How to Read and Draw Strike And Dip Symbols on a Geological Map
Strike and dip symbols are how geologists communicate the 3D orientation of rock layers on a 2D map. They look simple—basically a T-shaped mark with a number—but getting them right matters because an erroneous symbol can send an entire cross-section interpretation in the wrong direction. I have spent years field mapping and correcting other people's maps, and the most common mistake I see is people confusing which way the dip points. The strike is the compass direction of a horizontal line drawn on the tilted plane. It is always measured relative to true north, not magnetic north. If you are using a compass without accounting for declination, your strike values will be consistently wrong. The dip is the steepest angle of descent measured perpendicular to the strike, going from horizontal down to the plane. A vertical bed has a dip of 90 degrees and its strike is essentially meaningless since any direction is horizontal along that plane. A flat-lying bed has a dip of 0 degrees, which is why you sometimes see a circle with a dot in the center instead of the usual T symbol—that is the standard representation for horizontal strata.
Understanding Strike And Dip Symbols in Practice
Here is how the symbol actually works on paper. You draw a long line in the direction of the strike, then from the center of that line you draw a short perpendicular tick pointing in the direction the layer tilts. Next to the short tick you write the dip angle. A bed striking due east at 30 degrees south would look like a horizontal line with a short tick pointing downward on the page and the number 30 beside it. That convention assumes north is up on your map, which is standard but not universal, so always check your map projection before interpreting anyone else's symbols. I once spent two full days redoing a structural analysis because someone on a previous mapping team had flipped the dip direction on about thirty percent of their measurements. The strike values were correct, but the dip ticks were pointing the opposite way. The structure looked like a reasonable anticline until I overlaid the cross-section and realized the limbs were dipping into each other in a way that made no geological sense. The fix was to go back to outcrop and remeasure, but it also showed me that relying solely on symbols without checking the original field notes is a fragile workflow. One thing beginners consistently miss is that the dip direction is always 90 degrees clockwise from the strike direction when you are using the standard right-hand rule convention. If your strike is 045 degrees, the dip direction is 135 degrees. Some regions and some companies use the three-point method instead, recording strike, dip, and dip direction as three separate numbers rather than relying on the tick mark to convey the dip direction visually. That approach eliminates the possibility of a flipped tick being misread, but it requires more data per measurement and slows down field documentation significantly.
Another nuance that causes problems is what happens when the plane is nearly vertical. When the dip approaches 90 degrees, the strike line becomes extremely sensitive to small compass errors. A two-degree error in dip near vertical translates into a massive shift in where that plane intersects with anything else. I usually recommend treating any measurement above 80 degrees dip with extra caution, verifying it with at least three distinct outcrop exposures rather than taking a single reading and moving on. There is also the issue of overturned folds, where the bedding has been tilted past vertical so that the dip direction on one limb appears to contradict the overall structural grain. A student once submitted a map where every measurement on the southern limb of a fold had the dip tick reversed because they were visually drawing the symbol to match the apparent younging direction rather than the actual measured dip. The symbols looked cleaner but the data was wrong. The workaround is to measure the dip first, decide on the tick direction strictly from that measurement, and only then worry about whether the result looks structurally intuitive. If you need a template or symbol guide to keep in the field, most geological survey organizations publish downloadable symbol sheets. The British Geological Survey and the USGS both offer PDF reference cards that cover strike and dip, lineations, fold axes, and other common structural symbols. Search for the specific survey you are working in, since some countries have minor regional variations in how they format the notation. The underlying geometry does not change, but the visual style sometimes does.
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