Wacky compass readings

Yesterday I took a few compass bearings from a spot on a hiking path that ran high on top of a dam. Before leaving home, I’d written down bearings taken from my map. When I field-checked them (yes, I corrected for declination), I found that the readings were all over the map, pun intended. I’d never had such crazy results before.

At first, I thought I must’ve based my prep work from the wrong location. But I doubt it. Besides, the readings were not off by a consistent amount, as would be the case if I’d simply erred in locating my position. They varied by hugely differing amounts from what they should’ve been.

I checked for metal on my body (none) and nearby, and could find none. UNLESS there was magnetic ore under the path or along the dam??? Something buried under the tread?

Do dams have metal underneath the exteriors? In this case, it was rock showing but (I guess) it was mainly concrete. I wonder if rebar could cause such wildly erratic readings.

Lots of rebar
How big a dam was this?

If it is concrete then there is rebar. If it’s a big dam, probably tons of it.

And maybe big metal pipes in there too.

Here’s proof it’s due to rebar
The amount of rebar in concrete is often pretty substantial, and it can have a huge effect on a compass. I have a magnetic dash-mounted compass in each vehicle that I drive, and when crossing highway bridges, the compass oscillates in a crazy manner. Illustrating just how strong the “pull” of the rebar in concrete can be, the compass doesn’t just oscillate in the leisurely manner that a pocket compass would do if set askew and then allowed to zero-in normally. Instead, the compass spool gets violently “yanked” this way and that as the car crosses the bridge. Of course, the compass veers all over the place because the car is in motion relative to the bridge, but if the car were stationary on the bridge the compass would simply lock-on to some erroneous reading and stay there. It’s worth noting that this happens even on bridges having pre-cast beams and cast-in-place guardrails, and such bridges have no major steel components at all, so it really is the rebar that’s the cause. In fact, the compass starts going crazy at least 30 feet before the car even hits the bridge deck, so I have no doubt you could walk on a footbridge quite high above a concrete dam and still your compass would be affected.

Sudden jerky needle movement
I noticed that at least one time. Must’ve been something magnetic nearby even though I could not see it.

I didn’t even think I was exactly on the dam at that point, but since the riprap was evident still, the concrete must’ve been there, too. With low water the typical case when I’ve been up there, it’s not obvious just how big this dam really is.

For anybody who’s curious, it is Chatfield Reservoir’s dam, and my observation point was the tiny loop at the end of the paved path. You can continue past that point, on a dirt path.

Cause for some uncertainty
Concrete is not normally protected by rip-rap, so I wonder if there was really concrete right nearby. Rip-rap is usually placed on earthen embankments. Your other idea of there perhaps being a magnetic material in the ground might be possible too.

If that happened to me, I’d suspect that if there were electrical turbines operating under the dam that they might be the cause of the effect. Great big electromagnet would be more likely to deflect a compass than re bar, I’d think.

Compass is finicky

– Last Updated: Nov-26-11 11:49 AM EST –

Is the situation repeatable ?
Has it happened more than once using different compasses ?

Double check whatever you might have been wearing.
Belt buckle, carabiner on backpack, etc., etc.

A well built compass picks up on the slightest of items

How about all the electrical stuff coming off a dam? Theres probably so much metal its unreal

I’m Going With This…
The poles are on the verge of a reversal. We’re overdue!

Big DUH on me (and lesson learned)
First, the DUH part:

The bearings I’d written down the night before, I had taken from a parking lot location. I did NOT write down where the “from” was; I only recorded the bearings TO the landmarks.

When we actually went hiking the next day, I looked around and changed my mind about where to take bearings from in the field. The parking lot had some cars and metal posts so I thought that might not be the best place. Unfortunately, when I took the field bearings from the end of the paved trail, I totally forgot about the change; I just compared with my written numbers from the night before. And it was a big enough location move to have made my original readings extremely out of whack.

Now for the Lesson Learned part:

Even though I had packed the map with me on the hike, I didn’t bother checking against that again. From now on, if ever I have any doubts about my notes vs field results, I WILL re-check with the map!!!

My husband says he thinks that dam is earthen, not concrete—which someone astutely picked up on due to the presence of rip-rap. Doesn’t rule out the possibility of hidden magnetic materials (there is, for example, a concrete dam tower, though it was not close to me). However, in the name of full disclosure, I have to post that when I later checked my field results against the map–this time using the correct position–the seemingly wacky results were mostly good. I say “mostly” because there was one that was still a fair bit off. Possibly that was the time when I noticed the needle jerk.

This is embarrassing. I hope my Lesson Learned will prevent someone else from making a similar mistake. Big dope slap coming my way…

just curious …
… were you blazing your own trail , off public trail through the forest ??

One thing I know about using a compass … you have to know where you are (a fix on a map/topo) to find which way go to the next fix .

In the general direction is OK if you have significant geographical or man-made landmarks to work with .

You can trust your compass & topo , it doesn’t lie … at least not for long .

No worries
At least you weren’t in in a place where you needed to navigate accurately. Lessons learned from every mistake!

While such things don’t happen often
you always have to be aware that your compass reading can be off because of some metallic influence. You should every so often take a back bearing since you probably will be far enough away from the magnetic source so as to give you an indication of a problem. If your back bearing doesn’t point back to where you came from you know that there is a problem.

Back bearings
I’ve done that on the water but not on land where I’m almost always on a trail.

Since the place in question is nearby, I’ll return and check it out again with map and compass. Not because I actually need the compass there (yikes! I’d hope not) but as practice and to try to find any anomalies with magnetic stuff.