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Magnetic Anomalies. A magnetic compass points towards the magnetic North Pole because of the Earth's Magnetic field. If anything distorts that magnetic field, this is called a "Magnetic Anomaly."  Magnetic Anomalies are particularly important to cave cartographers because they work underground, in close contact with the sources of the magnetic distortion.
Magnetic Anomalies can be caused by things like minerals in the rocks, void spaces and lightening strikes. Magnetic anomalies can be large-scale covering hundreds of square mile. For example, the image to the right shows large magnetic anomalies around Carlsbad Cavern. Large-scale anomalies aren't much of a problem for cavers because they are consistent and unchanging over the entire breadth of a cave.

On the other hand, small-scale anomalies that are isolated to a small portion of a cave can cause severe and difficult to identify problems. These types of anomalies can cause big distortions in a map because compass angles change drastically as the survey moves in and out the area of anomaly. Dale Green has documented anomalies of 6 degrees caused by a large void, 18 degrees around a lighting strike and 12 degrees in a lava tube.[1,2]  

USGS Magnetic Anomaly Map

These changes can happen in less than 100 feet. Imagine what happens to your compass readings as you move into and out of an area where the North has been rotated by as much as 18 degrees!
Correcting Magnetic Anomalies. It is possible to correct magnetic anomalies. John Halleck has written extensively about the techniques. [3,4]  As John points out, these techniques are widely known to Land Surveyors and cavers could use them in caves with magnetic anomalies. I wrote this document to a give a simple description of the concepts and step-by-step procedures to use the technique.

The First Station Anomaly. To illustrate the concepts, we'll use a simplified cave that is a series of shots going due East. Here is what the first shot looks like:

I'm assuming that Station-A is at the entrance and has access to the outside world. The first step is to find the magnetic anomaly for this station. To do this we need to use a external reference point that isn't affected by the Earth's magnetic field. This could be stars, the sun or nearby landmarks. But the easiest way to calculate the anomaly is with a GPS receiver.

To do this, find the location of Station-A with a GPS receiver. Then find another location about 50 feet away from Station-A that is due East, West, North or South from the Station-A. In this example, I've chosen a location that is due North of Station-A. Mark the location with a cairn or have a person stand on the location. Now do a compass shot to the location. The difference between the actual azimuth and measured azimuth will be the magnetic distortion for Station-A.

In this example, I've chosen a location that is due North of the cave. N shows the direction to True Magnetic North and N' shows the direction to distorted Magnetic North. As you can see, distorted North is 5 degrees West of where it is supposed to be. When you sight your marked location, the compass needle will be pointing 5 degrees West of North, so your marker will appear to be at +5 degrees. To calculation the magnetic anomaly, you would subtract the apparent location of your Marker from the actual location:

A = Actual Direction To Marker
B = Bearing To Marker
D = Magnetic Anomaly

M = A - B
-5 = 0 - 5

Since Station-A has a -5 degree magnetic distortion, shot A-B will also have a -5 degree distortion. This means that North has been rotated 5 degrees to the West of where it should be. As a result, the angle between North will be 5 degrees bigger. In other words, the azimuth for shot A-B will be 95 degrees. However, now that we know the anomaly for Station-A, we can correct shot A-B, by adding the -5 degrees. That changes shot A-B to 90 degrees, the correct value.

Anomalies In the Cave. Once we move underground, we can no longer use GPS to find and correct anomalies, so we have to use a different technique. The key to doing this is the fact that we have corrected shot A-B and we can now use it as a reference to find the anomaly at Station-B. Since magnetic anomalies change over relatively short distances, we can expect that every station in the cave will have a different magnetic anomaly.

The example assumes that Station-B has an anomaly where magnetic North is rotated 10 degrees East. That means Magnetic North points in a different direction at Station-B than Station-A.

The way to find the distortion at Station-B is to do a backsight from B to A. Since the foresight has been corrected, any difference between the foresight and the backsight will be caused by the magnetic anomaly at Station-B. For example, if you did a backsight from Station-B to Station-A, you would expect the azimuth to be 270 degrees, the reverse of 90-degree foresight. But because of the magnetic anomaly at Station-B, the angle between North and the shot has changed and now the backsight is 260 degrees.

If the measurements have been done carefully, the 10 degree difference between the foresight and the backsight will be caused entirely by the magnetic anomaly. In the other words, we've measured the exact magnetic anomaly at Station-B from the difference between foresight and backsight values.

The magnetic anomaly at Station-B is the same for any shot starting at B. For example, if the next shot is between B and C, that the azimuth for that shot will be off by 10 degrees. In the example below, instead of 90 degrees, the shot will measure 80 degrees.

But, since we now know the error, we can add 10 degrees and correct it to the proper value of 90 degrees. This process can be continued for every shot in the cave. For example, now that we've corrected the anomaly in B->C, we can find the anomaly at Station-C. Once again, we do this by taking a backsight from C to B.

This time the magnetic fields has shifted 10 degrees to the West, so the backsight will measure 280 degrees instead of 270. If you subtract the backsight from the reversed foresight, you get a -10 degree anomaly for Station-C. Since we now know the size of the anomaly at Station-C, we can now correct the next foresight.

Step-By-Step Procedures. To simplify things here is a set of procedures to correct for magnetic anomalies in a cave survey:

I. Find The Anomaly At The Entrance Station. The first task find the magnetic anomaly at the entrance station. There are three steps:

A. Place A Marker At A Known Bearing. Using a GPS receiver, find a location 50 feet from the Entrance Station that is either due North, South, East or West. Mark the location with a cairn or have a person stand on the location. The main thing is to have a location that is easily visible from the entrance station.

The 50-foot distance isn't critical, it could be 25 or even 10, but longer distances will be slightly more accurate. Likewise, the direction isn't critical, as long as you know the actual compass bear to the marker. Never the less, it is a lot easier to place a marker due East than on a bearing of 25 degrees.

In cases where the Entrance Station is not visible to the outside world, you can choose a different location a few feet away. Just remember that magnetic anomalies can vary over short distances, so try to keep it as close to the entrance station as possible. In the worst case scenario, you may need to add another shot from the entrance station to the outside world.

If you don't have a GPS receiver, you can also use landmarks that are shown on topographic maps or even the sun or stars. However, this is much more complicated and requires extra calculations to find the actual bearing.

B. Measure the Bearing to the Marker. Now take a shot with your compass from the Entrance-Station to your marker. Do you best job measuring this angle because any errors will affect the whole survey. Do not do a backsight on this shot. The magnetic anomaly at the Marker will different from the one at the Entrance. We only care about the anomaly at the Entrance.

Important: since the GPS positions you are using will be relative to True North, you must adjust the Bearing with the Magnetic Declination to convert the compass reading to True North. You do this by adding the that Magnetic Declination for your location and the date. You can get the Declination for a particular place and date from the Compass Geographic Calculator that is available in the Project Manager under the "Tools" menu. For example, if the Bearing was -5 degrees and the Magnetic Declination was +10 degrees, the True bearing would be +5 degrees.

C. Calculate The Distortion. You can now calculate the magnetic anomaly at the entrance station. You just subtract the measured, bearing

A = Actual Direction To Marker
B = Bearing To Marker
D = Magnetic Anomaly

M = A - B

II. Adjust the Next Shot In the Cave. Since we now have the anomaly for what is now the From Station of th e next shot, we can adjust the foresight for the next shot in the cave. You do this by adding the anomaly value to azimuth value for this shot. You only adjust the foresight measurement. Once you do this, the foresight shot will be considered free of magnetic distortions and we will use it for the next step.

III. Calculate The Anomaly for the Next Station. To calculate the anomaly for the next station, we compare the foresight and the backsight of this shot. Since the foresight is now distortion free, only the backsight will contain the magnetic anomalies. Since the backsight is taken from the "To" station, the anomaly value applies to the To station.

Since the backsight is runs in the opposite direction as the foresight, the first step is to reverse the backsight by adding or subtracting 180 degrees. Next, you subtract the backsight value from the foresight. This will give you the anomaly value. 

A = Magnetic Anomaly
F = Foresight Azimuth
B = Backsight Azimuth

A = F - B

IV. Repeat. You now repeat Steps II through IV until all the shots in the cave have been processed.

If you are interested in more detailed and technical information about handling magnetic anomalies in caves, refer to the footnotes listed below:

Footnotes:

1. The Effects of Lava on Compass Readings: Part I
2. The Effects of Lava on Compass Readings: Part II
3. Foresights, Backsights, and Surveying with Magnetic Anomalies.
4. Internal Angles.
 

 
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