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.
 The
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.
|
.gif){kind=small}