FW: Disto-X Calibration Woes

Sun May 3 14:41:42 BST 2009

Bruce,
Your calibration procedure (with the stakes in 
the ground, not the one against your chest) 
appears valid to me. Given that the slopes 
appears good, I would suspect that there might be 
a metallic object within range of at least one of 
your 4 "basic" stations. I've always done my 
calibration in a forest (and once in a cave), as 
Beat suggests, never having "dared" doing one in 
my backyard, being worried about fences, 
electrical wires, etc... When being careful to 
always aim at exactly the same target (from 
exactly the same location) for all shots (I know, 
this is likely overkill), I've always had great 
calibration results (around 0.3). This is both 
with PocketTopo on my PC and the DixtoX calibration for Palm OS

I did do a number of fake calibrations in my 
house, and without much surprise, these were way 
off for the azimuth. Without being overly careful 
with these, I did attempt to remove any nearby 
metal objects. Obviously metal hidden here and there does interfere.

Did you try calibrating your DistoX in a totally 
different location? Perhaps one where you're 100% 
certain humans have not interfered with. If so, 
are you getting exactly the same results, or are 
you getting equally bad yet different azimuth discordance?

If anyone else in your area has another DistoX, 
calibrating both on the same course could help narrow down the problem.

Chris

At 05:40 AM 03/05/2009, Bruce Mutton wrote:
>[Re sending without the attachment as I think it bounced
.]
>
>Hi All (not sure if I have sent this message to the appropriate forums  I’m
>sure you’ll let me know)
>
>I’ve been the proud co-owner of a Disto-X for about two weeks, but am almost
>disillusioned that to date I have been unable to calibrate it
>satisfactorily.
>I have used it in a cave for a minor survey branch, and it seemed to work
>fine.  No pda yet, so mostly done the old fashioned way with paper.
>Perhaps some of you out there have had similar experiences and been able to
>solve them?
>
>On soldering together I eagerly went through a calibration procedure in my
>back yard holding the disto-X by hand against my chest and aiming roughly at
>a particular point for each shot.  I was standing about 3m from a steel
>framed trampoline (pictured in the attachment), so of course I wasn’t
>expecting a perfect result.  I got delta = 0.9 and compared with the
>‘acceptable’ value of delta < 0.5 I thought I’d done OK for a ‘rough enough’
>attempt.
>This is the best calibration delta I have ever had, ALL later more careful
>attempts have been much worse, and therefore the only one I have ever loaded
>into the disto-X has been the first one.
>
>I then thought I’d install some lithium non-rechargeable cells and do a
>careful calibration.
>
>I drove square wooden stakes into the ground in bare grassland.  There was a
>230 Volt power cable buried in the ground ~35m away.  There was a steel axe
>35m way, and I left my keys and cell phone 50m away.  Waving my spectacles
>and wrist watch over the instrument produced no effect (the bearing and
>clino constantly wander 0.1 to 0.2 deg with no disturbing influences
>present)  I figured these precautions were likely to result in less
>disturbance (by at least an order or two of magnitude) than the presence of
>a cavers headlight, SRT kit etc that are always likely to be within 0.5m to
>1.0m of an instrument held by a surveying caver.
>
>Using the stake to position the instrument carefully I took
>front-rear-left-right shots aiming at specific points.
>I took up-down shots resting the instrument on the vertical face of the
>stake, and used my hand to give the instrument something to ‘read’ for the
>‘up’ shots.
>For the ‘downward diagonal shots I used the same approach as for
>front-rear-left-right.
>For the upward diagonal shots I held the instrument near my belly button and
>aimed at my hand to give it something to read.
>
>Initially these calibration delta values were in excess of 2.5, but I traced
>that back to a nail in the wooden peg.  I went out and bought brand new pegs
>and checked them for magnetic effect on the disto-X.   The new pegs enabled
>values of 1.5 to 2.5.
>
>Processing the exported calibration files I find that the clino typically
>varies by 1 to 2 deg and the bearings by 2 to 6 degrees for each of the 4
>‘roll-about-long-axis’ values.
>I studied the time it takes the bearing and clino to stabilise when the
>instrument is placed on a firm surface.  This is a generous two seconds, so
>I always imagine a very unstable or under damped compass while I wait before
>pressing the ‘dist’ button.
>I also imagine that the movement of the instrument as I push the button is a
>problem  hence the wooden pegs.
>
>No improvement with the above approach so I made up the wooden guide
>pictured in the attachments.
>It enables me, in a low tech kind of way, to ensure the long axis of the
>instrument body is always pointing in the same direction for each of the
>‘roll’ shots and usually results in no detectable movement as the button is
>pressed.  What I found is that the laser spot moves about 40 to 70mm as the
>instrument is rolled about it’s axis (after accounting for the offset
>position of the laser source in the instrument) on a target about 5m away.
>This equates to a misalignment of the laser of about 0.5 to 0.8 deg relative
>to the body of the instrument.  This seems to me to be a reasonable
>construction tolerance, and one of the effects that the first four carefully
>TARGETED sets of shots are intended to account for.  It also means I can not
>rely entirely on my wooden guide, because to do so would be to ‘lock in’ the
>‘laser-disto body’ misalignment.  I need to focus the laser spot on a
>particular point  and I believe this is what is intended by the
>instructions.
>
>If I place the disto-X in the guide, and make repeated shots with an
>identical orientation, I get readings for bearing and clino that are within
>0.1 deg typically, and occasionally 0.3 deg.  These compare directly with
>the values quoted two paragraphs above, and are an order of magnitude
>smaller.  So repeatability of readings is good with constant position of
>instrument, but not with ‘roll-about-long-axis’.
>
>If I rotate the disto-X 180 deg in plan (display upwards always) the back
>bearings and forward bearings are also highly repeatable, but always about 8
>deg out (ie 016 deg maps to 188 deg).  The clinos match perfectly (ie -5 deg
>maps to +5deg). This may be as a result of my ‘delta = 0.9’ calibration
>making the compass rose non-linear or something else?
>
>So, any advice?
>
>One other thing that doesn’t have direct bearing on the calibration, but
>does become annoying after a time.
>There seems to be no way to remove a calibration shot from either the
>disto-X or the pocket topo calibration application.  This means that a
>single accidental shot screws up the whole calibration process, and to clear
>it and be able to start from the beginning again it seems to have to
>automatically download (at a painfully slow pace of about 1 shot per second)
>into the pocket topo application so that it can be deleted/discarded.
>
>Also I’d be interested to learn how much more numerical emphasis is placed
>on the first four calibration directions.  It’s too soon for me to detect a
>trend, but it seems that even a minor transgression in the later shots can
>screw up what should otherwise be a reasonable calibration attempt.  This
>appears to be contrary to my interpretation of the instructions.
>
>Beat, perhaps you could distribute a synopsis of the calibration theory, the
>mathematical process and the meaning of the various numbers produced when a
>calibration is exported.
>
>If you’ve got this far, thanks for spending the time

>Bruce
>
>
>
>
>--
>Cave-Surveying http://lists.survex.com/mailman/listinfo/cave-surveying

--------------------------------------------
Christian Chénier
Gatineau (Québec), Canada
(819) 772-8824 