Computing a US historic magnetic declination

Calculating a US historic declination value To calculate a US historic magnetic declination, you simply fill out the "Calculate Historic Declination" form under the "US Historic Declination" tab of the Magnetic Field Calculators page, then click "Calculate" at the bottom. Below is an explanation of the each of the sections contained within the calculator form.

Location

Longitude and Latitude: Entered in either "decimal degrees" or "degrees minutes and seconds (space separated integers)":

If you do not know the longitude and latitude of the location you are trying to use, try looking it up using the "Lookup location" form on the right-hand side of the calculator, which will populate the longitude and latitude values in the calculator form for you.

If the "Location lookup" can't find your location, you can try to look up your location by going to the "U.S. Gazetteer". If you lookup your location using the U.S. Gazetteer, please enter the latitude and longitude you find directly into the calculator form on the left. If you are entering the location in degrees, minutes, and seconds, please enter the values separated by a single space. Remember, 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 in the box! The box should be a positive value 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, which have been pre-populated for you. 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: The year you wish the declination calculations to begin with. (Defaults to the current date).

End Date: The year you wish the declination calculations to end with. (Defaults to the current date).

Step size: The interval that you would like your date range to be calculated with. 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

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

Using the programmatic interface

The calculator provides an easy way for you to get results in HTML, XML, or CSV programmatically. To use the programmatic interface, you just have to url-encode the form parameters into a GET request including the result format you want.

The following is a list of parameters and the base url needed to perform a US historic magnetic declination calculation programmatically.

BASE URL: http://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'

EXAMPLE:

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

Interpreting the results

Declination: Declination is 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 Environment Center . For more information on Earth's magnetic field parameters, please visit our Frequently Asked Questions section.

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