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Cave Surveying and Mapping

Cave surveying is the process of measuring a cave so that an accurate map of the cave can be made. Without an accurate survey any resulting map will be suspect and of limited use for determining the relationship of the cave to surface landforms or other caves.

SURVEY TEAM

Surveying is usually done by a team of three or four people, each performing a specific role. Most survey teams are glad to take along a novice surveyor as long as they posses the basic caving skills required for the survey trip. While the makeup of the team and roles assigned in a survey team will vary from project to project, and even from trip to trip, a typical team might consist of the following roles:

Lead Instruments

  • Sets new stations for the survey (an important job - selecting stations is a bit of an art)
  • Takes along one end of the tape (usually the 'dumb' or zero end) for distance measurement
  • Takes backsights to the previous station with compass and clinometer.

Rear Instruments

  • Keeps one end of the tape (usually the 'smart' end that is read) for distance measurement.
  • Takes foresights to the new station with compass and clinometer

Sketcher

  • Usually the team leader or most experienced team member
  • Does a sketch of the cave to scale, marking the locations of the survey stations and including notes for the cartographer and later survey teams
  • May do some or all of the functions of the Book person

Book

  • When a fourth person is available, records measurements taken by the instrument readers and checks foresights & backsights to make sure they agree (usually within 2 degrees)
  • May supplement the sketcher by sketching cross sections of the cave at each station

Taking backsights is a way of detecting errors and blunders in reading instruments. Readings are taken between two survey stations (A and B) from A to B (foresight) and then from B to A (backsight). This can be done with one set of instuments and one instrument reader or, as suggested above, with two instrument readers using two sets of instruments. By checking for agreement of the readings within some specified amount (generally 1 to 3 degrees) the effects of magnetic minerals or hardware on the compass, or mistakes by the instrument readers can be immediately discovered and corrected. Fixing errors that inevitiably crop up is much more difficult after the fact.

The National Park Service has a nice overview of the cave surveying process used by surveyors in Wind Cave on their Wind Cave National Park web site. This process is an example of one done without using backsights, still fairly common. Be sure to move your mouse over the photos where indicated to see more detail.

SURVEY INSTRUMENTS

Unlike surface surveyors, cave surveyors need to be able to work in a variety of difficult situations where most traditional surveying instrument would be too fragile and bulky. A variety of instrument are used for cave surveying, but the Suunto compass and clinometer seem to be by far the most commonly used instruments in the US.

Photo: Suunto compass & clinometerThe instruments shown at left are the Suunto KB14 Compass and PM5 Clinometer, with optional rubber covers. Suunto also makes a combination version with both instruments in a single housing. Brunton makes a similar set of instruments, as well as a Pocket Transit favored by many geologists and some surveyors.

You can probably borrow instruments for your first couple of trips, but if you start surveying frequently you are going to want your own set of instruments. A set of instruments like the Suuntos shown above will typically cost around $200+ (USD) and can be found at companies that specialize in forestry supplies or at some speleo vendors. Shop around because prices can vary substantially.

The other instrument required is the survey tape, usually a fiberglass surveying tape marked in inches and tenths or metric. Tape length is commonly 50 or 100 feet (15 or 30 meters) in length depending on the type of cave you are surveying. The tapes come on plastic reels, but cave surveyors more often just buy the refills to avoid the extra expense and bulk of the reel.

The sketcher and book person will often use either a small plastic loose leaf binder or a bound notebook with waterproof paper marked with a columnar format for data and grid pattern for sketches. A few inexpensive mechanical pencils and a small protractor complete the kit.

CAVE MAPPING

When the survey data gets back to the cartographer, the numbers are usually entered into a specialized cave survey software package that will process the data to produce three dimensional line plots. A recent trend is the use of software that will 'morph' scanned images of the sketches to corresponding station points on the line plot, providing a quick way of creating working maps for the next survey trip (Carto is currently the best example of this type of software). The actual production of the final map is still a lot of hard work, though many cartographers have abandoned pen and ink drawing for computer drawing packages.

To learn more about surveying and cave mapping, two books on the subject, A Guide to Cave Mapping by John Ganter and On Station by George Dasher, make excellent references. Both these books are available from the NSS Bookstore and speleo vendors.

Book cover: A Guide to Cave Mapping

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  Copyright 2002-2004, Bob Robins
  Last Updated: August 9, 2005