Where is the north direction
The north direction
Where is north actually? At first glance, that seems like a very simple question. North is the direction the compass needle is pointing. Many people who have dealt with it so far think like this or something similar. But if one delves deeper into the matter, one is confronted with several north directions.
For the cartographic mapping of the earth's surface, three different north directions are distinguished: true north, magnetic north and grid north.
Geographically north is where everyone suspects the direction - at the geographic North Pole. It is the north direction that the Pole Star shows us and the point at which all meridian lines in the northern hemisphere meet.
Magnetic north is the north direction the compass needle is pointing. This direction is location dependent and does not coincide with the North Pole. It "changes" all the time.
North grid is defined as the direction in which the grid lines of a Gauß-Krüger system or a UTM grid system in the northern hemisphere converge.
(Sketch by the Cologne District Government)
Since there are now different north directions, we also denote the angles between these directions. We differentiate between declination, needle deviation and meridian convergence.
The declination (with information on measurements with a compass)
The angle between true north and magnetic north is called the declination or Magnetic declination designated. Magnetic field lines (Isogons: Lines with the same declination influence the north direction. Unfortunately, these lines are not identical to the meridians, but change every year. They “wander” westwards with the magnetic poles. Local magnetic disturbances also influence the declination. It is therefore checked at regular intervals using magnetic measurements.
The declination depends on the geographic coordinate. I am happy to provide help with which you can easily determine the current declination:
The geographical coordinates of a point can be obtained from this address:
Pay attention to the local situation (west / east, north / south)!
With these coordinates you go to the following link:
the coordinates are entered here, please note the entry east and west!
Example: Munich is approx. 48 ° north latitude and 11 ° east longitude. If you accidentally enter 11 ° west longitude, you are in the Atlantic. Have fun.
(Declination in Munich: June 2009: 1 ° 53 'East - June 1900: 10 ° 38' West!)
And here is the current example (January 2013) using the declination calculator of the Helmholz Center Potsdam:http://www-app3.gfz-potsdam.de/Declinationcalc/declinationcalc.html
How do I determine the declination as westerly or easterly declination from the map and with compass measurements in the field?
To do this, I measure two directions in the terrain with a high-quality bearing compass (reading accuracy 0.5 °) and thus determine the angle between the target points. At the same time, I also measure the directions on the map with a map protractor and determine the angle. Now I get two angles. We call one angle terrain, the other angle map.
The following applies:
|Western magnetic declination:||Angle Terrain> Angle Map|
|Eastern magnetic declination:||Winkel Terrain |
The difference between the setpoint and the actual value is then the declination.
Terrain angle 1 = 90 °
Map angle 1 = 102 °
Eastern magnetic declination 12 °
Terrain angle 2 = 60 °
Map angle 2 = 45 °
Western magnetic declination 15 °
The declination through the ages
This is how the declination values look like in 2010:
and in the past the following values resulted:
How do I now set the declination on my compass?
The declination is e.g. given or determined as "10 ° West".
Now I set my declination correction to 10 ° West to compensate for the declination. Here you have to make sure to enter the correct direction! Western declination here.
Instead of pointing to the north marking on the housing, my north arrow now always points to the marking of my declination correction.
(here my compass alpine from K&R)
If the geographic coordinates in the declination calculator are exactly! are to be entered, then required on the decimal conversion of the geographical coordinates. The conversion from degrees to minutes and seconds works as follows:
Given: the format is given in degrees °, minutes', seconds' '
Wanted: the decimal degree
The degree can be converted just like the time:
Given: 51 ° 38 '52' '
Solution: 51 ° + 38 '* 1/60 + 52.0' '* 1/3600 = 51.64777 ...
The meridian convergence
The angle between true north and grid north is called Meridian convergence designated. The meridian convergence at a certain point on the earth's surface is dependent on the respective cartographic mapping and the position of the point. The exact value of the meridian convergence is calculated (and not measured!). The meridian convergences are equal to zero at the respective main meridians.
The maximum values of the meridian convergences at the border meridians depend on the geographical latitude and become larger and larger towards the north. The meridian convergence is a result of the mapping of the ellipsoid surface into the 3 ° (Gauß-Krüger) or 6 ° (UTM) wide grid strips. Its value depends on the geographical latitude and the distance of the assumed center of the leaf from the main meridian.
Example: In the geographical latitude of North Rhine-Westphalia between about 50 ° 30 'and 52 ° 20' north latitude, the meridian convergences in the UTM meridian strip system at the respective western and eastern border meridians have maximum values of about -2 ° 40 'to + 2 ° 40 ', near the pole it has a value of approx. 3 °, at the equator 0 °.
The calculation of the meridian convergence α works with a map as follows
Attached is the sketch of a topographic map with UTM coordinates (grid with GiN) and with geographic coordinates and GeN.
The grid line (here red) intersects the graticule of the geographic coordinates (here blue) in one point. A triangle (green) is created.
Now I measure the distance g and the corresponding distance a on the map and can use this data to determine the meridian convergence.
Tan (α) = Gegencathete / A.nkathete = g / a
Meridian convergence (α) = arctan (g / a)
Usually the grid lines are shown on the topographic map, the geographic north line is the line of the map frame or a parallel to it.
The needle deviation
The angle between magnetic north and grid north is called the needle deviation. In the needle deviation, the influences of declination and meridian convergence are superimposed. The values of the needle deviation for a certain point on the earth's surface are therefore subject to the same amounts of change as the declination. In the topographic maps, the needle deviation plays a major role.
At the central meridian, grid north coincides with true north. The needle deviation here is therefore 0 degrees. The greater the distance to the central meridian, the greater the meridian convergence. It is indicated on good topographic maps, but I can also calculate it.
(Sketch by the Cologne District Government)
When I work with the declination, I always consider the systems true north (meridian lines) and magnetic north (compass). If I orient myself with a map with a UTM grid, I also have to take into account the meridian convergence. This is the angle between Grid North and True North.
For detailed knowledge, especially for regions with a large magnetic declination!
When we work with topographic maps, we always have at least two different coordinate systems as a basis: Geographic coordinates and UTM coordinates.
Since our compass always points to MAN, we have to take into account various influences when north-facing: The Declination and the needle deviation. Why?
The northing of the map in the geographical coordinate system
The compass needle always points to MaN, the meridian line to GeN. So when I use the meridian coordinate system, I have to use the compass to go north to the meridian line invest. (see example) and set the declination (magnetic declination).
I presented this briefly a few pages ago. If you want to know more about this, I recommend my first “Orientation Guide” with a compass, map, step counter and map meter. You can do this freely under
Download orientation made easy.
In our latitudes we currently have a low declination that we can neglect: That is why I have not set it on the compass.
The northing of the map with a compass in the UTM coordinate system
The needle deviation is the angle between GiN and MaN. When I work in the UTM system, I of course place my compass on the grid line and have to take the needle deviation into account. This angle is given on good topographic maps (see below), but I can safely ignore a deviation of 2 ° with a compass reading accuracy of 1 °. When orienting myself, I can practically not keep the direction exactly. (At least I don't). Therefore it is not set here either.
Of course, the declination also plays a role here as part of the needle deviation. That is why there are annual changes here too.
Here I read the value of the needle deviation for the map sheet. However, this only applies to the date on which the card was printed. So depending on the date, I have to calculate how the current value will appear. The annual change is indicated. That's why I always work with the latest maps.
To illustrate the north directions using the example of the Gauß-Krüger meridian strip
Here is a very clear sketch of the three north directions in the Gauß-Krüger meridian strip system.
Note the Magnetic North Pole (MgN) and the Geographic North Pole (GeN). Grid north (GiN) is determined by the grid. The main / middle meridian and the boundary meridian are also shown very nicely.
(Sketch from Lukas Wener, Wir Kartographen, perpetuum Publishing)
Example of compass work in the UTM grid
Enclosed I am adding a practical example for the work in the UTM grid with the needle deviation. I must not forget that I have to pay attention to the declination and the meridian convergence here.
Topographic map 1: 25000, map sheet 6533 Röthenbach a d Pegnitz
Longitude (east): 11 ° 11 ′ (11.2333 °)
Latitude (north): 49 ° 29 ′ (49.4833 °)
For the Calculation of the current declination I enter this data into a declination calculator, my favorite is the declination calculator from the Helmholtz Center in Potsdam. You can get it here as a link:
You currently get (July 2011) the following value: +2°1´ eastern declination (2.0166 °).
The Meridian convergence for orientation you get a simple calculation from the topographic map (I already described how this works in the first manual).
In my card sheet:
Distance of the grid line to the meridian line on the left map sheet from the upper edge to the intersection of both lines:
Δ 8.5mm (distance between grid line and meridian line at the top of the map)
Height of the card edge: 300 mm (from the top edge to the intersection of both lines on the card)
Tan α = (8.5mm / 300mm) = 0.02833 °, α = + 1.623 °
According to a general convention, the meridian convergences west of the main meridians are negative and east positive.
According to the geographical longitude of the point with 11 ° 11 'we are clearly east of the main meridian 9 ° of the zone field 32U. This is limited by the meridians 6 ° and 12 °.
So I get the Needle deviation by means of a simple invoice, unless this is stated on the card sheet. The needle deviation is the angle between GiN and MaN.
Meridian convergence (angle between GeN and GiN) is + 1.623 °. Declination (angle between GeN and MaN) is + 2.016 °
The needle deviation (angle GiN and MaN) is therefore 0.393 ° and can currently (2011) be neglected here in Germany.
A sketch is always helpful here to determine the value with the correct sign.
In order to avoid errors in the determination and calculation, I always north my topographic map at the edge of the map (meridian line).
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