I've not got round to installing my DistoX board in the laser range finder yet and have been putting it off because I've heard that the compass is very sensitive to the presence of metal objects.

Being a mine explorer, we tend to be walking on rails all the time, passing iron spikes, air pipes etc... Has anyone on here used there DistoX in mines and with what results, the mines we go down, your surrounded by metal most of the time in the levels, easily within 30-40cm.

I did speak to Beat about it and he seemed to think it would be a big problem.

Would be grateful if any one could voice experiences? 

We resurveyed Gough's Cave (show cave) using a Distox. It would not work in most of the cave unless the electricity supply to the lights was turned off and in the sections with significant iron handrails along the passage sides we had to abandon it in favour of a Leica total station, i.e. not do a magnetic survey at all in those parts.

Taking a bearing from both ends of a leg will very quickly show whether you have a problem or not.

Peter's suggestion s good.  I have some experience of surveying in lava tubes where significant magnetic anomalies are present, note that the Disto X will be no more or less sensitive to these than any other form of compass.  If bearings are taken each way at each station, the it is relatively simple to check whether or not there is a disturbance to the Earth's magnetic field at that station by checking that the bearings to the adjacent legs are 180 degrees to the bearings from the adjacent stations.  Obviously if the bearings are reciprocal (or near enough for the level of accuracy you are trying to achieve) then all is hunky dory.

If the bearings are not reciprocal, then not all is lost.  Assuming you have at least some stations where there are no magnetic anomalies then you can still use the data you have collected using the back bearing method to create a good survey.  Assuming that the anomaly produces a constant variance at a given station, then the difference between the angles recorded to the adjacent stations is still accurate.  This included angle can then be added/subtracted from an adjacent known bearing.  In theory this process can be followed along a series of stations with magnetic anomalies.  Care has to be taken in  ensuring you don't mis-apply the included angle to create a mirror image though...

Anyone doing a good survey will be doing back bearings anyway, regardless of the suspected presence of magnetic anomalies.

With respect, both Ed and Peter are missing the point. Mr Mike isn't talking about occasional suspected anomalies, he's talking about vast quantities of steel lying all over the place. In such circumstances, magnetic surveys, be they by distox or more traditional compass, are simply not on and the only answer is to use non-magnetic instruments, such as theodolites.

Thanks for the input everyone. Graham is right about what info I am after. I have surveyed with laser range finder, traditional compass and voice recorder, to very good effect. Metal does effect a normal compass, but move it away 30-40cm or so and any deviation disappears. BUT from what I can gather with electronic compasses, you need metres not cm of seperation, which makes it a mess in the mines of Nenthead. Every reading would be off, and every reading would be affected to a different degree ( ) depending on how much metal is around each leg.

Surveying mine galleries using a traditional compass, with a line of continuous heavy cast iron plate rails on the floor did not cause significant problems for me. We did have a fair number of closed loops back onto the affected galleries which allowed minor anomalies to be corrected. We also had the luxury of being able to separate the compass from the iron by at least a metre and a half.

Given that both electronic compass and mechanical compass are indicating the direction of the local magnetic field, i.e. that in which they are placed, it is really rather difficult for me to understand why one would be less affected by an 'anomaly' than the other. Think about it.

Indeed, Graham. Never doubted it for one moment.

With respect, both Ed and Peter are missing the point. Mr Mike isn't talking about occasional suspected anomalies, he's talking about vast quantities of steel lying all over the place. In such circumstances, magnetic surveys, be they by distox or more traditional compass, are simply not on and the only answer is to use non-magnetic instruments, such as theodolites.

Click to expand...

Graham, I do not believe I am missing the point at all.  The method highlighted in my previous post will work as long as a single "good" magnetic leg can be obtained.  Of course this can if necessary even be outside the cave/mine.  Even with every station affected by magnetic anomalies, the "included" angle measured by the difference in magnetic bearings taken at each station is still accurate.

I was involved in an experiment to this end (unfortunately never published) in the Arnaker lava tube in Iceland, where surveys were made using non-magnetic methods and magnetic instruments to measure included angles.  The resulting survey centrelines were almost identical.  The included angle method was also faster and required much less kit to be lugged to and through the cave.  It also had the added interest in highlighting where the anomalies were.  Therefore I do not concur that non-magnetic instruments are necessary to carry out an accurate survey in caves/mines where there are significant disturbances to the earth's magnetic field.

As to Peter's riposte to my previous post I was merely pointing out that more than just highlighting where magnetic anomalies lie, that the use of back-bearings also provides a method to overcome them as well.

Take a look at this. It should answer all your questions. http://home.utah.edu/~nahaj/cave/survey/internal-angles-example.html

Sounds a bit like a method called 'Kreuzschn?re' in the 19th century mine surveying books in Germany.

Check pp 677 -  680 ?383 in the book below.

http://books.google.de/books?id=g_UGAAAAcAAJ&pg=PA176&dq=h%C3%A4ngekompa%C3%9F+bergbau&hl=de&sa=X&ei=5AjfUeDxKYjfswapooCwBg&ved=0CDIQ6AEwADgK#v=onepage&q=schnur&f=false

With a DistoX and probably a compass, in a place with anomalies, the difference method (fore and back sites) does not always work, rotating the compass through 180 degrees did not give a reading that was 180 degrees difference, sometime by tens of degrees, the roll and yaw of the compass seemed to affect the result.
I decided that in really bad places you would have to make sure that the compass was in exactly the same plain as you rotate it, which made it far easier to use other techniques.

The DistoX magnetic sensors are located off centre, at the front lower right of the unit near the laser optics.
Note also that the axes of the three orthogonal magnetic sensors are not coincident.
Bear this in mind when trying to measure close to magnetic anomalies. The DistoX sensors are very sensitive.

I will assume you do not have a tactical grade inertial measurement unit so the alternative is to measure magnetic bearings into and out of the magnetic anomaly area and use turned angles in between.

Turned angles could be measured using a small transit - mine passages usually have room to operate one of these. There are some relatively low cost ones available.

Alternatively mount a Suunto compass onto a small non-magnetic tripod/mount which permits the compass to be rotated on the axis of the magnetic card. So long as the tripod/mount is not moved and you do not introduce any magnetic signatures of your own, the magnetic card will experience the same magnetic field for the back station and to station readings and the resulting turned angle will be valid. This technique does not lend itself to large vertical angles so you may wish to add the perspex rod modification to your Suunto compass to aid vertical observations.  A scaffold clad dig benefits from this approach but how you get observers to read the to and back stations whilst keeping the compass mount in exactly the same position in a confined space is the fun bit !