DistoX - Best calibration/correction method

Rob

Well-known member
As part of the Castleton Caves Survex Project we are resurveying Giants, a project that will take many trips. Each trip we are re-calibrating the DistoX, and we are also taking a test shot near the entrance after each trip.

Each trip gives us one .svx file, to which we are applying a *calibration value taken from http://www.ngdc.noaa.gov/geomag-web/ to allow for the magnetic deviation.

We are getting (small) variations in the test shot each time. Should we adjust the *calibration values of each trip to make these test shots all the same?
 

andrew

Member
Depends on what you mean by small. Magnetic deviation does vary hour to hour, quick look at yesterday and it was 0.3 degrees, but I believe it can be more than a degree, this can all happen within a very short time period, so it will vary during the trip. Then there will be errors in taking the reading, things like not on the correct point of origin, and not hitting the exact point.

If your instruments are taking readings within 1 degree of reality (considering all the factors, human and otherwise) you are probably getting good results, if the calibration leg is changing by more than a degree each time, something is up, and would lead to me thinking about the methodology, and whether results of each shot could be considered consistent. The other problem could be a bit of metal near the calibration point, rebar in buried concrete.
Using traditional instruments did not get that within 1 degree consistently ever, generally I find people are too fussy about the numbers, and not fussy enough about a consistent methodology, which I feel leads to better results. One example was a team that made sure they had instruments that agreed on the calibration leg within a degree, but then did not use fixed (as opposed to permanent) points  (oh that probably will lead to lots of debate....)

Andrew
 

Rob

Well-known member
Thanks for that Andrew. Test shots are all within 0.5? of each other, so maybe we should just calibrate based on the mag dev and not adjust based on the test shot. The difficult decision will come if the test shot is +1? out. What do other people think?


richardg said:
Why do you have to resurvey Giants?
Whilst the paper survey looks good, the 1970s raw data that makes up the castleton .3d file here (http://www.peakdistrictcaving.info/caves-of-castleton-survex-project.htm) has significant errors, leading to a loop error with Oxlow of +70m.

We've now almost finished the main line resurvey, with only the Chamber of Horrors still on the old data. Unfortunately we still have a 30m error...
 

SamT

Moderator
Its not inconceivable that the chamber of horrors could account for a significant error. Its not the easiest place to faff about surveying.
 

badger

Active member
something we do when first starting we checked reading against certain points of our body to see if anything we had on was effecting the readings, lights, belts boots.
we also found on one particular leg in a survey we had a funny reading which turned out to be metal rebar in the concrete.
however do you need to re calibrate every time, if at the end of every session you are checking by doing the same bit, why not do that check at the beginning if it is within 1 degree is that then ok?
we always do a check first forwards backwards side to side on all four axis (hope that makes sense)
if not  within 180 degress, length similar and angle similar then we would recalibrate
 

John S

Member
Rob said:
Thanks for that Andrew. Test shots are all within 0.5? of each other, so maybe we should just calibrate based on the mag dev and not adjust based on the test shot. The difficult decision will come if the test shot is +1? out. What do other people think?


richardg said:
Why do you have to resurvey Giants?
Whilst the paper survey looks good, the 1970s raw data that makes up the castleton .3d file here (http://www.peakdistrictcaving.info/caves-of-castleton-survex-project.htm) has significant errors, leading to a loop error with Oxlow of +70m.

We've now almost finished the main line resurvey, with only the Chamber of Horrors still on the old data. Unfortunately we still have a 30m error...

I do not use the look up table as local rock influences are not taken into account in the table. It is worth checking to see if some of your rocks have a magnetic influence. In Agen Allwedd there is one orange layer of ochre type stuff that can pull the compass off a couple of degrees. So I have to try and stay a reasonable distance from that bed. 

I use a distoX original, and change the batteries quite often, so have set up a calibration course in a wood using plant pot markers screwed to the trees to get the 56 readings (I used brass screws). This means I can repeat the calibrations with different sets of batteries and can compare these. I have calibrated several sets on a day, marking and dating them, so I can swap batteries underground quickly. I have noted that even with sets both giving a delta of 0.25, I get a fixed line calibration 0.5 degrees different. (12 readings averaged, 3 on each face). When I swap batteries back,(aligning marks for battery rotation) I get it to 0.1 or 0.2 of the previous set. So I use the line calibration which I got set out and know accurately to grid north.

So if you have a standard calibration line, take a set of reading before each trip and use these rather than the look up table.
0.5 degrees over a kilometer is quite a bit, so cutting this possible source of error in half is worthwhile. If you take the readings you can later decide not to use them but you cant do it the other way around. My large loops have an error of about 0.5% of loop length, if that is any help to compare your error, try to get it under 1%.
 
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