What is local attraction in land surveying?

Local Attraction in Land Surveying: A Surveyor’s Tale

Okay, let’s talk about something that can really throw a wrench into your survey: local attraction. If you’re using a compass, and let’s face it, sometimes you have to, this is one gremlin you absolutely need to understand. Basically, local attraction is when stuff nearby messes with your compass, pulling the needle away from where it should be pointing – magnetic north. And believe me, that little wobble can snowball into big errors if you’re not careful.

So, what exactly causes this magnetic mayhem? Well, it boils down to magnetic interference, plain and simple. Think of it like this: your compass is trying to follow the Earth’s magnetic field, but if there’s something else nearby with a stronger magnetic personality, the compass gets distracted.

You’ve got your usual suspects in terms of causes. Nature itself can be a culprit. We’re talking iron ore deposits lurking underground – those can really tug on a compass needle. Certain mineralized rocks can also cause issues, creating magnetic hotspots that throw off your readings. I remember one time, surveying out in the boonies, I kept getting weird readings. Turns out, I was practically standing on a vein of magnetite!

But it’s not just Mother Nature playing tricks. We humans are pretty good at creating our own magnetic messes. Ever try using a compass near a bridge? Forget about it! All that steel will send your needle spinning. Railway tracks? Same deal. Underground pipes and cables? Yep, they can cause interference too. And don’t even get me started on power lines – those things are like giant magnets!

Heck, even your own gear can be a problem. I’ve seen guys get thrown off by their own watches, keys, or even the steel toes in their boots! Rule of thumb: keep any metal away from your compass.

So, how do you know if local attraction is messing with your survey? Here’s the trick: fore bearings and back bearings. A fore bearing is what you read when you’re looking forward, and the back bearing is what you read when you look back. If everything’s perfect, the difference between those two readings should be exactly 180 degrees. But if local attraction is present, you’ll see a discrepancy. Let’s say you’re shooting a line, and your fore bearing is 90 degrees. When you turn around and shoot the back bearing, it should be 270 degrees (90 + 180). If it’s, say, 268 or 272, you’ve got a problem.

Alright, so you’ve detected local attraction. Now what? Well, you’ve got a couple of options to fix things. One way is to use corrected included angles. The basic idea here is that even if the compass bearings are off, the angles between the lines in your survey are still good. So, you calculate those angles and then use them to figure out the real bearings.

Another method, which I find a bit more precise, is the error computation method. This involves figuring out exactly how much the compass is being thrown off at each station. You start by finding a line that isn’t affected by local attraction – a line where the fore and back bearings do differ by exactly 180 degrees. Then, you use that line as your baseline and calculate the errors at all the other stations. Once you know the errors, you can apply corrections to your bearings. It can be a bit tedious, but it’s worth it for the accuracy.

A few extra things to keep in mind: Don’t confuse local attraction with magnetic declination! Declination is the difference between true north and magnetic north, and it’s a whole different beast. Also, be aware of isogonic and agonic lines – these are lines on a map that show areas of equal magnetic declination.

And finally, remember the precautions: keep metal away from your compass, and if the whole area is a magnetic mess, maybe consider using a different surveying method altogether!

Look, local attraction can be a pain, but it’s just part of the job. The key is to be aware of it, know how to detect it, and know how to correct for it. Do that, and you’ll be well on your way to accurate surveys, no matter what magnetic gremlins are lurking nearby.