Draw the Three North Needle

Function Description

The standard directions commonly used in surveying and mapping are the true meridian direction, the magnetic meridian direction and the coordinate longitudinal axis direction, which are abbreviated as the true north direction, the magnetic north direction and the coordinate north direction, that is, the three north directions, as shown in Figure 1. On medium and small scale topographic maps, it is usually required to draw three north directions.

Figure: Schematic Diagram of Three North Needles

Coordinates North

Coordinate north, also known as map north or grid north, refers to the "upper" direction indicated by the vertical grid line on a map. In the Gaussian plane rectangular coordinate system, the direction of the ordinate axis or the direction parallel to the ordinate axis is the direction of the coordinate north.

True North

The direction of the North Pole through any point on the ground is called true north. For a map, the north direction of the Center Meridian of the map sheet is usually taken as the true north direction of the map sheet.

In the Gauss-Kr Krüger projection, all meridians, except the Center Meridian, are projected as arcs converging toward the pole. Therefore, except for Center Meridian, the projection of all other meridians has an angle with the coordinate longitudinal line, which is the convergence angle of the meridian.

Figure: True North Direction and Meridian Convergence Angle

Meridian Convergence Angle

The meridian convergence angle is the angle between the true meridian of a point on the surface of the earth's ellipsoid and the Prime Meridian of the projection zone where the point is located, that is, the angle between the coordinate north and the true north of the Gaussian plane rectangular coordinate system. Relative to the true north direction, when the north end of the coordinate longitudinal axis is located in the east, it is called east deviation, and its angle value is positive; when it is located in the west, it is called west deviation, and its angle value is negative.

At present, the meridian convergence angle can be calculated by the formula method and the look-up table method. In Application, the meridian convergence angle is automatically calculated by using Map Center Point coordinates through the following formula.

Formula Method

GCS (B, L) is known to compute the meridian convergence angle. According to the approximate formula for calculating the meridian convergence angle in Principles and Methods of Digital Mapping published by China University of Mining and Technology Press:

L: Longitude of GCS; LO: Longitude of Prime Meridian; △ L: Longitude difference between a point and Prime Meridian; B: Latitude of GCS.

For example, the GCS of the Map Center PointP in the layout is: B = 18 ° 18 '32. 82820 ", L = 109 ° 18' 36. 94903". Find the meridian convergence angle of the point P in the 19th zone of the 6 degree zone. LO=6°×19-3°=111°γ=（109°18′-111°）× sin18°18′32.82820″=-0°31′50.59″

Magnetic North

The direction of magnetic north is the north indicated by the compass, mainly because the poles of the earth's magnetic field do not coincide with the geographical north and south poles, so the north indicated by the compass is magnetic north rather than true north, and the specific location of magnetic north changes with time. The geomagnetic poles are close to, but do not coincide with, the north and south poles, one at about 72 ° N and 96 ° W, the other at about 70 ° S and 150 ° E.

The geomagnetic north pole is the point on the earth's surface where the direction of the earth's magnetic field is vertically downward. It is not the same as the geographic North Pole and is constantly changing, moving at a rate of 20.5 meters per day. The distance between the magnetic north pole and the geographic north pole is about 1500 km. The position of the magnetic north pole is also constantly changing in a day. Its trajectory is roughly an ellipse. The magnetic north pole is 40 m northward on average every day. If you want to draw the magnetic north direction in daily mapping, you can calibrate the magnetic north direction by determining the size of the magnetic declination.

Figure: Schematic of magnetic north direction

Magnetic declination

Declination is the angle between magnetic north and true north. The angle between the magnetic meridian and the Prime Meridian. Relative to the direction of magnetic north, the deviation to the east of true north is called eastward deviation, and its angle is positive; the deviation to the west of true north is called westward deviation, and its angle is negative.

Magnetic declination is measured by magnetic declination meter. At present, there are two ways to measure magnetic declination: one is the long-term continuous tracking observation of fixed geomagnetic stations, and the other is the irregular measurement of field mobile measuring points. Because of the high standard of site selection and infrastructure construction of fixed geomagnetic stations, it is not suitable to build too many, so the combination of a small number of geomagnetic stations and a large number of field observation points is the best mode of geomagnetic field measurement.

In general, the actual measured value of the magnetic declination will be marked in the map when drawing, so when marking the magnetic north direction, the user can input the magnetic declination obtained during mapping to mark the precise magnetic north direction.

You can also enter the time and specific longitude and latitude through the data provided by the World Magnetic Model (WMM), based on a magnetic declination value obtained by the model monitoring station. The WMM was developed jointly by the National Geophysical Data Centre (NGDC, Boulder CO, USA), now the National Centre for Environmental Information (NCEI), and the British Geological Survey (BGS, Edinburgh, Scotland). The model is produced every 5 years and the current model will expire on December 31, 2019. For details, please refer to Geomagnetic model of the world .

Operation steps

  1. Select a map in Layout for which you want to draw a North Arrow.
  2. Click Object Operation tab-> Object Drawing group-> North Arrow-> Three North Arrow.
  3. Click and drag the mouse at the position where the Three North Arrow needs to be drawn in the current Layout to obtain the Three North Arrow drawn based on the selected map. The program will automatically calculate the meridian convergence angle according to the Map Center Point coordinates and mark the true north direction according to the meridian convergence angle. The magnetic north direction is valid only when the magnetic declination is specified by the user, otherwise the magnetic declination will coincide with the true north direction.

    Note: The Planar Coordinate System does not support the drawing of the Three-North Arrow. When the map coordinate is Planar Coordinate System, the Three North Arrow button is not available.

  4. Double-click the Three North Arrow object; or select the North Arrow, right click, and select the Attribute item in the pop-up Context Menu to pop up the NorthArrow Property "window. The attribute window displays the following parameters: magnetic declination, Rotation angle, width, height, etc.
    • Meridian Convergence Angle : The angle is automatically calculated by the program based on the Map Coordinate System and using the Map Center Point coordinates. It is not editable. When the meridian convergence angle is 0, it is because there is no Center Meridian in the current Map Coordinate System, and the true north direction coincides with the map north direction.
    • Magnetic declination : The default value is 0, and the unit is degree. The user is required to manually input the magnetic declination of Current Map. The program deflects based on the true north direction according to the input magnetic declination angle to represent the magnetic north direction. Deflection to true north is positive in the east and negative in the west.
    • The user can change the size of the Three North Arrow by setting the Rotation angle, height, and width. See "Draw a North Arrow" for a description of the parameters.