tape as a backup you can use that.

Organization
  • The sketcher is the person “in charge” of the survey.
  • The sketcher is usually also the slowest person on the team, because they have a lot to do. Help the sketcher by giving them information about the passage, such as “there are 2 leads at station 10 – one is a horizontal crawl that goes at least 2 meters, the other is a dome that pinches out after 4 meters.”
  • The whole team is responsible for safety, just like normal caving. If someone is cold or tired, it may be time to end the survey for the day.