Compass Tutorial

Selecting a Compass

There are many brands and types of compasses available with a variety of features and prices. For rock art recording, essential features are an adjustment for magnetic declination and a built in 90-degree clinometer.¹ Look for something lightweight, rugged enough to survive several accidental drops, and small enough to easily fit into a pocket. Inclusion of these features will probably limit your selection to the old fashioned kind that work without batteries.

Other common features that you may or may not require are:

• Meridian orientation lines
• Hinged body to fold more compactly
  • cover clip
• Notched sighting mirror
• Ruler in metric and/or inches
• Built in magnifying glass
• Scales for topo maps in various sizes
  • 1:25,000 (most common)
  • 1:50,000
  • 1:24,000 (rare)
• Silicon feet to grip topo maps
• Luminous points
• Lanyard
  • Screwdriver
  • Adjustable slider

In addition to the above, there are a wide variety of other imaginative features available. Avoid any compass models that have each quadrant marked 0-90 degrees on the bezel instead of the standard 0-360 degrees -- these may have the letter Q within the model number. Be aware that some inexpensive compasses may have a clinometer that measures less than 90 degrees -- these are not useful for rock art recording.

Because a compass clinometer is frequently hard to read, some people may prefer a separate clinometer that is larger and easier to read. These may be hard to find and are frequently not called clinometers or inclinometers. You may find units in building supply houses that are used to measure the pitch of a roof. One inexpensive unit is called the Empire Polycast Magnetic Protractor. The main disadvantage of a separate unit is that it is one more piece of equipment that must be carried over the rocks and through the bushes and you may run out of hands and pockets.

You should expect to pay about $25 to $100 for a compass. If you know nothing about compass features, a useful exercise is to compare the features of your selection against a Silva Ranger Type 15,² a Brunton Nexus Elite 15TDCL, or a Suunto MC-2DL navigator. List prices for these models are about $80, discounted prices may be as low as $40.

The most common failure of a compass is for the fluid supporting the needle within the dial to leak out and an air bubble to appear. Should this happen, your compass should be returned for repair or discarded.

Note that while every rock art recorder should know how to use a compass, not every rock art recorder may need a compass -- you may only need one compass per recording team.

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Setting the Declination

Before using a compass, you must adjust it to compensate for the fact that the earth's magnetic north is located in Hudson Bay in Canada, and not the north pole. The magnetic declination varies depending upon your location. In Arizona it varies from 10 to 13 degrees East, but in parts of Alaska it may be 90 degrees East and in Maine it may be 17 degrees West. The traditional way to determine the appropriate declination is to consult a topo map. If you know a U.S. zip code, a faster way is to visit this NOAA web page.

If your compass has a hinged body, open it flat and hold it or lay it on a table with the lanyard (string) closest to your body. If you have no lanyard, determine the top of the compass as best you can and hold or place the top away from your body.

Attached to the body will be a rotating dial marked with degree numbers (usually from 20 to 360 in increments of 20). Turn the dial so that 360 is at the top of the dial precisely opposite of the marker imprinted on the compass body.

If there is a lanyard, there may be a tiny metal screwdriver attached to it and and a tiny corresponding screw embedded in the top (or bottom) of the rotating dial. Within the dial, there will usually be a floating needle with a red tip that points to magnetic North and a larger outlined arrow fixed to the rotating dial.¹ Turn the screw so the arrow normally fixed to the rotating dial coincides with the magnetic declination for your location.² For example, if your topo map shows magnetic North to be 12 degrees east, the "fixed" red arrow must point to 12 degrees East of North.

Assuming you know the direction of true North, you may test your new setting. With 360 on the rotating dial opposite the marker at the compass top, stand and hold the compass body level with the lanyard close to your body, and then rotate your body so the red tip of the floating needle aligns perfectly with the red arrow fixed to the rotating dial. Look directly forward -- the direction you are facing should be true North.

Turn the rotating dial so that 90 (or the mark between 80 and 100 usually labeled E) is at the top of the dial. Again, look down at the compass and rotate your body so the two red arrows align perfectly and look up -- you should be facing East. Practice with the dial facing South (180) and West (270).

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Compass Basics

If you have not used a compass before, at first it will seem awkward to have the fixed arrow not pointing at North or 360, but pointing off to the side to reflect the local magnetic declination. Over time you will become adjusted and this will seem quite natural. When using a compass there are just two basic functions to master:

1. Set the desired bearing number at the top of your compass dial and rotate the compass until the arrows align.
2. Point the compass at your target, rotate the dial until the arrows align, and then read the bearing number from the top of the dial.

When recording rock art, you will always be using function 2 above.

Measuring a Bearing

In rock art recording, a common requirement is to produce topo charts that map the location of the various panels, boulders, or clusters. To start, you need a datum point. A datum point will frequently be a prominent rock art panel that is easily accessed or it may be a permanent stake in the ground or other prominent feature. Multiple panels may be mapped from the datum point creating a star-like topo chart. In addition to the primary datum point, large rock art sites may have several secondary datum points, resulting in topo charts with multiple stars. When recording panels on a cliff face, every panel may be a secondary (or the starting primary) datum point resulting in a topo chart that appears as a jagged string of panels.

Whatever the case, a panel bearing is always taken from a datum point to the target panel. In addition to the number of the target panel, the recording form must reflect both the datum number or name and the bearing in degrees measured from true north.

Before taking bearing measurements, the recorder must make herself aware of potential magnetic interference from the local rocks, a steel measuring tape, belt buckles, power lines, or other sources. If possible, it is a good idea to stand away from any potential sources of magnetic interference and make a mental note of a landmark on the horizon close to magnetic North (or South) before beginning to measure a site. Then as each measurement is taken, do a quick check to insure the needle is pointing in the expected direction.

To measure a bearing, you will normally stand near the datum point and walk around the datum point until you, the datum point, and the target panel are in direct alignment.

If you have a hinged compass with a mirror, the normal way to measure a bearing is to open the hinge to a 45 degree angle, hold the compass flat in front of you at eye level with one hand and point it at your target, and then using the mirror to view the top of the dial rotate the dial with the other hand until the two arrows align. Once the arrows are aligned, flip the hinge open a bit more so you can read the number at the top of the dial. This is the most accurate method, but requires a compass with a hinged mirror.

If you and a partner are measuring the distance between the datum point and the target panel with a non-magnetic tape, then an alternative method is to completely open the hinge so the compass body is flat, hold the stretched measuring tape and the compass in one hand with the compass body parallel to the tape, rotate the dial until the red-tipped arrows align, and read the bearing number from the top of the dial. Be sure to hold the compass well away from the datum point if there is a possibility it may cause magnetic interference.

A third method is to open the hinge completely, hold the compass body flat at waist level, point the top of the compass at the target panel, rotate the dial until the red-tipped arrows align, and read the bearing number from the top of the dial. When using this method the compass will be held close to your body -- be aware of possible magnetic interference from belt buckles, metal buttons or rivets in clothing or keys in your pockets.

Measuring the Elevation Change

Measurement of the elevation change from the datum point to the target panel is not a common measurement. If the rock art site being recorded is a relatively level boulder strewn field, then elevation changes will usually be near zero and the measurement can be omitted. However when working on sites composed of scree, cliff faces, or caves, the inclusion of an elevation change can be a useful measurement.

Elevation change measurements can be made using the clinometer feature of a compass. The clinometer feature consists of a tiny arrow on a pendulum that rotates around the dial based upon the pull of gravity and degree markings inside the dial that go from 0 to 90 degrees. Frequently there will be two 90 degree scales inside the dial for ease of use.

By convention, a perfect horizontal measurement is 0 degrees. If the target panel is directly above the datum point the elevation change is +90 degrees, and should the target panel be directly below the datum the measurement would be -90 degrees. Thus, all elevation change measurements will be in the range of -90 to +90 degrees. You can think of this measurement as being similar to road signs that advise you of 6% down-grades or 7% up-grades on the road ahead.

When using the clinometer, it is a good practice to use the long edge of the compass body as the pointing device or measuring surface. On most compasses, this will require rotating the compass dial so the 90 degree mark is at the top of the compass. Then, when a long edge of the compass body is placed on a horizontal surface such as a table top (you may have to flip the compass body so the clinometer degree measurements are at the bottom) you should obtain a reading of 0 degrees.

To measure an elevation change, try to position yourself between and off to the side of both the datum point and target panel. Then with the compass dial set as described above, hold the compass body on edge and position the long edge of the compass body to approximate the angle of elevation change from the datum to the target panel and read the degree measurement. Because there are no positive or negative measurements on the clinometer, you will have to remember to add the negative sign on the recording form for all downward measurements.

Measuring the Facing

If the world were perfect, all rock art panels would be perfectly vertical, flat, and have clearly defined edges. If this were true, then facing and inclination measurements could be made easily and accurately. Let us begin by describing how to measure the facing in a perfect world.

Assume the rock art panel to be measured is on the wall of a room. The facing is the direction the panel is facing. For example a picture hung on the North wall of a room is facing South, or 180 degrees. To obtain a reading, completely open the compass body, stand at the wall and press the bottom edge of the compass against the wall, rotate the dial until the two red arrows align, and read the degree measurement from the top of the compass.¹

In the real world, the above technique should not be used because many rocks are magnetic and holding the compass close to the rock will yield an inaccurate reading. In addition, the rock art panel may be rounded, jagged, bumpy, or horizontal. A better technique is to stand facing the panel several feet away and hold the compass body backwards (lanyard toward the panel), rotate the compass body to approximate the panel facing, then rotate the dial until the arrows coincide, and finally take the degree reading from the top of the compass.

An alternative method for advanced users is to hold the lanyard toward your body while making the measurements and then take the reading from the bottom of the compass dial. Choose the method that works best for you and stick to it.

If the rock art panel is perfectly horizontal and has an inclination reading of 0 (like a table top) or 180 (like a ceiling) then the facing is recorded as 0.

If the measured facing is true North, it is recorded as 360 (not 0).

For all panels with an inclination of less than 90 degrees (a shifted table top), the facing is the direction of the slope of the panel. For all panels with an inclination of more than 90 degrees (an overhang or shifted ceiling), the facing is the opposite direction of the slope. To picture this, draw two rock art elements near the top and bottom of a sheet of paper, fold the paper in half, then open it so the bottom half has an inclination of 45 about degrees and the top half has an inclination of about 135 degrees. The facing of both halves will be the same.

You may encounter situations where a boulder has shifted or fallen and the present facing of the panel is not of the same orientation as the elements on the panel. In this case, the rule is to measure the facing of the rock rather than the elements, and make a note on the recording form. For example, the quick sketch will show the panel elements in their natural orientation and add a noted arrow showing the present facing direction.

In many cases the panel being measured will have an irregular or rounded shape. In this case, you should try to measure the average facing of the individual elements. If there is one dominant element and several minor elements, an alternative is to measure the dominant element's facing.

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Measuring the Inclination

By convention, a horizontal surface similar to a table top is measured as 0 degrees, a vertical wall as 90 degrees, and a horizontal surface such as a ceiling as 180 degrees. There are no negative inclinations nor any inclinations over 180 degrees. This measurement is consistent with the way facing is measured, so that a floor is facing upward or 0 degrees, a wall is facing outward or 90 degrees, and a ceiling is facing downward or 180 degrees. But rather than continuing down from the ceiling to the opposite wall to get a reading of 270 degrees, by convention we would start over from the floor with its 0 degree inclination and measure the opposite wall as 90 degrees of inclination.

Measuring inclinations from 0 to 90 degrees is straightforward. Open the compass fully and set the dial to 90 degrees or East. Lay the side of the compass on the surface to be measured and take the reading from the clinometer.

Measuring inclinations above 90 degrees is more difficult because most clinometers only go up to 90 degrees. The easiest method is to hold the short end of the compass against the panel, take the reading and add 90 degrees. For example, if you were measuring a panel with a slight overhang, you would discover that when you hold the long edge of the compass against the panel the pendulum needle of the clinometer will swing past 90 degrees and off of the scale. Rotate the compass to bring the short end against the panel and take a reading, say 20 degrees, and then add 90. Record the total, 110, as the panel inclination on the form.

Accuracy

Although compass manufacturers will advertise some of their products as being accurate to within 0.5 degrees, few rock art recorders will obtain that level of accuracy in their measurements. When measuring the bearing from a datum point to a panel, you should strive to be accurate to within 2 to 3 degrees. Measurements of elevation change, facing and inclination will be less accurate, possibly within 5 degrees. But in many cases, rock art panels will have many irregular contours so that the inclination and facing measurements will become very subjective and different people will obtain readings that vary by 10 degrees or more.

By far, the most common errors will be caused by the magnetic interference from rocks. When this happens, measurements will often be off by 20 or 30 degrees in either direction and the resulting ground survey will be nearly useless. A second common error for inexperienced recorders is taking a reading that is the exact opposite of the desired direction resulting in a 180 degree error.