This is part two of a five part series called Mapping Caves, where the basic steps to underwater cave cartography are outlined in a way that both divers and non-divers alike will understand. To start at part one, click here.

Introduction

The first step to underwater cave cartography is the survey and exploration process. In short, divers need to know where the cave goes before they can map it.

Laying the Guideline

When divers enter a newly discovered cave for the first time, they must carefully lay a permanent guideline along their path as they work their way through a maze of unexplored passages.

This guideline serves two purposes: first, it acts as a navigational aid to lead divers back to the surface, and second, we use this line to collect survey data.

Surveying the Guideline

The survey is done by recording the depth, azimuth, and distance at each station along the guideline.

Some definitions:

1. Station: A fixed-point where the guideline makes a change of direction or depth such as a tie-off.

2. Distance: The distance between two stations. Normally measured using a combination of pre-knotted line and a 2 ft/50 cm string. Prior to diving, knots are tied into the line at 10 ft/3 m intervals and divers use their survey string to measure hand-over-hand to stations that are in between two knot intervals.

3. Azimuth: The angle from magnetic north between two stations. Measured using a compass such as the Suunto A-30.

4. Depth: The inclination (angle from horizontal) between two stations. Measured by recording the depth at each station using a dive computer.

Using the Data

The survey data is then plotted using software that compiles a basic line plot, or stick-map, of the cave system.

By linking this line plot to GPS coordinates taken at the entrance of the cave, the stick-map can be imported into a geobrowser such as Google Earth to view the cave passage as it relates to other nearby cave systems and the surface.

Unless divers plan to make a more detailed map showing walls and floor details of the cave, the cartography process is finished at this point - most underwater caves are not mapped beyond this step.

Common cave survey programs include: Walls, Compass, and Ariane's Line. Ariane's Line has the advantage of being the only program specifically created for cave divers.

In 2016, cave explorer, Sebastian Kister, introduced a device called the Mnemo, which can be clipped to the guideline and records this data at the push of a button. The Mnemo is quickly becoming a standard piece of gear for underwater cave survey. More info.