Navigation

ABSTRACT
This section describes the principles of navigation and applies it to the special case of underwater navigation. A description is given of the two types of underwater compasses: direct reading and indirect reading compasses. The usage of underwater compasses is addressed and a step by step instruction is given.

Navigation principles

One of the most important and valuable abilities of the diver should be the ability to navigate. Navigation is the process of knowing where you are and going where you want. The word 'navigation' originates from the Latin word 'navigare', which means 'sailing a ship'. Using navigation the diver finds it way under water and, more important, finds his way back. Being able to navigate has following advantages:

• Knowing where you are and where to go reduces anxiety and stress. Disorientation leads to stress since you do not know whether you are moving to or from your destination.
• It avoids swimming long distances at the surface. Usually, being lost means you have to go to the surface to look where you are or when you run out of air. Swimming at the surface might be undesirable in case of strong currents or in sailing-routes.
• Navigation helps planning your dive. It saves time and air reaching your goals efficiently underwater.

This section describes a number of generic navigation techniques. Next sections apply this to diving. There are a lot of good sites on navigation. Refer to the links for a few of them. Essential ingredients of navigation are

• Knowing your position. At some point you have to know your position. From this position you travel to other positions.
• Measuring distance. In some way you have to know the distance you travel.
• Measuring course. When you travel, it is in a certain direction or course. You have to know this course.
• Referencing. During traveling you can establish your exact position by referencing. This means you reference your observations with e.g. a map.

Measuring position

Measuring position is not common during diving. For the completeness of this section it is explained though. Using a map and a compass you can pinpoint your position by measuring the angle towards two visible landmarks that can be identified on the map (e.g. church towers). By drawing the lines on the map with the measured angles through the landmarks, the crossing of the lines indicates your position. In the example to the right the heading is measured towards a buoy (303°) and towards a house on the shore (41°). The lines with the measured angles are plotted on a map. The crossing of the lines indicates the position where the measurement was performed.

Measuring distance

For a diver there are several ways to estimate the distance covered.

• Counting kick cycles. In one kick cycle each leg has performed a full stroke cycle. It is necessary to swim in the same tempo.
  
• Measuring time. The time elapsed can be used as a measure for distance. It is necessary to swim the same tempo. If the diver stops for a while, you have to correct the measurement for this. Use a watch for this. Many dive computers stop counting dive time at shallow levels.
• Measuring cylinder pressure drop. The amount of air used is an alternative for measuring dive time, and hence distance if the diver swims at the same tempo continuously. This can only be performed when the diver remains at the same depth, since air usage is depth related. If the diver stops for a while, you have to correct the measurement for this.
• Measuring arm span lengths. Short distances can be measured fairly accurate by using arm span lengths. Swimming close to the bottom, you 'walk' with your hands over the bottom. Reach forward with one arm, put the hand on the bottom and move forward. When the hand is under your upper leg, reach forward with the other arm. One arm span length is measured by a full cycle of each arm. Roughly, one arm length equals the length of the divers body.
  
• Using lines or tape. The most accurate method is using marked lines or tapes. It is applicable on relatively flat areas and for short to medium distances.

With the exception of the line/tape method, the methods measure distance relatively. This means distances measured in such way can be compared to other distances measured in the same way. It is possible to convert the measurement to absolute distances (measured in meter, miles, etc). For that, the diver has to calibrate himself. For the use of kick cycles following steps are needed:

1. Swim a section of which the distance is known (e.g. in meter) and count the number of kick cycles.
2. Calculate the average-distance-per-kick-cycle the by dividing the distance by the counted number of kick cycles.
3. When an unknown section is covered counting the number of kick cycles, the diver can calculate the distance by multiplying the number of kick cycles by the average-distance-per-kick-cycle.

Following factors may influence the accuracy of the kick cycle, time and pressure measurement methods:

• Current reduces or enhances the speed of the diver. This influences the distance per kick cycle, time unit or pressure drop unit.
• From own experience: the diver swims slightly faster at the end of the dive (i.e. returning to the entrance).
• From own experience: In cold water the diver gradually gets colder during the dive. When the diver gets cold he uses more air.
• Depth heavily influences the amount of air the diver uses.

Measuring course

Measuring course is usually performed using a compass. Another chapter is dedicated to Compass Navigation.

Referencing

Dead Reckoning and swimming patterns

Dead Reckoning or Deduced Reckoning is the process of starting at a known position and travel to another position only by measuring distance and course. No other observations are used to establish your position. Traveling at sea typically is done by dead reckoning, since no landmarks (seamarks?) are present.

Dead Reckoning is the method that is often used during diving. Usually the diver swims a pattern, measuring distance and heading of each section. The goal is to return to the starting point again eventually. In the example to the right two famous patterns are shown. The to-and-fro pattern is a fairly simple pattern: just head back in the opposite direction. Swimming the square pattern means the diver changes direction 90° each time after swimming a certain distance. More advanced patterns are possible, but they require some more preparation and calculation.

When you end up at the coast it is often wise to add some extra error to the position where you intend to end up. In this case you are sure to end up left or right to your target. In that case you know in what direction to proceed along the coast. If you would not add error you do not know whether you end up left or right from your target. Hence you do not know which direction will bring you to your target. In the example on the right, the diver makes sure he ends up to the right of the target. In this case it is important, because if he ended up to far to the left, he would miss the way out entirely. The error is indicated in green.

Natural Navigation

Natural Navigation is navigation purely based on observations of the environment. No instruments are used.

Observations before the dive

Observation of the dive site before the dive provides a lot of information that is usable for navigation.

• Direction of waves, current and tidal flow. These directions are usually constant during the dive. These directions can be used for referencing direction.
• Direction of the sunlight. This direction can be used as reference for direction. It is most usable at the start and end of the day (sun is low) because the direction of the light is most distinguishable.
• Coastline, formations and objects at the coast. Reefs, buoys, piers, sandbanks are references for position. Waves breaking outside the coast indicate presence of shallow sandbanks or reefs.
• Information and knowledge about the site. Local divemasters can tell a lot. Maps, documentation, depth profiles are other sources provide information as well. The ships sonar can provide bottom contours.

Observations during the dive

Following natural references can be used for navigation during the dive:

• Direction of sunlight. The direction of the sunlight can be a reference for direction. Sunlight casts shadows. These can be used as well.
• Direction of current and waves. Current can be used as reference for direction. Note that the direction of currents can change. Waves produce swell (movement to and fro) under water. This movement can best be sensed in shallow water. Waves move towards the coast. The to and fro movement is roughly perpendicular to the coast. Moving along this direction to shallower water will generally bring you to the coast. Knowledge about the dive site is required.
• Bottom depth profile. The bottom usually follows a distinctive profile. This profile can be used as reference for direction (and location). You can often swim parallel at the coast by swimming at fixed depth along a slope.
• Bottom composition. Bottom composition changes (e.g. coral to sand, sand to vegetation) can be used as references for location. Ribs in the sand near the coast are usually parallel to the coast. These can be used as directional references.
• Sound. Sound produced by your boat, buoy or anchor chains may be references for position.
• Landmarks. Landmarks like rocks, wrecks, etc at the bottom can be great references for position. If their depth is known you can even look for them in murky water by following this depth till the landmark is reached.

Compass Navigation

A compass basically is a magnetic needle that can rotate freely. The earth has a magnetic field. The magnetic north and south pole roughly correspond to the earth geographic North and South Pole. Since equal magnet poles repel and opposite magnet poles attract each other, our magnetic needle will align itself with the earth magnetic field: the south pole of the needle will point to the earth's north pole, the north pole of the needle will point to the earth's south pole. The angle of traveling with respect to the direction indicated by the needle (measured clockwise, always positive) is called magnetic azimuth. With a Compass Reading you measure this magnetic azimuth. The azimuth usually is measured in degrees (0-360°). Other units are mils (0-6400 mil) or grads (0-400 grad). On diving compasses degrees are used.

Direction is measured with respect to the North Pole. This is called the Geographic North or True North. Direction with respect to the True North is called the True Direction. Compasses do not exactly point to the True North. A Compass Reading exhibits Compass Error. Compass Error is due to following facts:

• Magnetic Variation or Declination. The earth magnetic north pole is located near the northern islands of Canada, at approximately 78.9°N latitude and 103.8°W, about 1200 km from the geographic North Pole. A compass points to the magnetic north pole and not to the geographic North Pole. The difference between the True Direction and the compass heading is called Magnetic Variation. The amount of variation depends on the location on earth. It even changes in time, since the magnetic north pole moves a few km per year. Variation is expressed as e.g. 2°25' West. This means at this position the compass needle points 2°25' to the west with respect to true direction of the True North. Variations can be as large as 20°
• Deviation. Ferrous (iron, steel) objects, magnets, flowing electrical current (magnetic field!) influence the reading of the compass. This results in an error in the compass readout. This error depends on the compass heading. Most ships compasses can be adjusted to eliminate entirely or partly the compass deviation. Deviation left is documented on a deviation card. For various compass directions the deviation is documented (usually in a deviation vs. heading plot).

Good maps provide information about variation. In the figure besides the information from a Dutch map is shown. 'GN' stands for Grid North. Grid North indicates the direction of the grid on the map (at the map center). The arrow indicates the location of the Magnetic North. The Grid Magnetic Angle (indicated by the arc) is the difference between the Grid North and the Magnetic North. It is 4.5° West (usually rounded to a half degree). The star indicates the True North. Grid Convergence is the difference between the Grid North and True North. It is 1°22' West. Hence the magnetic variation (angle between Magnetic North and True North) is 4°30' -1°22' = 3°08' W. Since the variation is to the west, 3°08' has to be subtracted from a compass reading to obtain the True Direction. The timestamp of this deviation is given. The annual change of the Magnetic North is 8' East. Each year the Magnetic North moves 8 minutes (8/60 degrees) to the East. Per year, this amount has to be subtracted from the variation and Grid Magnetic Angle. In 2003 the Grid Magnetic Angle is 4°30' - (2003-1985) 8' = 2°06'. Note: Grid Convergence remains constant in time.

Compasses exist for the Northern Hemisphere and Southern Hemisphere. At the Northern Hemisphere the south pole of the compass needle (points to the North) is pulled towards the earth (magnetic north pole) whereas the north pole of the compass needle is pushed upward. At the Southern Hemisphere this is opposite. Good compasses are compensated for this.

In comparison to ship compasses, diving compasses are fairly inaccurate. As a diver we do not worry about compass error. Since a diver only travel small distances the compass error will result in only small absolute deviations in the divers location. Furthermore, a diver usually uses his compass for relative direction (in contrast to using a compass together with a map for absolute directions).

Diving compasses

On a diving compass following parts are discerned:

• Housing, usually filled with fluid. Two types are usual: a type worn on the wrist and one integrated in the divers console.
• Magnetic needle or card, always pointing to North. Though it is common practice to state that the needle or blade rotates in the housing, it is better to think of the needle or blade being steady, always pointing North. The housing rotates around the needle. The card contains an arrow indicating North. Thereby it often contains markings for East, West and South and/or a degree scale.
• Degree scale with degree markings on it. It is on the housing or on the rotating bezel, depending on the type of compass.
• Rotating bezel containing markers: usually two notches at one side and one notch at the opposite side.
• Lubber line indicating the direction of movement. On a diving compass it is usually printed on the window straight over the card. On some compasses it might be an arrow printed besides the window.

Most compasses are filled with fluid. This serves to purposes. First, it dampens the motion of the needle or card. Secondly, it makes the compass withstand the pressure at depth.

Sometimes there is a small window at the front side of the compass. Using an index on this window, the diver can directly read the course from degree markings on the card. For this, the degree markings on the blade have to be inverted: 0 degrees is at the South indicator, whereas 180 degrees is at the North indicator.

Diving compasses come in two flavors:

• Direct reading compass. On this type of compass the degree markings increase counter clockwise and fixed on the compass housing. The magnetic needle or card always indicates the direction to which the lubber line points. The rotating bezel only contains an index that points to the degree markings on the housing.
• Indirect reading compass. On this type of compass the degree markings increase clockwise and on the rotating bezel. If the rotating bezel is rotated such that the needle corresponds to the zero degree marking, the upper side of the bezel (often indicated by the lubber line) indicates the direction of the lubber line.

Indirect Reading Compass	Direct Reading Compass

Note: there is a bit of confusion in articles on the Internet between 'direct reading' and 'indirect reading'. These terms are not used consistently. I think the definition above is the right one: on a direct reading compass the course can always be read, independent of the position of the bezel: it is indicated on the scale by the North indicator of the needle or card.

Using a diving compass

When using an underwater compass following points are important:

• Make sure to keep ferrous (iron, steel) objects away from the compass. The needle is attracted to ferrous material and the readout is influenced.
• Line up your body with the lubber line. For wrist worn compasses this is accomplished by placing the hand of the arm on which you are wearing the compass on the elbow of the other arm, which is held straight in front of you. Hold the compass at eye level so you are sighting over it instead of down on it. If the compass is in a console, hold it in front of you and line your body up with the lubber line.

Reading out a direction

Often a diver decides which course to swim when he or she is standing at the waterline at the dive site. In many cases the environment defines the course. The diver picks a natural target to swim to, e.g. he wants to swim parallel to the coast. Using a compass the diver measures the course to the target. Under water he swims the to the target.

Point the compass to the target. This is done by imaginary extending the lubber line to the target.	On the compass the course can be read by looking through the front window of the compass and using the scale on the card. In this case 150° is read.	On a direct reading compass the North arrow points to the course (150°) on the degree scale on the housing. For this it is not necessary to rotate the bezel (shown in dark blue). The bezel can be rotated to indicate the coarse as an aid to memorize the course

Point the compass to the target. This is done by imaginary extending the lubber line to the target.	On the compass the course can be read by looking through the front window of the compass and using the scale on the card. In this case 150° is read.	On a indirect reading compass the bezel (dark blue) has to be rotated so that the index (double notches) line up with the North indication on the card (or needle). The upper end of the lubber line indicates the course (150°) on the bezel degree scale.

Swimming a course

This section describes the procedure to swim a predefined course. Distinction is made between a direct reading compass and an indirect reading compass. In the examples below, the diver intends to swim a course of 240°.

Direct reading compass: starting position. The magnetic card points to the North, the bezel is in the default position.	Rotate the bezel such that the double markers points to 240 degrees.	Now rotate the entire compass such that the North Arrow on the magnetic card points between the double markers. The lubber line now points in the 240 degrees direction.

Indirect reading compass: starting position. The magnetic card points to the North, the bezel is in the default position.	Rotate bezel until the 240 degrees mark on it corresponds to the lubber line.	Now rotate the entire compass such that the North Arrow on the magnetic card points between the double markers. The lubber line now points in the 240 degrees direction.