Help: U.S. Historic Magnetic Declination Calculator

Known Issues

Certain Browsers: We have noticed that the calculator does not work on certain combinations of Windows OS and Internet Explorer. Should you notice this issue, please send us your versions of Windows and IE to geomag.models@noaa.gov along with a short description of the issue. An immediate workaround is to use Chrome or Firefox browsers.

Use Calculate U.S. Historic Declination under the U.S. Historic Declination tab of the Magnetic Field Calculators page, then click Calculate at the bottom. See below for form section explanations.

Location

Latitude and Longitude: Enter "decimal degrees" or "degrees minutes and seconds (space separated integers)":
If you do not know the longitude and latitude, use Lookup Latitude / Longitude in the box to the right of the calculator, which will populate the longitude and latitude values in the calculator form for you.

If Lookup Latitude / Longitude can't find your location, search for an address at USGS Earth Explorer.

For degrees, minutes, and seconds, separate the values with a space (there are 60 seconds in a minute and 60 minutes in a degree, therefore 35° 30' 0" is equivalent to 35.500). Do not enter the N or W designation. The value should be positive and the hemisphere should be designated by selecting a value from the choices to the right of the text field. N stands for northern hemisphere latitude, and W for western hemisphere longitude. The USA is (mostly) located in the northern (N) and western (W) hemispheres, and have been pre-selected. Latitude ranges are limited from 25° North to 52° North, and longitude ranges are limited from 130° West to 65° West.

Date range

Start Date: Year to begin the declination calculations (defaults to the current year).

End Date: Year to end the declination calculations (defaults to the current year).

Step size: The interval to calculate the dates above. For example, if you select 1800 for your start date year, and 1850 for your end date year, and a step size of 10, your will get declination values for 1800, 1810, 1820, 1830, 1840, and 1850 (every 10 years).

Result Format

The format you wish to view the results in. This allows you to view the results in a separate browser window (HTML), XML, or as comma separated values (CSV).

Using the programmatic interface

The calculator programmatically provides results in HTML, XML, or CSV. To use the programmatic interface, url-encode the form parameters into a GET request and specify the desired result format.

A maximum of 50 connections/second is allowed at any given time. If exceeded, the calculator ceases requests for 10 seconds. Bulk calculations should be requested serially and not in parallel. If the requests start returning with errors then your script should go to sleep for 5-10 minutes before trying the next request.

EXAMPLE:

An XML document with U.S. Historic magnetic field values in Boulder for 2000 through 2010:
https://www.ngdc.noaa.gov/geomag-web/calculators/calculateUshistoric?lat1=40&lon1=-105.25&startYear=2000&endYear=2010&resultFormat=xml

BASE URL: https://www.ngdc.noaa.gov/geomag-web/calculators/calculateUshistoric

PARAMETERS:

Parameter	Required	Default	Description
lat1	yes		decimal degrees or degrees minutes seconds: -90.0 to 90.0
lon1	yes		decimal degrees or degrees minutes seconds: -180.0 to 180.0
startYear		current year	start of calculation interval: 1750-2015
endYear		current year	end of calculation interval: 1750-2015
resultFormat		html	format of calculation results: 'html', 'csv', or 'xml'

Interpreting Results

Declination: The angle of difference between true North and magnetic North. For instance, if the declination at a certain point were 10° W, then a compass at that location pointing north (magnetic) would actually align 10° W of true North. True North would be 10° E relative to the magnetic North direction given by the compass. Declination varies with location and slowly changes in time.
*Accuracy for the declination is generally within 30 minutes (0.5 degrees) of arc.

*Solar disturbances can cause significant differences between the estimated and actual field values. You can check the current solar conditions from NOAA's Space Weather Prediction Center. More on Earth's magnetic field parameters: Frequently Asked Questions.

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