Direction
Description
Details
Direction
Distance
Designation
-
The cardinal directions or points:
-
North = 360° / 0°
-
East = 90°
-
South = 180°
-
West = 270°
-
Directions that fall between the cardinal points are called inter-cardinal points like South-West (SW) or North-East (NE). Sometimes you see West Northwest (WNW) which falls between the inter-cardinal directions. That means the direction is more West of North West.
​
-
Azimuth- A direction that is always relative to North. That is, North is always 0° and your azimuth is an angle clockwise from North. When noting an azimuth direction always indicate if it’s referencing the magnetic field of the Earth, by using the word “Magnetic”, or use the word “True” if it is in reference to true north. Example: When reading a compass that indicates 42°, say “42 degrees magnetic”. When reading a map angle of 55° relative to True North, say “55 degrees true.”
-
Bearing- A direction relative to some reference.
​​
-
Heading- Typically used to indicate a magnetic azimuth.
“Bearing” and “Azimuth” are generally used interchangeably. Formally, “bearing” is relative to north or south (S 45 W means 225 degrees). Azimuth is formally the angle measured clockwise relative to North.
Example: This compass shows 50°. The Azimuth is 50°. A Bearing would be 50° East of North.
​
By now you may have noticed there’s something special about “North.” North is the starting point for all directions and without a starting point, angles don't really give you an absolute direction. North is a convenient reference to start from. There are three different kinds of north that must be understood. They’re not difficult, but you do need to know the differences.
​
Most maps depict NORTH as the top of the map. However you cannot assume that. Look for a compass rose (a compass rose is a pictorial representation of the cardinal points on an object. See below left picture) or look for a star on the magnetic declination diagram (below right).
Compass Rose Declination Diagram
​
-
North always represents ZERO degrees and is your starting point. To find other azimuths, start by facing north, then turn clockwise (when looking downward) a certain amount. That amount increases as you turn clockwise. When you turn a full circle, you start over at 0.
Degrees and Mils: Going from North to other directions
-
A full circle you turn can be divided many ways. The most common way is to use degrees. A full circle can be divided into 360 equal divisions called degrees. Starting with 0° for North, degrees increase in number as you turn clockwise until you turn all the way around to your starting point; a full circle is 360°.
-
Each degree is further sub-divided into 60 equal smaller parts called “minutes” (‘).
-
1° = 60 seconds.​
-
-
Each minute is further sub-divided into 60 equal smaller parts called “seconds” (“).
-
1 minute = 60 seconds.​
-
-
One degree is therefore equal to 3,600 seconds (60 minutes x 60 seconds in each minute).
-
Angles (azimuths) can be in decimal format called Decimal Degrees (DD), like 34.38°, or DMS (Degrees °, Minutes ‘, Seconds ”) like 34° 22’ 40”.
Example: 34°, 22 minutes = 34° + (22/60 = .37) = 34.37°
34°, 22 minutes, 40 seconds = 34.37° + (40/3600 = 0.01) = 34.38°
7.5 minutes = (7.5/60 = .37) = 0.125°
For a review on units, look here:
​
-
Another way to specify azimuth is called mils and is used frequently by the military. “Mils” is short for milli-radians.
​​
-
There are 6,400 mils in one full circle. Then 1° = 17.8 mils.
We will stick with degrees.
Different Norths
-
Maps are drawn relative to True North because Magnetic North is constantly changing and re-printing maps for changes in Magnetic North isn’t very convenient. So topographic map azimuths are specified in true directions.
​​
TRUE NORTH
-
True North (TN): The direction to the geographic North Pole. Also called “geodetic north.” All Longitude lines converge at the North and South pole so all lines of longitude (meridians) are True North lines. If you were to draw a line from any point on Earth directly to the North Pole, that line would be a True North line or a meridian.
​​
-
The top of maps usually always point to True North. True North is properly noted by a star ★ (see pic above) in the declination diagram. The left and right edges of a topo map (and the lines connecting similar longitudes from the top to the bottom edges) are always longitudinal lines so they always point True North and South. True North is parallel to the axis of rotation of the Earth.
-
In the field though, it is very difficult to find True North, but very easy to find Magnetic North since compass needles always point to Magnetic North. Magnetic North is fairly close to True North and is a convenient starting point in the field if you have a compass. But Magnetic North is not exactly True North, and if you’re doing land navigation, using the wrong one can get you lost quick.
MAGNETIC NORTH
-
Magnetic North (MN): Approximates (but not always equal to) True North in the field. Magnetic North is one of the poles of the Earth's magnetic field. Compass needles point to the magnetic poles so compass needles align with magnetic north. The Earth's magnetic field is created by the liquid iron core of the Earth and because of that, the north magnetic pole always changes position. Currently, Magnetic North appears to be "unusually" heading towards Siberia (Newsweek, March 2023).
​​
Does "7.5 minutes" sound familiar? US Topographic maps are referred to as 7.5 minute quadrangles. It means that the area covered by that map is 7.5 minutes or 0.125 degrees (1/8 degree). If the radius of the Earth is 3,598 miles on average, that means the area of a US Topographic map is about 64 square miles (~7.8 miles N/S and E/W). Don't worry about how I derived that, but if you're interested, I will be happy to share it with you.
-
You need a way to convert True North from maps to something useful in the field with a compass. That conversion is the map’s Magnetic Declination. Because Magnetic North changes all the time, Magnetic Declination does as well.
-
The magnetic declination (or variation) at the time a map is created is shown next to the datum. Magnetic Declination is just the difference between True North and Magnetic North. Magnetic north is always changing position because the Earth’s magnetic field changes. Refer to online sources for most current magnetic declination (www.magnetic-declination.com). It is important to have the most current information for your Orienteering events!
-
East Declination: When the Magnetic North points East of True North and is usually denoted with “+” positive numbers..
-
West Declination: When the Magnetic North points West of True North and is usually denoted with “-” negative numbers.
-
Directions on a map are always TRUE NORTH unless otherwise stated. That’s because the position of magnetic north is always changing and this prevents needing new maps.
​​
-
The World Magnetic Model (WMM) is a mathematical model that is used to predict the move of the magnetic poles.
-
Usually only good for 4-5 years, then needs re-modeling.
-
​​
-
The Agonic Line is the line of zero magnetic declination. If you're on this line, true north = magnetic north. Currently (2022), central Arkansas is very close to the agonic so your magnetic compass reads very very close to true north in central Arkansas. The map below is a bit old but it demonstrates the concept.
-
Isogonic Lines are lines of equal magnetic declination.
-
The Back Bearing or Reverse Azimuth is the direction opposite of where you are heading. Following a Back Bearing or Reverse Azimuth will bring you back to where you started.
​​
GRID NORTH
-
Grids, specifically UTM (Universal Transverse Mercator), are convenient ways to locate a position on Earth and will be discussed later in more detail. Basically, cartographers overlay a grid of lines like an X-Y graph that run North/South and East/West over maps and assign coordinates to the grid lines. The grid lines that align North/South point to what we call Grid North when going northward.
​​
-
The problem with this is it's impossible to perfectly overlay a flat square grid on a 3 dimensional spherical object like Earth. Imagine laying a piece of paper on a basketball; it doesn't lay flat. So there's some distortion.
​​
-
The distortion that results from laying a flat grid on a spherical object leads to misalignment in the grid's North/South lines from the Earth's True North direction.
​​
-
This distortion can be minimized by making the grid small compared to the area of interest. Think of this like putting a postage stamp on the basketball; it lays better than a big sheet of paper.
​​
-
The difference between a map's Grid North and True North is given in the declination diagram on maps. Usually, it is much smaller than 2 degrees. This difference is only due to the curved surface of the Earth.