Magnetic declination on nautical charts

Controlling the declination or magnetic deviation on a nautical chart is essential in order to avoid losing the course in navigation. To do this, it is necessary to take into account a series of notions and basic knowledge related to cartography, topography and the Earth’s magnetic field, among which are the normal changes that occur in a more or less constant way.


Know what magnetic declination is

The magnetic declination is the angle between the geographic north and the magnetic north, since the two are not located in the same area, but they do have a certain proximity. This angle varies over the years and also varies depending on where we are, it is not the same in all places. Therefore, it is necessary to establish certain parameters to calculate it.

Geographic north pole

The geographic north pole is the one we all know. It is, together with the geographic south pole, one of the points on the earth’s surface that coincides with its axis of rotation. The earth rotates around itself on this axis. At present, there is no land mass in this area, there are only blocks of ice; however, at the geographic south pole, lies Antarctica. Millions of years ago the continental configuration was different due to continental drift and plate tectonics.

Magnetic north pole

The magnetic north pole is measured by compasses and depends on the earth’s magnetic field. This magnetic field is generated at the core of our planet, consisting mainly of iron and nickel, and varies over time. There are geological records that speak of important inversions in the magnetic field caused by variations at the Earth’s core. Thus, the magnetic north almost never coincides with the geographic north, which means that, if these differences are not corrected, the compasses will not point to the “real north” of the Earth. And this is what magnetic declination corrections take care of.


Factors influencing magnetic deviation

Everyone knows that a compass always points north because of its magnet. If it becomes demagnetized, it will stop working and the same is true if it is not calibrated before its sale. For everyday use, a normal compass is very useful, but in navigation and also in aviation it is very important to correct magnetic deviations continuously. To do this, it is necessary to take into account the factors that influence the magnetic field measurements and that can alter the behavior of compasses, reducing their accuracy:

  • Time: it is necessary to take into account that the magnetic deviation changes over time due to the movements of the Earth, in addition to those of the core.
  • Magnetic declination and topography: the angle between magnetic and geographic north varies with our location and the hemisphere we are in due to the curvature of the Earth, as we get closer to the north, measurements can become more inaccurate.
  • Geological structures: There are underwater geological structures in the oceans that in some cases can alter the behavior of compasses.
  • Mineral deposits: the presence of large concentrations of iron-rich or magnetic minerals significantly alters magnetic declination, which can cause significant problems in navigation.


Calculating magnetic declination

The deviation in the magnetic field can be determined with the help of nautical charts. The nautical chart is a specific map used in navigation that shows the depth, the peculiarities of the seabed and the magnetic variations in each sector for a given year. All this helps to minimize the risks of navigation and facilitates the calculation of the declination for a given point.

To determine it, a number of basic aspects must be taken into account, which can be easily seen on the chart:

  • If magnetic north is to the right of the geographic, the declination will be positive. If it is to the left, it will be negative. On the map it is reflected in degrees. This value must be noted.
  • It is necessary to take into account the time elapsed between the year of the nautical chart and the year in which you want to calculate.
  • The annual magnetic variation for the point in question must be known.

Once obtained, the elapsed years are multiplied by the annual variation and the degrees noted on the first step are added or subtracted. Although it may seem complex, the process is faster and simpler than it seems, it is only necessary to know how to read a nautical chart.

In order to carry out these calculations, we can also rely on a magnetic declination calculator. On the web you can find some applications that can be downloaded for a quick calculation. These values can be compared with those obtained after following the steps mentioned above, to increase the accuracy or to double-check them.


The current magnetic declination

Since magnetic declination depends on latitude, exact and precise numbers cannot be given to the current deviation. What can be defined is that, every 100 years, the change is between 2 and 2.5 degrees, which is equivalent to about 60 km. This is very little, considering the total size of the Earth.

It is also known that, at present, the magnetic north pole is in the geographic south and not to the north. This means that the compasses are actually measuring the magnetic south pole, which is located closer to geographic north. The last reversal of the magnetic field occurred about 780,000 years ago, meaning that geographic and magnetic north did coincide before, but humans were not there to see it, although all this is recorded in the rocks that surround us.

In summary, and by way of conclusion, knowing the magnetic deviation is basic in navigation for a correct location.