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compass reading for cave survey

Compass Reading for Cave Survey

The basics of understanding and reading a compass. This training is only intended to cover compasses. For more advanced map reading or inclinometer reading please see future training materials. Written and presented by Adam Sampson for the Kentucky Karst Conservancy .

  • What is a compass and how does it work
  • How to read a compass
  • Practice reading a compass
  • Objects that can interfere with your compass
  • Practice around sources of interference
  • Understanding backshots and calculating them quickly
  • Practice backshots with a partner
  • Practice on a 5 point compass course
  • Advanced: Closing loops to verify accuracy
  • Discuss next steps
what is a compass
What is a Compass
  • Compasses are devices that measure the magnetic field wherever you are.
  • Compasses are affected by the earth, flowing electricity and some metals like steel or iron.
  • The magnetic field of the earth is horizontal near the equator and more vertical near the poles. The strength is also not constant.
  • The magnetic north and "true" north are not the same and you have to write which you are using, and on what date you measured .
  • Magnetic north changes a small amount daily.
reading a compass
Reading a Compass
  • We use compasses to determine which direction one object is from another. Often we are seeing what direction a landmark is from ourself.
  • We use a compass to help us compare information on a map to what we can see around us.
  • We can use a compass to read maps or draw maps.
  • Use a map in person with a mentor to see more about this. (Teaser for more advanced material)
reading a compass1
Reading a Compass
  • Survey compasses are broken into 360 directions called degrees(°).
  • Magnetic north is 0°, east is 90°, south is 180°, and west is 270°.
  • Compasses are a magnetic needle floating in water. You can notice this by tilting your compass and watching the dial move around.
  • Compasses must be held level in order to get a reading. Otherwise they get caught touching the glass or metal and can't spin freely.
  • You read a Suunto compass by looking through the lens on the end. This shows you accurately down to 1° increments.
  • Looking in the lens the top number is the direction you are facing (front shot), the bottom number is your backshot.
  • Line up your compass by looking through your viewport and then using your eye to look above or below the compass. Repeat until the sighting line in your compass is lined up with your target and then read which number that is.
practice reading a compass
Practice Reading a Compass
  • Look through the viewport on your compass and get used to counting the markers between the numbers. Notice how many markers there are between the numbers.
  • Pick two objects to line up between and take a shot. Wooden fence posts, trees, or rocks make good objects. Avoid metal or electricity. Everyone take the same shot so you can compare results. Repeat until comfortable.
  • Use a different compass and repeat number 2. Notice any difference? Compasses can be different.
  • Try taking the shot from close to your starting object. Now stand several steps from the starting object and take the reading again. Is it easier to get a good reading when up close or when far away? Does it matter?
  • Take a reading with your compass level. Now while looking at the numbers tilt the compass up at a 45 degree angle. Turn around and see how the compass doesn't turn freely. Tilt the compass back to horizontal and see how the compass adjusts. Try at different angles that aren't level.
  • Look at a map of your local area. Find something on the map and what direction it is on the map. Use your compass to point at the object. Good objects might be hills, buildings, radio towers, creeks, roads, etc. (Teaser for more advanced material)
  • Look around and find something near you that would be on a map. Find out what direction it is and then look on the map to find it on the map.(Teaser for more advanced material)
magnetic interference
Magnetic Interference
  • Compasses are deflected by magnetic fields. Usually this is from the earth. But electronics make magnetic fields too. And some metals (such as iron and steel) bend the magnetic fields that pass through them.
    • Electronics will deflect a nearby compass. The amount of electricity and length of wire determine this. Think about lights, distance meters, power lines, motors, phones, etc.
    • Steel and iron will deflect a nearby compass. Don't worry about the just happens.
    • Some metals don't cause enough effect to worry us. The ones that we don't worry too much about are copper, aluminum, silver, gold, lead, magnesium, platinum, and tungsten. Most wedding rings aren't a big deal.

Compasses on steel carabiners...

practice around interference
Practice Around Interference
  • While looking at a compass bring an iron or steel pipe close to your compass. See how the compass deflects towards the pipe.
  • While looking at a compass bring a variety of lights near the compass. See how the compass deflects towards the lights. Which of your lights causes more interference? Does turning the light on or off have an effect? Does your light have metal parts in it that cause interference beyond the electricity?
  • Look around and see if you can find any other objects that cause interference. Examples include power lines, fences, etc. See how they may effect your compass.
  • Take a shot from a piece of steel or light that causes interferenece. First take the shot from up close. Then have your partner do the same. Now back up a foot and take another shot. Compare how much your measurement changed. Does it matter whether the object causing the interference is north, south, east, or west? Try a couple of shots in different directions and from close and far distances.
understanding backshots
Understanding Backshots
  • We use backshots to decrease the chance of an error. By comparing backshots we are double checking our work. Errors here tell us to think about what be causing problems and changing them.
    • Interference, tilt, or hard to reach survey points can cause one side to have high error.
  • Our backshot is the compass opposite of our front shot.
    • When telling people your reading call out "frontshot" or "backshot" before the number. This helps keep things straight later.
  • Before taking backshots you have to make sure both compasses read the same. If your compasses don't agree on a frontshot, the backshot won't agree either.
  • If your shot is under 180 degrees then you add 180 to find its opposite. If your shot is over 180 degrees then you subtract 180 to find its opposite. We don't use numbers over 360 or less than 0.
    • Example: 191 degrees - 180 = 11 degrees.
calculating backshots quickly
Calculating Backshots Quickly
  • You can do an easy check before trying more math
    • How close is the right (ones) digit? Don't overthink this part. Remember if it is more than 2 degrees apart we are going to throw this data away.
      • Example: 191 and 8 -- 1 and 8 have a minimum difference of 3 on the last digit. Data isn't good enough.
    • Remove the right (ones) digit on both numbers. Are the remaining numbers close to 18 apart?
      • Example: 191 and 8 -- 19 and 00 are 19 apart. Close enough to look closer.
      • Example: 350 and 132 -- 35 and 13 are 22 apart. Something is wrong.
  • Tips and Tricks
    • Some people use this trick. For a shot under 180 you can add 200 and then subtract 20. For a shot over 180 you can add 20 and then subtract 200.
      • Example: 191 + 20 - 200 = 211 - 200 = 11
      • Example: 73 + 200 - 20 = 273 - 20 = 253
practicing backshots
Practicing Backshots
  • Set up two points between you an a buddy. Have one person shoot front shot and the other shoot back shot. Do the math and see how far off your results are.
  • Switch places and repeat the first exercise.
  • Replace one of your points with something that is magnetic so that it only affects one of you. Take shots and calculate how far off your readings are. See how far back you have to be until your readings are within 1 degree.
practice on a 5 point compass course
Practice on a 5 point compass course
  • Your instructor should set up a course for you to practice on. This course should have a number of small issues for you to overcome.
    • On each shot consider the following:
    • Am I in a safe spot to be doing this?
    • Am I in a good spot to take a steady reading or am I shaking on my tiptoes? Do I even have enough room to take the shot?
    • Am I keeping my compass level?
    • Am I keeping sources of interference such as lights and carabiners away from my compass?
    • In my team, do our frontshot and backshot match?
    • Are there any other sources of magnetic interference around me? How can I deal with them?
advanced closing loops
Advanced - Closing Loops
  • To close loops we usually break our information into X and Y coordinates to make things easier.
  • To calculate our X and Y coordinates we use a bit of geometry.
  • We start with coordinates of Point 1 with coordinates of X1 and Y1.
  • We will end calculating coordinates of Point 2 with coordinates of X2 and Y2
  • New X = Starting X + Change in X
  • X2 = X1 + sin(Angle)*Straight_Distance
  • Y2 = Y1 + cos(Angle)*Straight_Distance
  • MOST calculators will do this for any angle. But you should practice thinking about whether your answer makes sense.
    • Example: 91 degrees is almost all X.
    • Example: 181 degrees is almost all Y.


Point 2





Point 1



advanced closing loops1
Advanced - Closing Loops
  • Too close a loop you figure out the X and Y coordinates of all your points. When you calculate your last shot back to your starting point you will see how much error you have.
  • (Your Straight Distance is actually affected by your inclination and has to be calculated. More info to come in Inclinometer class. Pretend our shots are level here.)
  • X-change = sin(Ang)*Strt_Distance
  • Y-change= cos(Ang)*Strt_Distance
  • [Hint: Most Cell Phones can do this with the stock calculator in landscape mode.]
advanced closing loops2
Advanced - Closing Loops
  • If you want to see what it is like to account for inclination (next class), the equations are:
  • Z-change =
    • sin(incline)*straight distance
  • Horizontal Distance =
    • cos(incline)*straight distance
  • X-change =
    • sin(Frontshot)*Horizontal Distance
  • Y-change =
    • cos(Frontshot)*Horizontal Distance
what s next
What's Next?
  • Learning to read an inclinometer (clino).
  • Learning to use a compass and a map to walk an orientation course.
  • Learning to use a compass and clino to draw shots on graph paper.
  • Learning the basics of cave survey sketching.
  • etc...