Magnetic Declination and Grid Variance

Understanding the three different north references is one of the most difficult things a new navigator has to learn. Most likely your map and your compass are using different north references. Compasses are inherently magnetic instruments, and most commonly measure angles relative to Magnetic North. North lines printed on your map are most likely aligned with either Grid or True North. The difference between Magnetic North and Grid or True North changes depending on your location. It also changes with the passage of time.

This page allows you to look up current values for Magnetic Declination and Grid Varience for any location. Then you can see what changes as you select various combination of north references to use on your maps and with your compass.

When you have settled on the combination that you plan to use, you can add a custom made Declination Reference Sheet to your shopping cart. We will print it out using a color laser printer onto Rite-In-The-Rain(tm) paper. You will get a folded pocket sized weather proof reference sheet, made just for you.

Click on the images for a larger picture.

More information on using various north references with your map, compass and GPS can be found in our written and video tutorials.

Reset

Step 1 - Location

Your internet IP address was used to find your approximate location. Use any of the location inputs below to set a different location.


Latitude and Longitude in decimal degrees. Negative numbers for western longitudes and southern latitudes. No need for more than 1 or 2 digits after the decimal place.

Step 2 - About Your Map

The top of the page direction on you map corresponds to...

USGS maps are typically True North up. Military maps are typically Grid North up

The north reference lines you use for plotting corespond to...

UTM, USNG, and MGRS grid lines are easy to use for plotting. They are Grid North lines. Lines of longitude are True North lines. Few maps have Magnetic North lines printed on them. If you want to use them for plotting, you will probably need to draw them onto your map.

Step 3 - How you set up and use your compass

What north reference is your compass set to read...

If your compass is not adjustable for declination, select Magnetic North. Readings taken with your compass will always be relative to Magnetic North.

If your compass is adjustable for declination, you can choose what north reference to use. It's your responsibility to check that the setting is correct for the north reference you plan to use at your current location.

Remember to always include the north reference with any bearings you take with your compass. Bearings without a north references are useless.

By default we round values for magnetic declination, grid varience, and compass bearings to the nearest degree.

Your are probably fooling yourself if you think you need more than one degree precision.

Your Declination and Grid Variance Diagram

To understand how bearings using different north references are related it helps to draw an additional line onto the declination diagram representing the bearing to the target. On this diagram it's labeled "Tgt."

Remember that bearings are measured as angles starting at their north reference and measuring in a clockwise direction to the target. You indicated that bearings on your map will be measured relative to Grid North. In the diagram the angle between Grid North and the target is shown in red and measures 70°

Using a compass set up to measure magnetic bearings, the bearing to the target measured with the compass would be, 79°. Shown on the diagram in blue.

The difference between the Grid bearing and the Magnetic bearing is the 9° angle shown in green. This angle is a combination of the magnetic declination (the True-to-Magnetic angle) and the Grid Variance (the Grid-to-True angle), rounded to the nearest degree.

Grid-to-Magnetic = Grid-to-True angle - True-to-Magnetic angle.

Since a bearing measured relative to Grid North is smaller than a bearing measured relative to Magnetic North , you would add to a Grid bearing to convert it to a Magnetic Bearing.

Your Conversion Formulas

You have chosen different north references for plotting bearings on the map and taking bearings from your compass. Conversion is required to move bearings between your map and your compass.

Compass (Magnetic) to Map (Grid)

Grid = Compass (Magnetic) - 9°

Map (Grid) to Compass (Magnetic)

Compass (Magnetic) = Grid + 9°

A few things to keep in mind

Local Magnetic Anomalies

In some areas there may be significant localized magnetic fields typically caused by magnetic rock deposits such as iron ore or lava flows. 3-4 degrees of anomalous declination is common near these areas. In extreme cases a compass may be rendered useless.

It is a good idea to confirm the local declination by comparing compass sighted and map plotted bearings between two known points.

Add this reference sheet to your shopping cart

Item description Quantity Price Quantity
Custom Declination Reference Sheet
Custom Declination Reference Sheet
Part Id: Cust_DecRefSheet, Made in United States
1+
$1.95