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
