SharedGeo Magnetic Declination Diagram Service

This free utility will generate a PNG or SVG file representing a properly formatted declination diagram for a specific map.

Background

In cartography, a declination diagram is used to provide a graphic representation of the variations between true north, grid north, and magnetic north on a map. True north is the center of the earth’s rotation. Grid north is the orientation of the map projection coordinate system. Magnetic north is the point at which the earth’s magnetic field points vertically downward in the northern hemisphere; a compass north needle points to this location. Magnetic north changes over time due to changes in the magnetic field of the core of the Earth. Thus, a properly drawn declination diagram is not only unique to each map, but it also changes over time for the depicted area. It follows then that having a correctly drawn declination diagram on a current map is critical for anyone using a map and compass to navigate.

Although many utilities exist for calculating the declination angles, creating a suitable declination diagram manually can be tedious, especially when creating a map series that spans a large area. This is because, as previously mentioned, the declination diagram will be unique to each map, representing the differences between the three norths at the center of the map at a specific point in time. However, the SharedGeo web service creates a declination diagram automatically, returning a scalable image that can be inserted into a map layout using mapping software. This utility uses the NOAA World Magnetic Model (https://ngdc.noaa.gov/geomag/WMM/DoDWMM.shtml).

SharedGeo offers two versions of this free magnetic declination diagram service.

Dynamic Declination Diagram Service

This version can be used in two different ways.

A properly formatted URL in a web browser returns the declination image as a PNG or SVG file, which can be saved and added to a map, and
As a dynamic image using the URL directly, if the mapping software supports that. Further, when generating a map series using desktop GIS or automation scripting, the extents of each map can include the URL with the coordinates for the center of the map to dynamically generate the declination diagram for each map as it is automatically created.

The base URL is: https://mdd.sharedgeo.org/mdd-gen?, but that URL requires several additional arguments to work, including date, lat, lon, and zone where:

date – a decimal date. The whole number is the year and digit after the decimal represent the portion of the year corresponding to a specific month. Minimum required entry is the year (whole number). Decimal month decimal can be calculated using the formula ((month-1) / 12) which creates the results below:

January = .0 April = .3 July = .5 October = .8

February = .1 May = .3 August = .6 November = .8

March = .2 June = .4 September = .7 December = .9

Note: The current magnetic model is valid through 2025.
lat – latitude in decimal degrees of the center of the map
lon – Longitude in decimal degrees of the center of the map (positive = East, negative = West)
zone – UTM zone of the map
of – Output format – PNG or SVG – PNG is the default, if this argument is not specified.

Example: https://mdd.sharedgeo.org/mdd-gen?date=2019.0&lat=43.6276212901&lon=-92&zone=15

Two additional arguments are supported to adjust the size of the output image; width and height. Requesting a larger image effectively creates higher resolution images and, therefore, finer line widths and text characters when the image is reduced in size.

Full documentation of the service can be found here: https://mdd.sharedgeo.org/USNG_Magnetic_Declination_Diagram_Service.pdf

Single Declination Diagram Service

This web form allows a user to directly enter values to produce a single declination diagram. The direct URL is: https://mdd.sharedgeo.org/mdd-gen.html. Entries as described above apply.

Parameter Name Input Value

Date (decimal years) :
(ex. 2020.5)

Latitude :
(ex. 45.123237)

Longitude :
(ex. -93.124354)

UTM Zone :
(ex. 15)

Output Format :

Width (px) :
(ex. 450)

Height (px) :
(ex. 600)

About SharedGeo

Founded in 2008, SharedGeo is a 501(c)3 Minnesota based nonprofit with the mission of helping government, nonprofit, education, and corporate entities use mapping technologies and share geographic data for the public good.

Our past and current projects span:

Creation of unique and affordable approaches to the nation's most difficult geospatial challenges.

Learn more: www.sharedgeo.org

January = .0	April = .3	July = .5	October = .8
February = .1	May = .3	August = .6	November = .8
March = .2	June = .4	September = .7	December = .9

Parameter Name	Input Value
Date (decimal years) : (ex. 2020.5)
Latitude : (ex. 45.123237)
Longitude : (ex. -93.124354)
UTM Zone : (ex. 15)
Output Format :
Width (px) : (ex. 450)
Height (px) : (ex. 600)