If there is a surge at the bottom, and the ripple crests are perpendicular the direction of the surge, then the wave crests will be parallel to the ripple crests. Ripple crests, like surge, may be interpreted with a 180° error. Dip and strike of bedrock Many rock formations have characteristic angles known as dip and strike. Dip is the slope of the strata from the horizontal, and strike is the general direction of the strata in the horizontal plane (very roughly). These characteristics will usually be similar in the rocks above and below the water in a locality, so they can be used to estimate direction. Ridges above and below water are often parallel, and gullies and valleys may well extend under water for considerable distances. Ecological variations Different areas may for a wide variety of reasons have different ecologies. A diver who is familiar with an area can use the diversity variations and patterns to provide orientation cues. There is often variation of ecological zoning with depth, but a diver is expected to be aware of the depth all the time anyway. In some places the seaward side of big rocks may have different species from the shoreward face because of the greater exposure to wave action. Sea fans and sponges are filter feeders, and may grow into a fan shape at right angles to the usual current or surge direction, to get the maximum volume of water flowing past them. Using a compass How a compass works The magnetic compass indicates the local direction of the ambient magnetic field, which is usually that of the Earth. This is usually a reliable and consistent feature and is very useful as a navigational aid as it is not affected by visibility, pressure, or the presence of water. An important concept is that the compass card should not turn, even though it appears to always “swing” to magnetic north. The housing that holds the compass card turns around the card, which remains pointing in the same direction (Magnetic North) all the time. There are occasions when the card does turn, but this is when it has been stuck or the compass is turned over, and the card is unable to remain aligned with the magnetic field. True or geographic north True north is the geometrically accurate direction along the surface of the earth toward the North pole of the planet’s