Cave survey and making maps

One of the major objectives of our project is to create high-quality maps and other cartographic assets for the cave systems we explore.

The bread and butter of cave exploration is the “stick map.” This is a map that can be produced by measuring azimuth (compass heading), distance, and water depth, at each of a series of stations that are formed by tie-offs or placements to cave formations as our line passes through the cave. We make a tie-off or placement each time the line changes direction or depth, so if we survey the line, we end up with a cave map that has a series of jagged but straight lines that represent the path of the stations through the cave, rather than the cave walls themselves. This path is called the transect. A complete cave map is made up of one or more transects, each bisecting each other where lines meet at “T” intersections or approach, but do not cross at “jumps.”

Lines, Ts and Jumps are important in surveying, but also in safety. We wrote about the significant role that they play for safety here. Surveying takes place after the line is run in the cave, so that we have a precise pathway to survey, but also so that we can practice safe guideline practices.

Historically, stick maps were drawn by writing a set of compass, distance (counting knotted line), and depth (measured by manometer) measurements in a diver’s wetnotes, and then producing the map by hand from this data. Later, software allowed maps to be drawn by a computer after a bunch of data entry.

But recently, this has all become faster with a new device called the MNemo, which is produced by Sebastien Kister, a cave diver in Playa del Carmen, Q.R. Our exploration team is using a couple of MNemos, together with Sebastien’s software called Ariane’s Line, which integrates with the MNemo to dramatically streamline the survey and mapping process.

If the azimuth, distance and depth data is supplemented with just a single GPS coordinate at the start of a transect, then satellite images can be overlaid on the stick map, showing where the cave passes under the ground level that most of us see normally. Caves can pass below anything-jungle, buildings, highways, even bodies of water and the ocean.
Depth and distance data can be used to produce a Profile view of the cave, showing distance versus depth.
Stick maps can be very extensive and complicated; here is a stick map of Nohoch Pek courtesy of Ariane’s Line.

Moving beyond the basic stick map, we can augment our azimuth, distance, and depth data with a cross-section measurement at each survey station. To take a simple cross-section, we can measure left, right, up, and down (“LRUD”) from where the line crosses the station by using a very large tape measure.

We might also note shapes of the cross section so that we can adjust the segment tensions to produce an accurate cross section, rather than just a basic tube.

A stick map plus cross-sections allows us to produce a three-dimensional cave map. This can be manipulated in Ariane’s line or exported as a “.obj” file for use in CAD programs or even for representation in VR or 3D graphics engines like Blender or Maya. 3D maps are fun to play with and really give a much better sense to people of what the volume of the cave feels like as one swims through it.

A 3D model we built of Cenote SD, shown in the video here.

Traditionally, the apex of cave survey is the full cartographic map. These maps are meticulously created from careful measurements and detailed sketches of each significant feature in the cave. Full cartography of even a moderately sized cave can take years, and just one or two beautiful maps could be the work of a lifetime for someone sufficiently dedicated to it.

A very fine example of full cartographic mapping is David Mayor’s current Taj Mahal Cartography Project. David outlines his process here.

Work in progress from David Mayor’s Taj Mahal Cartography Project

Photogrammetry is perhaps the current frontier of cave mapping. In this technique, a series of overlapping photographs are processed by photogrammetric software to generate a high-resolution three-dimensional model of the cave. This technique has the potential to speed the creation of high-resolution cartography, while also moving the output formats from traditional two-dimensional maps, to interactive models or even virtual reality experiences.

Photogrammetry does have some significant challenges, and there isn’t yet a consensus on the best techniques for cave cartography. But a few practitioners are pushing this technology to see what it can do. Right now, it seems that a primary limitation is computer processing time–and as we know in technology, those limitations recede rapidly as they tend to follow Moore’s law or something like it.

We are experimenting with photogrammetry in our project to see what is possible. Already, we’ve been able to make some useful models of small parts of the SD cave system.