Introduction

There are three different definitions of north commonly found on cave maps: True North, Grid North, and Magnetic North. Each type of north is represented by its own style of arrowhead or symbol, and the differences are an important consideration for divers to be aware of.

True North

True north is the geographic north pole of the earth. It is defined as a line from any point on the earth’s surface to the north pole. For example, all lines of longitude run towards true north where they eventually intersect at the north pole.

On a map, the direction of True North is often shown as a line pointing to a 5-point star (the north star, Polaris), or a double-sided arrowhead.

Grid North

Grid North applies to map projections with a flattened grid of the globe, and it is defined as the direction north established by the vertical grid lines.

The difference between Grid North and True North is normally very small and is often ignored for cave navigation purposes. This difference can be shown in degrees as a Grid Convergence value next to the north line.

Grid North is shown as a line pointing towards a GN symbol.

Magnetic North

Magnetic North is defined as the direction towards the earth’s magnetic pole in the northern hemisphere, as indicated by a magnetic instrument. For example, a compass points towards magnetic north.

Magnetic north is shown as a single-sided arrow on a map. It may sometimes be labeled as MN, and often includes a magnetic declination value from the time of publication.

What is Magnetic Declination?

Magnetic declination is the angle of difference between magnetic north and true north. It is expressed in degrees east or west of true north and varies depending on your location.

It is important to keep in mind that the earth’s magnetic poles are constantly moving. As of January 2021, the northern magnetic pole is offset nearly 400 kilometers / 250 miles from true north and moving at a speed of over 50 kilometers / 30 miles per year. A map that shows only magnetic north can become significantly inaccurate depending on its location and how long ago it was published.

It is best practice to always include the magnetic declination and date alongside an arrow pointing towards magnetic north on a map.

Finding Magnetic Declination

Use NCEI’s Magnetic Declination Calculator to determine the declination of a location.

For example, using the above calculator you can see that the current magnetic declination for Toronto in January 2021, is 10° 18' W of true north. This means that a compass in Toronto will actually point over 10° W of true north.

I am located near Tulum, Mexico, and the current magnetic declination here is 1° 44' W which means that my compass is pointing 1° 40' W of True North.

How can I apply this to a cave map?

You can use a protractor to draw an updated magnetic north arrow onto an old cave map. Unless the map is very old, the difference will be minimal but it is still a fun experiment to try at home.

For this example, I will use the map of Cenote Naharon, drafted by James G. Coke between January 1988 and October 1989.

Step 1: Locate true north on the map.

Most cave maps will have both a true north arrow and a magnetic north arrow. As mentioned above, the true north arrow will be marked by a 5-pointed star or a double-sided point. If your map already has true north clearly indicated, proceed to step 2.

As the map of Cenote Naharon does not reference true north or what the declination value was at the time of publication, you will need to determine where true north would have been during that timeframe.

Part 1: Determine the declination at the time of publication.

1. Look for a draft date that can be used to determine the approximate declination using historical declination maps such as the one offered by NCEI here: Historical Declination Viewer (noaa.gov).

2. As per the instructions of NCEI's historical declination map, I have set the date to 1989 which is the final draft date of the Naharon map.

3. Navigating to the Tulum area of Mexico which is where Cenote Naharon is located, we can see that it would have been approximately midway between the declination lines of 3° E and 2° E in 1989.

Part 2: Draw true north onto your map.

Using the chart above, let's say that the location of Cenote Naharon had a magnetic declination of 2° 30' E from true north during the year 1989.

1. Use your protractor to draw a line 2.5° W of the map's magnetic north arrow to show the direction of true north. (There are 60 minutes in 1 degree, so 30 minutes is 0.5°).

Step 2: Determine the current magnetic declination.

1. Use NCEI's declination calculator to determine the present-day declination of Cenote Naharon in Tulum, Mexico.

2. Enter the present date and the coordinates of your cave entrance, then select Calculate.

• According to the calculator, the magnetic declination of Tulum is now 1° 44' W.

• Interesting fact: There has been a shift of approximately 3° 14' in the 32 years since Cenote Naharon was mapped.

Step 3: Update magnetic north.

1. Convert the magnetic declination from degrees and minutes to decimal degrees. Remember: there are 60 minutes in 1 degree.

   • In the case of the Naharon map, the present declination was 1° 44' W, which equals approximately 1.75 decimal degrees. I will round up to 2° for the purpose of this example.

2. Use your protractor to draw a new line for magnetic north based on the present declination.

   • Tulum is now 2° W of true north, so draw your new line on the west side of true north.

Finished.

This new line represents the present-day direction of magnetic north for Cenote Naharon.