Dash mounted compass calibration

mep1811

Gentleman Adventurer
I have a Silva dash mounted compass. http://www.touring-gear.com/products/Silva-Type-C58-Compass.html

While my GPS has a compass, I do like the at a glance direction of travel and the old school addition to my 32 year old truck.

I have tried like crazy to calibrate it but once I set it, the compass is fine for a short time then it is out of calibration. I know there is metal in the dash but the compass was designed for that hence the calibration.

I must be missing something. thanks
 

Tembo

topless adventures
I have an old Silva from a BV206 military vehicle mounted in my truck. I have a GPS but since most of my trips are out in the Sahara desert, other than bearing and distances to the next waypoint, a GPS doesn't offer much. So, I use the Silva to do most of my navigation. But I had the same problem when trying to mount it. I found that wherever I tried to mount it on the dash, it suffered from too much magnetic interference from the engine. So, I mounted it on the console between the seats, where it had far less interference. It will never give a perfect compass direction, but I check it against my GPS regularly and it is always correct to within a few degrees...maybe a 5% variation. So, when setting off for a day, I will get my bearing from the GPS and then as I drive just refer to the Silva to stay roughly on course.

So, I would experiment with creative locations for the compass until you find one with minimal interference and then just accept that it is general guide rather than a 100% accurate nav device.
 

NMC_EXP

Explorer
I have used that sort of compass with mixed results. Location is important because the internal compensators can do only so much to correct for the vehicle metal and electromagnetic field.

To adjust the compensators I used the method for the last ditch whiskey compass in light aircraft called spinning the compass. Orient the vehicle to 360 degrees and adjust the compass. Then orient to 90 and adjust, then 180 and adjust, then 270 and adjust. Then repeat the entire process until it cannot be improved and make a note of the deviation from true. This should be done with the engine running and in an open area away from metal and power lines...etc.

I have found that a well adjusted compass in location A can be off significantly in location B. I assume mineral content (iron or magnetite?) of the specific area causes the readings to deviate.
 
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NMC_EXP

Explorer
I'm with you - I like having one of those to look at every once in a while as opposed to trying to read a GPS unit screen.

I'm going to try an aircraft magnetic compass in the FJ60. The automotive dash mount types seem to have problems in the Rockies.
 

martnH

Member
I have the same one Silva c58
Mine rattles a lot
Is your doing the same?
Did you connect the wire to the compass led?
Cheers

Sent from my LG-H870DS using Tapatalk
 

verdesard0g

Search and Rescue first responder
A magnetic compass is not ideal for a far moving vehicle as the magnetic lines of force are different all around the world. Have you heard of magnetic variation? The magnetic north pole is not the same as true north. So there is more to the equation than errors caused by local magnetic variations. Some places have a variation of over 20 degrees or more!
 

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