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January 15th, 2021
Release of Free Magnetic Declination Diagram Service

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St. Paul, Minnesota – SharedGeo is pleased to announce it has created an open source solution to a longstanding cartographic problem by creating a free web service to generate a magnetic declination diagram.

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. A declination helps a map user make adjustments in bearings when using a compass and a map for navigation.

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 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).

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 layout. It can also be added 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 created.

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

• Date – a decimal date. The whole number is the year and digits after the decimal represent the portion of the year corresponding to a specific day. There are many utilities online to convert dates of various formats to a decimal date. Minimum required is the year (whole number). Note that 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. Note that scaling a larger image effectively creates higher resolution images and, therefore, finer line widths and text characters when the image is reduced in size.

Although some GIS software has add-on components which will generate a declination diagram at an additional cost, and free websites can be used to calculate the declination angles, SharedGeo provides this free utility to generate a magnetic declination diagram image that can be directly incorporated in a map. More information can be found here: https://mdd.sharedgeo.org/

About SharedGeo: SharedGeo is a Minnesota 501(c)3 nonprofit dedicated to helping government, nonprofit, education, and corporate entities use mapping technologies and share geographic data for the public good. Past and current collaborative partners include federal, state, tribal, county, municipal, nonprofit, and corporate entities. www.sharedgeo.org

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