Using a fixed declination compass involves understanding how to account for the difference between true north and magnetic north without manually adjusting the compass for each location. Instead, the compass is designed to automatically incorporate a specific declination value. Here's how:
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Understand Declination: Declination is the angle between true north (geographic north) and magnetic north (where a compass needle points). This angle varies depending on your location on Earth.
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Fixed Declination: A fixed declination compass has an internal adjustment for a specific declination value. This value is usually printed on the compass. It does not automatically adjust to changing declination based on your location.
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Determine Local Declination: Before using the compass, you need to find the magnetic declination for your current location. This information is readily available from online resources (search "magnetic declination calculator") or topographic maps.
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If Local Declination matches Compass Declination: If the declination printed on the compass matches the declination of your location, you can use the compass like any other compass:
- Rotate the bezel until the desired bearing (direction you want to travel) aligns with the direction-of-travel arrow.
- Hold the compass level and rotate your body until the magnetic needle (the one that swings freely) lines up with the fixed declination mark on the compass housing. You do not align it with the red "north" marking as you would on a compass without fixed declination. Instead, you align it with the indicated amount of declination (e.g., 10 degrees east).
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If Local Declination does not match Compass Declination: This is where things get tricky. A fixed declination compass is only accurate in areas where the local declination matches the compass's fixed declination. If they differ, you need to apply a correction. The correction involves adding or subtracting the difference between the compass's fixed declination and your local declination to your desired bearing. This correction needs to be done mathematically before you use the compass. For example, if the compass is fixed for 10 degrees east declination, and your local declination is 15 degrees east, you would subtract 5 degrees from your desired bearing before setting it on the compass. Then, follow the steps above. This is NOT ideal, and it is better to use a compass that allows you to manually adjust for declination.
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Alternative to Correction: Alternatively, you can simply add (or subtract) the difference between the compass’s declination and the actual declination to each bearing you take in the field, which is cumbersome and increases the chance of error.
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Why Fixed Declination Isn't Always Ideal: Fixed declination compasses are less versatile than adjustable declination compasses because they only accurately reflect true north in areas matching their preset declination value. As you move to different locations with different declination values, you must make manual corrections. This makes them generally unsuitable for travel unless their pre-set value very closely matches your area of use.
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Understanding Scale: Be aware that some compasses use different scales (degrees, mils, etc.). Ensure you are using the correct scale when taking bearings.
In summary, to use a fixed declination compass effectively, first determine your local magnetic declination, then either choose to only use the compass in areas where the fixed declination is correct, or make the required mathematical corrections to your bearings beforehand.
