In Edmund Halley came up with the idea of showing declination as contour lines on a map ; he used this novel concept to produced the first declination chart of the Atlantic Ocean.
Declination charts have been produced on a regular basis ever since. The secular variation of the magnetic field causes declination to change with time.
Changes in declination can be quite large. At Yellowknife, NWT , for example, the declination is changing by more than one degree every three years.
On the other hand, at Ottawa, the yearly change in declination is almost zero. The diagram shows the change in declination at several locations in Canada. Magnetic declination also undergoes changes that are much more rapid than secular variation and are a result of magnetic activity.
These variations can be smooth and cyclic, with amplitudes of several minutes of arc in southern Canada, or, during magnetic storms, large and erratic. Changes in declination become increasingly irregular in both amplitude and frequency as one approaches the North Magnetic Pole , a result of the weak horizontal component of the magnetic field.
The number of times per year that a compass user will be affected by changes in declination caused by magnetic storms will depend both on the user's application and location. The diagram shows the percentage of days in a typical year during which magnetic declination will fluctuate by more than a given amount from its normal value. Most standard orienteering compasses have a precision of about 2 degrees. It can be seen that in Southern Canada users of such compasses will seldom experience fluctuations larger than 2 degrees.
However, someone on a canoe trip in the NWT would find his or her compass in error by more than 2 degrees on more than 1 day in 4. Canadian topographic charts contain a diagram in the margin which gives the declination for the year in which the chart was published. Beneath the diagram is a statement informing the user about the annual change of declination.
Here is another excerpt from Map Use :. So, this means that you can use the Rotate tool to approximate the angle of magnetic declination. In fact, you can approximate the angle for the grid north line, too — as long as the relationships in the declination diagram are correct relatively and the exaggeration is not too great! Now group all the graphic elements and move the complete declination diagram to its proper location on the page.
The bottom line is this — a declination diagram is a graphic that is meant to show the map readers the relative relationships between grid, true and magnetic north.
As such, it can be approximate. Additionally, with the revelation of the way that a north arrow works in ArcMap, we suggest you insert your north arrow as a marker symbol and rotate it manually to the angle of a meridian to show the direction of true north. Of course, if the meridians that span the extent of your map are not all nearly parallel so that true north is in the same direction across the extent of the map , then you should either: 1 not use a north arrow, or 2 place it along a meridian, perhaps in more than one location to show people that the meridians indicate true north.
And if you use a compass rose or any north arrow symbol that shows both north-south and east-west , then you should place it at the intersection of a parallel and a meridian again, perhaps in more than one location. Development reconsiders all deferred issues at the beginning of each release cycle. Read this article. Inline Feedbacks. Load More Comments. This implies a recent value of:. UTM Provides a constant distance relationship anywhere on the map. In angular coordinate systems like latitude and longitude , the distance covered by a degree of longitude differs as you move towards the poles and only equals the distance covered by a degree of latitude at the equator.
With the advent of inexpensive GPS receivers, many other map users are adopting the UTM grid system for coordinates that are simpler to use than latitude and longitude. The problem with grid north is that is coincident with true north only at the center line of each UTM zone, known as central meridians. The difference between grid north and true north can be over two degrees. This might not be so bad if it were not for the different conventions with respect to declination diagrams adopted by different countries.
A declination diagram on a topographic Canadian map or an Australian map shows magnetic north with respect to grid north, but a US map shows magnetic north with respect to true north. However, if you use declination from a USGS style declination diagram or any of the other sources below, you must make the meridian lines on the compass parallel with the edges of the map true north.
Canadian maps show a blue fine-lined UTM grid, while some USGS , scale maps show black grid lines, but the others only show blue grid tick marks on the map margins. The choice of grid lines or tick marks on the US maps seems inconsistent by year or by region.
Isogonic or declination charts are plots of equal magnetic declination on a map, yielding its value by visually situating a location, and interpolating between isogonic lines. Some isogonic charts include lines of annual change in the magnetic declination also called isoporic lines. Again, the older, the less valid.
The world charts illustrate the complexity of the field. A Brunton compass included a isogonic chart of North America, on a sheet copyrighted in The ,, scale series of World Aeronautical Charts include isogonic lines.
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