Making a New Grotto Map


Maps serve an important function at dive sites such as 40 Fathom Grotto, They are a vital part of our diver and Grotto Guide orientation programs. Equally important, they help us answer the ever present question, What’s down there?

Mapping Tools

Over the past several weeks, we and several of our guests have been involved in a joint project to create the most accurate map to date of what divers can see and experience in the Grotto. You can download the most current version of this map in Adobe Acrobat Portable Document Format (PDF). (Just bear in mind this is a “work in progress;” you will want to revisit our website from time to time to download the newest version.)


Any diver who has undergone Underwater Navigator or Divemaster training has most likely been involved in the making of at least one underwater map. Creating such maps generally involves swimming simple compass headings and counting kick cycles. The resulting maps may not be excruciatingly accurate; nevertheless, they are almost always better than having no map at all.

Cave Map

Cave divers take a lot of pride in the precision of the maps they make of various cave systems. In fact, they even have a grading system to reflect the degree of accuracy used to make their maps. The highest degree of accuracy in cave mapping is what is known as a Grade 5 Survey. This degree of accuracy is what we wanted to achieve in making the new Grotto map.

The Tools

While kick cycles and timed swims are good ways to make general distance estimates, they lack precision. Using a techniques such as arm spans increases accuracy; however, in the final analysis, nothing beats a fiberglass tape measure for getting distances that are accurate to the inch.

Measuring Tape

We use a Stanley 300-foot construction site tape measure. It comes on it own reel, has no metal parts to rust or corrode, and stands up well to underwater use.


Another important tool in our arsenal is the digitally accurate dive computers we all carry. At a steeply inclined site such as the Grotto, depth measurements are every bit as critical as distance and direction.


Compasses are also important — but less so for the methodology we use than you might think. We use a Suunto SK-7 on a retractor, as this gives us a top- and side-reading unit that can be passed from one team member to another, and is not constrained by being attached to a console or wrist. (Slate-mounted compasses also work particularly well for map making.)


The last item in our inventory is a large slate on which to record survey data. As you can see from the accompanying photo, the slate contains a rough diagram of the objects to be surveyed, the azimuths between them and the measuring points we use. It provides a place for the surveyors to record depths, distances and compass headings.

The Methodology

The key to our survey technique is the accuracy inherent in triangles. For example, if you draw a triangle that measures precisely three, four and five feet on its sides, all you need to know is the compass heading of a single side; once drawn, the direction of the remaining two sides will be accurate as well.


To use this method, we group survey points in threes — say, the bow of the shallow boat, the nose of the tow sub the the windshield of the airplane. For each of these three measuring points, we get the following:

Depth Pythagoras

Dealing with Pythagoras: Were the bottom of the Grotto perfectly flat, the only survey data we would need would be distance and bearing. The steeply inclined bottom of the Grotto, however, provides us with an interesting challenge. Case in point:

You can calculate this number by using Pythagorean Theory. In other words, the apparent distance (21.82 feet) squared, added to the square of the difference in depth (10 feet), must equal the square of the actual distance, i.e., (24 x 24) = (10 x 10) + (21.82 x 21.82).

Inclined Boat

We have a formula programmed into the same Excel spreadsheet in which we record survey data that automatically does this calculation for us. There are commercially available mapping programs that do the same.

Putting it All Together

Old Map

When CDA took over the Grotto, we inherited Eric Hutchenson’s excellent cutaway drawing and a decades-old, overhead map showing the general outline of the water line, the approximate position of several items and the direction of magnetic north. This overhead map has proven surprisingly accurate (many such maps are not), and gave us a good starting point from which to add more detailed data.


Using Adobe Illustrator, we plotted the distance and azimuths of our survey data, and added in scaled drawings of items such as the boats, plane, satellite dish and cars. Over this we plotted the current position of the new docks and training platforms.

Float Ball

Among the most critical factors in aligning all of the items properly was a buoy with a descent line running straight down to the tip of the satellite dish. We know the location of this buoy relative to the docks, and used it to position all of the other items on the bottom.

Map Detail

At this point, our survey has encompassed most of the major items from 90 to 140 feet. As time passes, we will survey and plot deeper items, as well as get more accurate data on the position of the walls. We invite you to return to this page on a regular basis to see how the map continues to evolve.