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Understanding Magnetic Declination: RED hyperlinks are external links Location of
Magnetic North... Magnetic north is only an approximation for true north, the difference between them varies across the country and across the world by latitude and longitude because the magnetic mass that your compass needle points to is not located exactly at the north pole. It is located today in Canadian Arctic waters and is in fact a molten magnetic mass that moves slowly over time under the earths crust, it also moves around slightly even within a day... You can ignore the daily movements since they are small and on average the location is 'stable', you can usually also ignore the changes over time - unless you have a very old map. Copyright Natural Resources Canada (Geological Survey of Canada)
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Copyright Natural Resources Canada (Geological Survey of Canada) In the last century the position of magnetic north has moved about 680 miles. From Arizona it has been moving mostly away from us and generally closer to the actual north pole. At the moment its moving about 25 miles/ year. On its current trajectory and speed it will end up in Siberia in about 50 years - but its path & rate is unpredictable so it's likely to veer before then. Magnetic North & Declination: This difference between different 'Norths' is called Declination. The difference between True North and Magnetic North is known as the Magnetic Declination Angle (often referred to as simply Declination). Magnetic declination angles east of true north are considered positive while magnetic declinations west of true north are considered negative - they are usually marked as positive or negative values on the map's declination key. In Phoenix, AZ magnetic north is to the east of true north by about 11 degrees (so the magnetic declination angle is +11 degrees). So Magnetic North is a variable approximation for True North and Grid North although closer is also still only an approximation for True North. You will see that the magnetic north on the map is usually many degrees offset from true north. Look for the Declination Legend indicating True North ('¬/TN/N' - the direction to the actual north pole), Magnetic North ('x/MN' - where your compass needle points) and Grid North ('o/GN' - the vertical grid lines on the map).
Example of a Declination Legend at the bottom of a USGS Map. GN applies accurately to the middle of the sheet. (for MN note the date is given) True North (N) is shown as "¬" Note that the Grid-Magnetic
Declination Angle (MN-GN) is
significant while the Convergence Angle (GN-N) is usually tiny - almost insignificant Usually MN-GN is drawn to scale while the GN-N usually is not. You can calculate the declination for a given location using an on-line declination calculator. A line called the 'Agonic Line' runs through the mid-west USA (Wisconsin, Iowa, Illinois, Missouri, Mississippi) where the magnetic declination angle is zero... places on this line are the easiest places of all to use a map and compass together! To the east of this line magnetic declinations are negative (magnetic north is west of true north) and to the west of this line magnetic declinations are positive (magnetic north is east of true north). There is of course a similar line on the other side of the earth that runs through Russia.
The angle you care about for translation between a Map and compass is the Grid-Magnetic Declination Angle - the difference between grid north marked on this particular map and the magnetic north shown on your compass (some people simply call this Grid Declination). These angles are marked on the map - usually the grid/true difference is very small and effectively almost negligible compared to the true/magnetic difference that is often quite significant - maybe as much as 30 degrees in some part of the USA. To translate accurate readings between a map & compass you must allow for the Grid-Magnetic Declination Angle that applies in this particular area - the map will directly tell you the amount that applies in degrees, on the declination key. Usually although 2 angles are shown: the Convergence Angle and the Grid-Magnetic Declination Angle - you only care about the Grid-Magnetic Declination Angle. Some (expensive) compasses allow for a Declination adjustment to be set that adjusts the compass reading to Grid North. Note if your compass has this declination feature you may not be able to use it as a protractor on the map once it is adjusted (check the instructions) and may then need a separate protractor for map bearing measurements. Without this feature you need to do the adjustment yourself by adding or subtracting the Grid Magnetic Declination Angle in degrees, or by marking your compass to convert it to Grid North readings. ACM 1/2007 Please - Report any broken (or inappropriate) links (we have no control over how external content changes) |
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