Discussion: Magnetic Rocks

I was exploring online about some areas in my region that seemed interesting, and I found one place that said:

If you choose to explore the interior, choose your route carefully. Compasses are not always reliable due to local magnetic influences in the vast expanse of rock.

That got me wondering, would that be a dealbreaker for an orienteering venue?

Are there existing orienteering maps with known magnetic disturbances?

Yes.

Kiruna in Sweden - the whole town is built on top of an iron ore deposit. When I ran there they warned us about compass use.

One O Ringen down south had a similar thing - the "Magic Mountain," a large iron ore deposit that stopped your compass working. They planned a really evil diagonal downhill slope leg for it too!

I think it was an issue for some of the events for the Zermatt 5 day a few years back. Nice and sunny every day so you just had to note the direction of the Matterhorn and use that or the sun to align yourself.

Saylor Park in Colorado is said to have areas of magnetic anomaly. That plus the fact that it's above 9000 feet make it challenging.

The Macedonia Map in CT has one place where there is a small pit and withing 20 meters of of, your compass always points to the small pit. Not really an issue because it is on an obvious ridge but it was found during field checking.

Iron rich laterite in breakaways in Western Australia can create local magnetic anomalies Kenine Hill at least has a few spots where it all goes crazy

Among other things I could name, there's an obscure little map in New York called Surebridge Mountain that has a section of maybe a square kilometer or so where your compass will typically be off by about 45 degrees.

In NSW, Australia you can download magnetic anomaly maps to your smartphone (iphone via Galileo app, Android via Rmaps app) so you can see if where you are is magnetic.
http://dwh.minerals.nsw.gov.au/CI/warehouse/view/m...

Richard, I hear that light poles can cause similar problems.

closer to home for you, pink socks - at Cleman mountain there are rock areas that have significant magnetic properties. Even more fun - there are broken rock slopes that provide you with a covering of magnetic dust on your hands and clothing, which then throws your compass off even after you have left the area.
At the NARC in 2012, I ended up using most of my water to wash off the dust, once I realized that my compass was liking the dust as much as the earth's mag field.
bottom line - lots of events are run in areas with magnetic anomalies, perhaps they best thing is just warn competitors if it is likely to affect the event.

It would be cool if we could develop a symbol for the map which showed where these areas are.

If we ever get to the point where we are doing smartphone orienteering, as a geologist I think it would be really fun to have a geological map layer in the GIS application. I remember at SOW 2006 in Zermatt they warned us that on Day 1 magnetic rocks made compass bearings a little wonky; so I didn't bother bringing mine that day, choosing to rely on the surrounding mountainous landmarks for orientation instead. Later I was told by other runners that it wasn't as big of a deal as it was made out to be.

In NSW, Australia you can download geology maps to your smartphone (iphone via Galileo app, Android via Rmaps app) so you can see the geology where you are.
http://dwh.minerals.nsw.gov.au/CI/warehouse/view/m...

For WOC93, we considered three options:
1) Mark the magnetically bad area with some kind of overlay symbol
2) Draw the north lines so that they indicate what the compass shows (which means they would not have been straight, they would have started heading north, then taken a curve to a 45 degree angle, then curved back to north again)
3) Keep the courses away from that hillside.

We opted for the simplest alternative.

I find these were really a pain in Croatia. My compass was spinning sometimes 30-49 degrees off course if I ran by them.