Mapping may seem an unnecessary skill – until you can’t find the specific parts of scattered wreckage, the small finger reef that always has nudibranchs or the artificial reef. Perhaps you are bottle collecting and need to identify artifact piles to try and establish a pattern. When you need to locate various sites underwater, it is extremely helpful to have a plot map showing their relative locations and distances.
Most local knowledge maps are made by divers who frequent a certain site. They want to be able to relocate a specific place and draw the maps for their personal information. Scientific divers studying an area draw highly accurate maps and, over a period of time, take organism census counts at the same area. Cave divers survey for personal interest and safety. In the interest of diver safety, cave penetration maps are rarely shared beyond the trained cave diving community.
It’s one thing to make a simple sketch on a slate in the midst of a dive and quite another to plan a survey. Before the dive, you need to:
- Select a survey diving partner
- Inventory your combined diving equipment
- Determine what additional gear to acquire
- Buy, borrow or make surveying supplies
- Narrow your objective for the survey
- Commit the time to plan, execute and record the survey
- Agree in advance how accurate and detailed a map you are committed to producing
- Decide who is doing what underwater, so you can master the needed skills by practicing on land
- Agree upon a set of hand signals that are simple, understood and able to be given with one hand
- Do not get discouraged when it takes several dives to survey a small area or accomplish a single task
For some surveys, it helps to make an exploratory dive just to scope out the area. This gives you an idea of how much line, the number of markers, the amount of air etc., that you may need to make the actual survey. Plan specific tasks or objectives for each dive. For example: “We will lay one baseline and take ten measurements on this dive.”
SURVEYING TOOLS. The basic tool box for surveying includes:
- A baseline anchored at each end
- Knotted line and reel
- Tape measure
- Markers for survey points
- Anchor points (nails, sand screws) and hammer
- Slate and pencil
Specialized surveys could require additional equipment. A marine biologist might want to count the number of starfish or do a summary of all organisms in an area. This type of survey is commonly done by laying a baseline on the bottom as a reference. Then one meter squares are marked off on alternate sides of the baseline. The census is done within the squares. Scientists often fabricate a PVC pipe square meter frame or meter stick to show the borders of their count area.
If the baseline is always laid in the exact same place, divers can position their meter squares and recount the same zone on many different dates. Comparative data can show trends that may be helpful in predicting the other areas of the reef or the future. To be valid, however, the grids need to be placed in the same location each time the survey is taken.
BASELINE: KNOTTED LINE. Nylon fishing line, size 21 or 24, works pretty well. It is thin enough to fit on a medium sized reel, abrasion resistant and not too stretchy. Very small diameter (numbers 30 to 36) braided nylon also works well. Remember to burn the ends so it doesn’t ravel.
Mark every 10 feet with an overhand knot. When you do the knotting as you go, it takes two people to keep the line taut and ensure the knots are precisely 10 feet apart. If you get sloppy, the error introduced keeps compounding itself.
One way to guard against “knot creep” is to lay out a 100 foot line, then fold it into 10 equal lengths. Use a felt tipped marker to indicate where the knots will go, then go back and tie the knots. You may need several reels of line, depending upon the size of the survey area. This knotting must be done in advance of the dive. A baseline is a simple knotted line laid out straight and then anchored at each end. For duplication purposes, it is important to place the baseline anchors at places you can relocate, such as an unusual rock formation. Or, you may be able to pound a nail in the reef as a fixed end point. In sand, use a sand screw to anchor the line. The metal stakes used for dog leash anchors work great.
If you leave a nail or sand screw in place for multiple dives, be prepared to lose it. Sand may cover up the screw. Other divers see nails or pitons in a reef and take them. If you leave an anchor point, put a marker tag on it identifying it as part of a survey. There are no guarantees it will stay put.
TAPE MEASURE. The best choice is a common builder’s 100 foot, fiberglass tape measure that comes in a crank up housing. You can buy this non-stretching, fiberglass tape for about $30 at home improvement stores.
The housing that comes with the tape works pretty well underwater but can get jammed with sand. Most of the cases have a ring, so you can clip the reel on your BC or tie it off at an anchor point. No matter which tape measure you select, it needs to be long and on a reel.
MARKERS. Markers are attached to each anchor point on a baseline or survey line. Each marker is labeled as a coordinate. If you were encircling a wreck with ten point-to-point lines, they might be labeled W1 through W10. If you found separated rubble piles on the east side of the baseline, they might be labeled ER1 (for E [east], R [rubble], 1) and up. There is no way to know in advance what the labels will be. So, you need generic markers that can be labeled underwater as you go.
Small pieces of slate material with a drilled hole and short string loop work well. Some divers like to attach the markers to a wooden clothespin. This makes it easy to clip on a marker or remove with gloved hands. If you ever see I laminate color sample strips going in the trash, the 1.5 by 3 inch sample chips make great marker stock.
If you can’t make markers in advance, a roll of gray duct tape will work. Duct tape can be pulled off a roll underwater and wrapped around a line; it will also stick to itself. A permanent, fine point marker will write underwater on duct tape. But, it is difficult to remove duct tape from a line after the dive. If you make acrylic markers in advance, you won’t have to clean duct tape and adhesive residual off the line later!
Whatever type of marker you select, have a spare writing instrument in your BC pocket. Sport divers often use a grease pencil with their slate. When surveying you accumulate lots of data and will find the blunt tip of a grease pencil takes too much space. Most divers switch to a pencil for finer lines and less smudging. The same writing instrument should work on your slate and the marker tags.
ANCHOR POINTS. Surveys need straight segments of line. The math gets too complicated if a line segment curves, So, you need to anchor the end of each section to a fixed point. Sometimes you can wrap line around a rock, other times you will need to introduce a pivot point. This could be as small as a 60 penny spike nail. In the sand, the best choice may be a sand screw. On really hard rock, try using concrete nails.
Use common sense to decide what will do the least damage to the environment and meet your needs. If you are doing a one day survey and removing all anchors at the conclusion of the dive, they do not need to be permanent. If you damage a seafan with a rubbing line, it may have been better to drive a nail in the reef. Care should always be taken to preserve and protect the environment. A carpenter’s claw hammer works well underwater and makes it easy to remove the nails with minimal damage. Buy a wooden handle hammer, drill a hole in it and attach a lanyard. Otherwise, you’ll put the hammer down and lose it. Some divers prefer a four to six pound sledge hammer for quicker anchor point fixing.
COMPASS. Accuracy of compass readings is critical in taking a survey. Even a one degree error means that when your data is transferred to a map it will not form a closed loop.
Select a compass that has easy to read degree marks. If your compass only has every five degrees marked, it will be highly inaccurate for surveying. Look for a well damped compass card that allows the arrow to swing slowly and smoothly.
Since the compass is going to be mounted on your slate, consider buying a backpacking compass. They are flatter, come with a backplate and are easy to use. They are usually liquid filled for smooth movement and have two degree increments for better accuracy.
SURVEY SLATE AND PENCIL. A survey slate is your instrument dashboard. Mounted on a regular slate are the instruments you need to have in constant view. At a minimum, this is a compass and pencil. Depending upon the nature of the survey, it could also include a depth gauge, bubble level and brass clip.
Assembly instructions are simple. When mounting anything to the slate, use brass or stainless steel screws or glue. Anything else may be magnetic and influence the compass readings. Adhesive mounting is the last choice, since a sharp blow may pop a glued item off the slate. Place each item you will be mounting next to the compass to be sure they are not magnetic. If the compass needle moves at all, that item is affecting the compass and you need to select another model gauge or other type of screw or nut.
Instruments should be arranged for easy reading, i.e., on the left side of the slate for right handers and on the right side of the slate for left handers. It is hard to write when you are working over a depth gauge.
Be sure the compass lubber line is precisely parallel to the edge of the slate. You will be holding the slate next to the survey line when taking readings. The edge of the slate becomes an extension of your compass, which yields much more accurate readings. If the compass is cocked, all the readings will be off-kilter.
RV supply stores sell small, cheap bubble levels. Mount the level so you can see it at the same time as the compass. Take the compass readings from a level survey slate for added accuracy.
Slate size is a matter of personal preference and ease of use. A 5 by 7 inch slate may fit in a BC. Larger slates have longer straight edges and aid accuracy. Smaller slates are easier to stow. Don’t get one that’s too small or you won’t have enough room to write survey notes. Either size should have a brass clip so you never set the slate down. Whichever size you choose should also have a holder, so the pencil is not left to dangle.
These pencils have pre-sharpened lengths of lead inside a hollow plastic shaft. If the lead point breaks, pull it out, open the pencil’s other end and drop in the no-point piece of lead. That pushes a new sharp piece of lead to the pencil’s writing end. You may need to put a nylon tiewrap nub on the pencil to be sure it will stay on your slate’s pencil clip.
Pencil holders can be tricky. They need to be tight enough to hold the pencil but not so tight gloved hands can’t extract it. A standard nylon tiewrap may work. (Drill a hole in the slate and poke the tie through, forming a loop, leaving the locking nub on the slate’s backside.) A length of surgical rubber tubing pushed over the eraser end of a standard pencil might suit you. Or, you can get creative at the hardware store and find a plastic clip that exactly fits your pencil preference.
Before you do the first survey, make a reconnaissance dive. Make simple pencil sketch notes about the configuration of the terrain. Armed with this information, you can decide where to lay baselines for the survey. Look for rocks or other natural structures that will make it easy to relocate anchor points.
You may want to start using the symbols that will be on your completed map. Identify major topographical features that can be used as reference points. See our sidebar for a list of common symbols.
Surveying and mapping dives require more preparation but you will be rewarded with a sense of accomplishment and a permanent record of places you want to relocate.
Once the novelty of breathing underwater wears off, most people use activities such as underwater photography to add a purpose to their diving. Surveying and mapping will give you the skills to demonstrate your knowledge of all the local hot spots. For a collector, mapping allows documentation of where you have searched and where you made finds. Northwest bottle collectors have unexpectedly found a treasure pile and, by mapping their next finds, been able to see the outline of a dock that formerly existed at the site.