Mapping Dive Sites

Mapping dive sites can be an important part of diving.  In it's simplest form a dive site map is nothing more then a bearing from a known object.  More complex maps can consists of a cross bearing (for high-accuracy marking of a dive site), to a set of bearings (to "tour" a dive site), to complex and highly accurate maps of a dive location.  For 99% of the diving most people do a map need not have more then a few bearings and landmarks to work from.  The main techniques of this types of mapping is covered in this article.

Index:

• Equipment: Things you will need
• Mapping From Water: how to map a site when you are in the water
• Mapping from land: mapping a site form the shoe.

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Equipment:
Before you can start mapping a site you will need some basic equipment.  Below is a description of some things you can use:

Underwater Compass:
At a bare minimum you will need an underwater compass.  This will allow you to take bearings to landmarks while in the water

Slate:
A waterproof writing slate is a valuable piece of equipment as it allows you to write down measurements and bearings while underwater.

Land Compass:
Land compasses tend to be more accurate than underwater compasses.  These compasses will allow you to measure bearings from shore, although you will need to somehow mark the site.

Marker Buoy's:
One way of marking underwater sites so they are visible on the surface is with a marker buoy.  These buoy's are anchored at the site you want to map and then released to the surface.  This allows easy spotting of the site from shore.  Many things can be used as a marker buoy - commercial buoys are available.  Or you can use your dive flag or lift bag.  Even a painted pop-bottle or milk container attached to a rope can be used.

GPS:
GPS (global positioning units) are one of the most accurate way's to mark dive site.  This system uses signals from a series of satellites to measure your position on the earth to within less then 10m (30').   However, at the time this article was written there were no GPS units that could stand the water pressure of diving, so GPS units have to be left on a boat.  In addition, GPS units cannot receive their satellite signals underwater, so they will never be of use for underwater navigation.

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Mapping from the Water:
Most of the mapping you will do will be from the water.  All you need for this mapping is an underwater compass, although a slate is useful if you have a bad memory.  Generally speaking all you need to do in order to map from the water is surface and take some bearings.  Described below are two methods of in-water mapping.

Single Bearing:
The easiest way to map a site is to surface immediately above the site and take a bearing to an obvious object on land.  Ideally you want to use something that will not be moved (tree, hilltop, lamppost).  Measuring a bearing is easy - it is covered in detail in both the underwater compass page, as well as in our land compass page.

There are two main problems with this first technique.  First, you have no idea how far from shore you are.  Secondly wind and current can carry you away from the dive site before you take your bearing.  The first problem isn't much of an issue - all you have to do is swim underwater along your bearing and eventually you'll reach your site.  Of course you can estimate the distance, either by guessing when you surface, or by swimming underwater and counting kick-cycles.

Current is a little harder to deal with.  The best way to deal with current is to use a marker buoy.  While taking your bearing you simply remain near the buoy.  This will allow you to measure the bearing accurately without having to worry about the current.

Figure 1: Diver surfaces from his dive.  In order to find the dive site the next time he takes a bearing to an object on shore.  Note: This method does not allow the diver to measure distance to shore, so to find the site in the future the diver will have to enter the water and swim the reciprocal bearing.

Cross Bearings:
Sometimes you will need higher accuracy then what a single bearing will give you.  This is especially true when you are a long ways from shore (i.e. while diving from a boat), or the dive site is small (i.e. a small shipwreck).  In these cases you can take cross bearings to accurately mark the site.  This is simple to do.  When you surface (or while one the boat) pick two landmarks on shore.  Ideally you want these to be separated by at least 60 degrees, but not more than 130 degrees.  Ideally you want an angle of 90 degrees, although it is better to pick two obvious objects with a less then ideal positioning then two not obvious objects at the ideal angle.  Once you've picked two objects you must measure the bearing to each.  That's it!  You can then navigate to the dive site using the techniques described in the navigation page.

Figure 2: Cross bearings accurately identify the location of a site.

Important Note:
The above two mapping methods will give you the bearings from the dive site to the objects on shore.  However, if you are on shore these bearings will not point towards the dive site - in fact they will point away from the dive site!  The reason for this is that the bearing you took points from the dive site to shore - so if you're on shore and take the same bearing you will be facing away from the dive site.  In order to use the bearing from shore you must "reverse" the direction they point - or in other words calculate the reciprocal bearing.  This is easy - if the bearing you took lies between 0 and 179 degrees add 180 degrees.  If the bearing you took is between 180 and 360 degrees you must subtract 180 degrees.  When I map a site I always write both the "site to shore" and "shore to site" bearings on the map.

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Mapping From Shore:
Mapping from shore is easier then mapping from the water.  You don't have to worry about currents moving you, or sighting a landmark while waves throw you about.  But in order for you to map from shore you need to mark the dive site with a buoy or some other marker.  One of the main advantages of shore-based mapping is that you can accurately measure distances, something which is difficult in the water.  You will want to invest in a proper land compass for shore mapping - the use of these compasses is described in detail in our compasses page.

There are two types of maps usually made from shore - single bearing and cross bearing maps.  Just like the maps you make from the water!  The difference is that you must first mark the dive site so you can see it from shore - a marker buoy is the best way to do this.  Once the dive site is marked you must then identify one or more landmarks you will be able to see from the water - a map based on landmarks a diver can't see is useless.  For mapping form shore you want landmarks located close to the dive site - ideally you should be able to stand by the object and see the dive site.  As such hill tops, or far away objects are not useful.  Instead trees, signs, buildings, lampposts and other objects located immediately next to the dive site are preferred.  To make your map go to your landmark(s) and take a bearing to the buoy marking the dive site.  If designing a map with cross bearings go to your second site and take the bearing from there to your dive site.  That's it - you now have a bearing, or cross bearings, to the dive site! 

Single Bearing + Distance:
As mentioned above, one of the leading advantages of mapping form shore is you can measure the distance from a land mark to your dive site.  In order to do this you will need a compass, a scientific calculator, and a long length of rope.  It is important to note that this technique is only accurate for measuring distances to near-shore objects - the farther away the dive site is the less accurate this will be.

The first step in this procedure is to take a single bearing to the dive site, as you would for a single bearing map.  Once this is done take the bearing and add (or subtract) 90 degrees from it.  Stretch out your rope on this new bearing.  From the end of the rope measure the bearing to the dive site - it should be different then the original bearing.  Ideally you want the new bearing to be at least 10 degrees different then the first bearing - if it is not you need a longer rope.  Once you have the two bearings you have all you need to determine the distance to the dive site. 

Do you remember your high school trigonometry?  I'm sure when you took that class you didn't think  you would ever use that little bit of knowledge, but you will!!!  Those measurements you've taken are enough to reproduce all of the sides and angles within a triangle.  Now one of those sides happens to be the distance from you landmark to the dive site, which is all we are really interested in.  Below is a picture of what we're doing.  We know B (this is the length of the rope), as well as angle c (90 degrees).  We can calculate the rest - although all we are interested in is A - the distance to the dive site.

Here's how it is done:

1. First we need to calculate angle b - this is simply the difference between the two bearings we took.
2. Next we need to calculate angle a - this is done by subtracting the angle calculated in #1 from 90.
3. Finally we calculate length A.  This is done using the following formula: A = Tan(a)*B
   • Tan is a calculation done by your calculator.  You will need to review your calculators manual to determine how to use its' Tan function.

Here's and example:

You attach a buoy to your favorite wreck and return to shore.  From the weird dead tree in the dive sites' parking lot you measure a bearing to your buoy of 100 degrees.  You then attach you 30m (100') rope to the tree and stretch it out at a bearing of 190 degrees (this is 90 degrees from your initial bearing).  From the end of your rope you take a second bearing to your buoy - this time it is 85 degrees.  Now we start calculating:

• First we determine angle b: this is the difference between our second and first bearing.  In this case that is 100-85 = 15 degrees.
• Now we determine angle a: this is simply 90 - the number calculated above (90-15 = 75)
• We can now calculate the distance to the ship wreak.  Keep in mind our rope is 30m (100') long.  Here we go:
  • First we calculate the value of Tan(75).  When you enter this into your calculator you should get a value of 3.372
  • Next we multiply the value of Tan(75) by the length of our rope - that is 3.372 * 30m = 101.16m (337')

That's it - the dive site is located approximately 100m (330') from our landmark at a bearing of 100 degrees.