Cave Surveying

Surveying a cave such as Wind Cave is a long and difficult task. The Wind Cave map is the result of tens of thousands of hours of work by cavers exploring and charting out the thousands of passages that we now know comprise Wind Cave. Just what does this surveying involve?

To survey a cave requires several pieces of equipment and the expertise to use that equipment in the dark, dirty, and often contorted conditions found underground. The equipment includes a compass, an inclinometer, a measuring tape, and a notebook to record the information that is generated by the survey team.

The survey team usually consists of a person to read the instruments (compass and inclinometer), one or two people to take the tape readings and push leads and a person to record the measurements, sketch the passage outlines, and inventory the features of the cave. Ideally, a survey team consists of four persons, but three and two person teams are fairly common at Wind Cave. No matter how many cavers are on the survey team, all of the following tasks must be done to complete a survey.

 

Once the survey team has gotten to the lead (unexplored or unmapped passage) to survey, the first task for the sketcher or book person is to set up the book. Most cavers use notebooks specifically manufactured for surveying. These notebooks are made with special paper that does not deteriorate when it gets wet. One page of the notebook is ruled with lines such as one might see on an accountant's ledger pad. This is where the actual data generated by the instrument and tape readers are recorded. On the opposite page in the notebook there is graph paper. On this page a sketch of the cave passage is drawn according to the measurements taken by other team members and the features of the cave are recorded according to their location in the cave passage.

The first page of the survey notes includes the date of the survey, the people involved and the tasks that each performs on the survey. The type of survey instruments used, the type of measuring tape used and the units of measurement on that tape, a scale for the sketch on the graph paper and a north arrow to orient the sketch are also included. The columns where the data will be written down are then labeled. These labels are: STATION, DISTANCE, AZIMUTH or BEARING, VERTICAL ANGLE or INCLINATION, UP, DOWN, LEFT and RIGHT.

 

To begin the survey, the tape person places one end of the measuring tape at the last point, called a STATION, of the previous survey. Each station in the cave is given a name, usually one or two letters followed by a number. The tape is then stretched out down the passage as far as can be seen from the previous station. At that point the tape is read and the information is passed on to the sketcher who records the reading in the notebook.

At Wind Cave a small piece of mylar with a piece of reflector tape is then placed at the new survey station with that station' name written on it. These station markers are used to avoid damaging the cave by writing on the cave's walls. Because Wind Cave is seldom flooded, these markers will remain in place unless disturbed by people.

 
Once the distance between survey stations has been recorded, the instrument reader sets up to take the compass and inclinometer readings. The compass is set above or below the survey station while the caver at the next station places their light on the next station. The instrument person can then use this light to sight in the compass bearing from their position to the next point. This reading is then recorded in the book as the bearing from that station to the next.
 
After the compass has been read, the inclinometer is used to measure the vertical angle or dip from that station to the next station. This reading is also recorded and allows for the calculation of the depth of the cave, and also provides a way to calculate horizontal distances which are required to plot the map of the cave.

The sketcher then estimates the distance from the station to the ceiling, from the station to the floor, and from the station to the left and right walls of the passage or room. These, too, are recorded and are used to sketch the passages or rooms.

 

Finally, using all the measurements, a sketch is made to scale, on the graph paper in the notebook. This sketch shows the configuration of the passage relative to the instrument readings and includes notes or symbols showing the features of the cave.

Of course all of these tasks must be done in the cave. Instruments often have to be read while the caver is squashed into tiny passages or is hanging over deep chasms. Distances between stations are often as short as one foot or less in tight crawlways or may be as long as 100 feet or more in large rooms. Overall, the average distance of a survey shot in Wind Cave is about 20 feet. To produce the Wind Cave map about 27,000 sets of readings had to be collected.

 

Once out of the cave all of the data must be reduced or analyzed. The cave data consisting of compass bearings, distances, and inclinations must be put into a form that allows a map to be drawn. The major problem with that data collected in the cave is that the distances measured in the cave are SLOPE DISTANCES and not HORIZONTAL DISTANCES which is the distance that must be drawn on the map.

Using trigonometry, and the data collected in the cave, the slope distances can be reduced to horizontal distances. This reduction is given by the formula: horizontal distance = Slope distance times the Cosine of the inclination. For example, if the distance measured from one station to another is 100 feet but the passage was going down a slope at a 30 degree angle, the horizontal distance would be 86.6 fee [100 x COS(30) = 100 x .866].

Similarly the depth of each station can be calculated using trigonometry. In this case, the rise or fall from station to station is given by the formula: Rise = Slope distance times the Sin of the inclination. In the example above, the second station would be 50 feet below the other station [100 x SIN(30) = 100 x .500]. By using trigonometry on each of the 27,000 sets of readings, the true horizontal distances and depths of each station in the cave can be calculated relative to a known point (the cave entrance).

Of course, al these calculations provide the information necessary to draw an accurate map if all the readings in the cave were done with perfect accuracy. This is never the case, especially when the instruments have to be read under the conditions found in the cave. To correct such errors, complex statistical corrections (called LEAST SQUARES) are performed on the analyzed data and in recent years electronic underground radio location devices called cave radios have been used to help correct errors in the cave surveys.

The Wind Cave Map today is produced by feeding all the survey data into a computer where complex trigonometric calculations are performed and the statistical corrections are done taking into consideration the locations proved by cave radios. It is the most accurate map of Wind Cave that can be produced by modern technology. Yet despite such marvelous advances, the actual surveying process still requires long hours in the cave, crawling and climbing around, searching for that ever elusive discovery...and the cavers who explore Wind Cave would not want it any other way.

Last updated: January 17, 2018

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