UK Caving

TECHNICAL FORUMS => Surveying => Topic started by: Bob Smith on September 25, 2007, 10:54:20 am

Title: Shetland Attack Pony
Post by: Bob Smith on September 25, 2007, 10:54:20 am
Does anyone have any Info on where i can find details about the Shetland Attack Pony surveying device?
Title: Re: Shetland Attack Pony
Post by: Peter Burgess on September 25, 2007, 10:58:05 am
http://www.shetlandattackpony.co.uk/ (http://www.shetlandattackpony.co.uk/)
Title: Re: Shetland Attack Pony
Post by: Bob Smith on September 25, 2007, 11:04:14 am
You are a star, googled this and nothing came up, thanks
Title: Re: Shetland Attack Pony
Post by: biffa on October 02, 2007, 04:32:33 pm
Also worth looking at is:
http://www.ietodd.co.uk/clino/
Title: Re: Shetland Attack Pony
Post by: AndyF on October 02, 2007, 06:18:53 pm
Shetland Attack Pony; LOL Great name...

I hope this project does well, it just looks like a great idea.... I'd probably buy one if I ever had anything to survey!!
Title: Re: Shetland Attack Pony
Post by: Cap'n Chris on October 02, 2007, 06:48:37 pm
Biffa's link leads to the fabled and dizzying land of Ubergeekdom. I visited it for a while but my eyes glazed over and my brain squealed while becoming gently frazzled.

I'm back in good old Numptyland now. Phew! 
Title: Re: Shetland Attack Pony
Post by: darkplaces on October 02, 2007, 11:34:50 pm
How can I get one and do they eat much grass?
Title: Re: Shetland Attack Pony
Post by: biffa on October 03, 2007, 11:40:04 am
Biffa's link leads to the fabled and dizzying land of Ubergeekdom. I visited it for a while but my eyes glazed over and my brain squealed while becoming gently frazzled.

I'm back in good old Numptyland now. Phew! 


Geeks rule.  Ha ha I squealed his brain.
Title: Re: Shetland Attack Pony
Post by: beardedboy on October 03, 2007, 02:12:03 pm
Also worth looking at is:
http://www.ietodd.co.uk/clino/

This unit looks quite a bit larger than the SAP, and only does clino readings, unlike the SAP, which does compass and clino.

However, it does look like you could knock one together yourself if you are sufficiently geekyfied, but it would prbably still cost about £70? once you had got the board built up etc. A bit cheaper than a Sunto clino.

The SAP is £250 but does seem to have some obvious advantages over traditional compass clino combinations, which only cost a little less.
Title: Re: Shetland Attack Pony
Post by: menacer on October 03, 2007, 06:21:02 pm
Has any one done any comparisons with for example the suunto tandem.
I have heard that the sap used against a tandem on an experiment run by only 1 person, had a larger error margin than the Tandem.(5 degrees to 2 i think)
I appreciate this was only one comment/experiment, has anyone out there done the 2 side by side for comparison.
Im not after a "its crap or its Ace " comments just real experiences...my tandem ...the compass bit has just packed up (sticks)after 2 good years... so im looking at replacing it...
thanks
Title: Re: Shetland Attack Pony
Post by: Horace on October 04, 2007, 11:57:46 am
I was involved in the initial comparison of the SAP against the suunto tandem where we surveyed the round trip in Bull Pot of Witches. 2 sets of readings were taken with the suunto instruments, one by an experienced surveyor and one inexperienced (myself), and one set taken with the SAP by another inexperienced surveyor. We used a disto to record one distance reading for all data sets. The error in the loop closure with the SAP was 1.08% compared to 1.8 and 2.7% for the two suunto readings.

A fuller description of this is in Compass Points 37 http://www.chaos.org.uk/survex/cp/CP37/CP37.pdf

I've not heard of the experiment you mention, but I suppose more comparisons have been made since there are 6 or 8 in use now. I think Langthwaite Pot on this forum might be able to give some more feedback on using it in a harsher cave environment than BPotW.
Title: Re: Shetland Attack Pony
Post by: Les W on October 04, 2007, 06:22:40 pm
I believe the SAP was used at the surveying competition at HE this year. It didn't win though as Peter Burgess felt the need to show of with his conventional set of instruments (0.2% ish, I think (I'm sure he will put me right) misclosure error)  :thumbsup:
Title: Re: Shetland Attack Pony
Post by: Langthwaite Pot on October 04, 2007, 08:10:15 pm
I've not heard of the experiment you mention, but I suppose more comparisons have been made since there are 6 or 8 in use now. I think Langthwaite Pot on this forum might be able to give some more feedback on using it in a harsher cave environment than BPotW.

Yep, you should probably take what I'm about to say with a pinch of salt because I'm a relatively inexperienced surveyor, but we used both an SAP and a Suunto Tandem to survey 'The Dark Side of The Grind' in Easegill this Summer. The passages are in general a surveyor's nightmare due to being small (often body sized), bendy, wet and very, very muddy!. Although unfortunately we didn't do any loops, in terms of ease of use (which I suspect strongly corresponds to accuracy in this sort of passage) the SAP was far superior. Not having to sight from the station was the major advantage, although the SAP was also easier to keep clear of mud. We did still have a problem with the lense clarting up, but I think Phil's working on that.

The SAP's were also used a lot on the CUCC expo to Austria this Summer, and again they were the tool of choice for most people (again, apparently due to their ease of use over traditional instruments)

George.



Title: Re: Shetland Attack Pony
Post by: Peter Burgess on October 04, 2007, 08:25:01 pm
I believe the SAP was used at the surveying competition at HE this year. It didn't win though as Peter Burgess felt the need to show of with his conventional set of instruments (0.2% ish, I think (I'm sure he will put me right) misclosure error)  :thumbsup:
Graham C. and I did the loop twice, each taking our turn with the instruments. By far the greatest factor affecting accuracy was the presence of large amounts of steel at various points around the loop. This factor, regardless of the instruments used, would have been the greatest source of potential inaccuracy. Graham achieved 0.25%, and I achieved 0.5% Maybe, had we used a SAP, we could have got it even tighter. There was another factor that also helped, but if I told you what it was, someone else might win next year, so it's staying a secret.  :tease:
Title: Re: Shetland Attack Pony
Post by: Cookie on October 05, 2007, 09:58:21 am
There was another factor that also helped, but if I told you what it was, someone else might win next year, so it's staying a secret.  :tease:

Back sights?
Title: Re: Shetland Attack Pony
Post by: Peter Burgess on October 05, 2007, 10:02:53 am
Nope. That's standard practice.
Title: Re: Shetland Attack Pony
Post by: beardedboy on October 05, 2007, 10:27:05 am
Yeah, the SAP came 2nd in the Hidden Earth surveying comp, with a closure of just over 2%. The major problem with using the SAP for that course is that it was very bright sunlight, so it was near impossible to see the laser dot. This is obviously not a problem inside a cave, or with the appropriate above ground kit (some sexy red glasses and a bit of 3M reflective tape) which we obviously didn't have!

The small advantage that we did have is that if there was a bit of metal in a preferred survey point you can just put a but of string between the survey point and the 'eye' on the rear of the SAP, then pull the SAP so the string is tight and you will still get an accurate reading because the SAP is still in the correct line.

Also a problem with these very purdy LED lights that are becoming more popular (Scurion, Stenlight, Nova, etc) is that they all use magnetic switches. With the SAP you can keep the device away from your head and the magnet and therefore keep accurate readings. Last time I talked to Phil he was also talking about the possibility of making the SAP beep when it detected a large change in magnetic field, but this isn't confirmed yet.
Title: Re: Shetland Attack Pony
Post by: Peter Burgess on October 05, 2007, 10:41:57 am
Yeah, the SAP came 2nd in the Hidden Earth surveying comp, with a closure of just over 2%. The major problem with using the SAP for that course is that it was very bright sunlight, so it was near impossible to see the laser dot. This is obviously not a problem inside a cave, or with the appropriate above ground kit (some sexy red glasses and a bit of 3M reflective tape) which we obviously didn't have!

The small advantage that we did have is that if there was a bit of metal in a preferred survey point you can just put a but of string between the survey point and the 'eye' on the rear of the SAP, then pull the SAP so the string is tight and you will still get an accurate reading because the SAP is still in the correct line.

Also a problem with these very purdy LED lights that are becoming more popular (Scurion, Stenlight, Nova, etc) is that they all use magnetic switches. With the SAP you can keep the device away from your head and the magnet and therefore keep accurate readings. Last time I talked to Phil he was also talking about the possibility of making the SAP beep when it detected a large change in magnetic field, but this isn't confirmed yet.

But would have been third if Graham and I had insisted on entering both our efforts to the competition!  :tease: At several points around the competition loop we did not take the bearing from the survey point, but from a point in line, either behind or in front of it (which I think is what you are describing). This is just as simple with a Suunto as it is with a SAP.
Title: Re: Shetland Attack Pony
Post by: MSD on October 05, 2007, 01:16:17 pm
I used the SAP this year in Austria. As already pointed out, a big advantage is that you don't have to put your head near any survey stations in order to sight. Another really big advantage is that (at last!) you can get an accurate compass reading on a steeply sloping survey leg. This saves loads of hassle and will surely give better results in many cases.

One thing I found hard, especially with very long survey legs, is that even a slight wobble of the instrument makes the dot dance about. Obviously all that is happening is that you are "seeing" the innacuracy which can result from not pointing the instrument steadily. I'm sure the same applies with conventional instruments, but it's kind of disconcerting to see the dot wobble about and I actually found it quite tiring on my eyes. Something to think about is that at long range a larger target is needed than someone's finger - get the person you are sighting to to hold up somthing a bit bigger (their hand held flat for example).

Mark
Title: Re: Shetland Attack Pony
Post by: Peter Burgess on October 05, 2007, 01:24:26 pm
A useful exercise to evaluate the SAP might be to have the same team survey a loop once with a SAP and once with Suunto (other brands are available) compass/clino, using identical survey points. Perhaps it's been done already. Actually, I expect it has.
Title: Re: Shetland Attack Pony
Post by: Dave H on October 05, 2007, 01:53:38 pm
Biffa's link leads to the fabled and dizzying land of Ubergeekdom. I visited it for a while but my eyes glazed over and my brain squealed while becoming gently frazzled.

I'm back in good old Numptyland now. Phew! 


Geeks rule.  Ha ha I squealed his brain.

I designed and built an electronic compass, yaw, roll and pitch sensor head using PICs and similar sensors (and a few more) around 1998/9 which fed into a sophisticated micro that actually crunched the numbers (which I didn't program). The application was for light aircraft and we only had to achieve an accuracy, resolution and repeatability of 1 degree in each axis. Accuracy and resolution were not a problem, but the repeatability over a wide range of conditions was the killer.
I would certainly say stick with measuring one plane of movement (such as the linked device) for an easier life! Having said that we did achieve the spec. with quite a bit to spare, but it really was prohibitively expensive for applications such as surveying (I think that the aircraft kit was being sold for $5000)  :o
If I ever had the time  :down: I'm sure that I could do something cheaper now, but I would want 0.5 degree specs. and for it to have a distance measuring laser pointer.

Up with the geeks! And yes, I am a rocket scientist (sort of)
Title: Re: Shetland Attack Pony
Post by: Martin Melzer on October 13, 2007, 07:47:58 pm

Also a problem with these very purdy LED lights that are becoming more popular (Scurion, Stenlight, Nova, etc) is that they all use magnetic switches. With the SAP you can keep the device away from your head and the magnet and therefore keep accurate readings. Last time I talked to Phil he was also talking about the possibility of making the SAP beep when it detected a large change in magnetic field, but this isn't confirmed yet.

Scurion does not use a magnetic switch!
I have developed a compass/clino module myself (see my Auriga project - http://www.melzer.ch/html/aurigaimport.html (http://www.melzer.ch/html/aurigaimport.html)) and I know how important it is to have no magnetic parts nearby.
For Scurion, we ran a finite element magnetic field simulation and found out, that using currently available magnetic field sensors, it is not possible to use magnets that are weak enough in order not to disturb the compass (I used a limit of 1% field disturbance in 10 cm distance). For that reason, we have the reletively complicated mechanical solution. Few swiss cavers would have bought the lamp if they would know that it ruins their surveying accuracy.

- martin
Title: Re: Shetland Attack Pony
Post by: menacer on December 22, 2007, 09:01:59 am
Borrowed one the other day for a little Mendip project i'd been promising to do for some time.Also had a disto.
It was a pleasure not to have to contort myself into funny position just to take a reading. Also there was no extra faffing with lighting the instrument just to take a reading.
Another good point, you can train it ahead to the area of the next survey station and see if youve got a good line of sight. (If you can see the red dot, you have)
Best of all, it is possibile to survey solo.
If one of the people surveying, cant fit through the squeeze or get up a climb the other can continue alone....
The downside of that is, Ive got no excuse to not return to all the bits of cave ive travelled alone, and remain unsurveyed....
Back to Draenan for this girlie....phhh

If anyone knows Phil and reads this, can you let him know I cant get the program to run in vista.(even with xp compatibility mode)
Title: Re: Shetland Attack Pony
Post by: Chris J on January 02, 2008, 02:24:21 pm
Surveying Solo... obviously need a disto as no one to hold the tape measure - but a good advantage to this.
Title: Re: Shetland Attack Pony
Post by: Cave_Troll on January 02, 2008, 04:30:21 pm
As long as you have a good base line, you can conduct a survey using just mag and dec, working in triangles, and then work out the distances using 3d trig.
Title: Re: Shetland Attack Pony
Post by: Bob Smith on January 02, 2008, 04:42:54 pm
As long as you have a good base line, you can conduct a survey using just mag and dec, working in triangles, and then work out the distances using 3d trig.

In theory that's great, but that's not really practical in most UK caves is it, fine if you are working with a sub minute instrument, with a stable setup, long survey legs etc, but it relies on intervisibilty of survey stations and it's an awful lot of hard work even for an experienced surveyor. A disto would be just fine and @ around £125 a pop not too far out of reach either.
Title: Re: Shetland Attack Pony
Post by: Cave_Troll on January 02, 2008, 04:49:51 pm
i did mean to put "in theory" sorry
Title: Re: Shetland Attack Pony
Post by: Rob on January 02, 2008, 05:26:46 pm
Surveying Solo... obviously need a disto as no one to hold the tape measure...

Earlier this week i prooved this wrong. With a nicley sized rock to weigh down the end of the tape and a bit of thought about station possitioning it is possible.

However I was in a stupidly tight bit of passage though so would have been much nicer with a disto / friend.  :down:
Title: Re: Shetland Attack Pony
Post by: graham on January 02, 2008, 05:33:28 pm
As long as you have a good base line, you can conduct a survey using just mag and dec, working in triangles, and then work out the distances using 3d trig.

In theory that's great, but that's not really practical in most UK caves is it, fine if you are working with a sub minute instrument, with a stable setup, long survey legs etc, but it relies on intervisibilty of survey stations and it's an awful lot of hard work even for an experienced surveyor. A disto would be just fine and @ around £125 a pop not too far out of reach either.

For the truly obscure, I believe that Alfie Collins did the 1950s survey of Redcliffe caves using a plane table. I've used that method to draw surface features but can think of very few UK caves where it might be feasible.
Title: Re: Shetland Attack Pony
Post by: menacer on January 02, 2008, 05:50:07 pm
Wow thanks for all that info guys...
So all I really needed was a slide rule, a protractor and a stick with a plumb line, a 6 inch ruler and a compass made out of  a bowl of water, a piece of cork, any magnet, and a needle.
Can anyone suggest a suitable parchment and quill for drawing it all up. :lol:
Oh and dont forget the large rock...
Title: Re: Shetland Attack Pony
Post by: CaversLass on January 02, 2008, 06:36:47 pm
I was at a lecture recently where Trevor Faulkner recounted his one man method for surveying caves in Norway. Apparently he finds it is necessary when on expedition to Norway as he is normally on his own (obviously a billy no mates).  ;D

He was getting fairly good results though using a tape and a rock, also using a magnetic compass and clino.
Title: Re: Shetland Attack Pony
Post by: ian mckenzie on January 02, 2008, 11:29:06 pm
An article in The Canadian Caver #67 on Isla Madre de Dios mentions "the marriage betwen Auriga, a cave survey freeware for PDAs running under Palm OS, and Easytopo, an all-in-one topobox... Easytopo generates a laser ray to point towards the next survey station.  Then, pressing the one-touch button, length, azimuth and slope are sent automatically to Auriga, where you can see your actual progression in plan or elevation view."

He was getting fairly good results though using a tape and a rock,
I've recorded data on rocks before, having forgotten to bring a notebook.  Makes for a heavy tackle-bag by the end of the survey...
Title: Re: Shetland Attack Pony
Post by: menacer on January 08, 2008, 02:01:59 pm
Psst.
I hear a batch may be ready for delivery by the end of next week...  :thumbsup:
Title: Shetland Attack Pony
Post by: menacer on May 29, 2008, 08:52:22 am
Phil gave me a handy tip for checking if your pony has been happily groomed fed and watered.
Find yourself a fixed to and from point, no where near any metal work and remove any wrist watches.

Take the same reading with pony faced up, 90 degrees left, upside down and 90 degrees right.
The readings should all be the same give or take a 2 degree error margin.
If they are wildly out its unlikely youll close any loops surveying...as discovered recently on a rods/bath survey trip on mendip.
Recalibrating it is relatively straight forward, the instructions are online on the shetlandattackpony.co.uk website.

As there are a few of us in mendip with Ponies im hoping to set up a permanent recallibration station (with 12 fixed stations of known compass clino) thing in the library at the Wessex...just need to ask the wonderful committee for permission ...  :kiss2:
Title: Re: Shetland Attack Pony
Post by: Cap'n Chris on May 29, 2008, 09:02:10 am
Stables at the Wessex? Whatever next?
Title: Re: Shetland Attack Pony
Post by: graham on May 29, 2008, 09:38:46 am
Would a pony not be affected in some way by being surrounded by plumbing & electrical gubbins inside Upper Pitts?

And wouldn't the distances be a tad on the short side for good calibration shots?
Title: Re: Shetland Attack Pony
Post by: menacer on May 29, 2008, 11:54:11 am
Would a pony not be affected in some way by being surrounded by plumbing & electrical gubbins inside Upper Pitts?

And wouldn't the distances be a tad on the short side for good calibration shots?
No  :)

http://www.shetlandattackpony.co.uk/downloads/manual2.pdf
read the bit on how to calibrate...as long as there are no electrical wires or pipes running across the middle of say the library it shouldnt be a problem....
Title: Re: Shetland Attack Pony
Post by: pete h on May 29, 2008, 08:12:59 pm
Would a pony not be affected in some way by being surrounded by plumbing & electrical gubbins inside Upper Pitts?

And wouldn't the distances be a tad on the short side for good calibration shots?
No  :)

http://www.shetlandattackpony.co.uk/downloads/manual2.pdf
read the bit on how to calibrate...as long as there are no electrical wires or pipes running across the middle of say the library it shouldnt be a problem....

central heating pipes maybe above ceiling and most certain to be wires as the light works.   :idea:
Title: Re: Shetland Attack Pony
Post by: SamT on May 29, 2008, 08:25:49 pm
surely your better off doing it outside somewhere  :coffee:

, prividing your not near any water mains - overhead cables, underground cables etc etc   ::)
Title: Re: Shetland Attack Pony
Post by: menacer on May 29, 2008, 08:38:24 pm
So nobody's actually read the calibration method from the maker of the device then.....guys you only need to read the 1st 4 points and all your questions/thoughts/ponderings will be answered...
Title: Re: Shetland Attack Pony
Post by: graham on May 29, 2008, 08:41:57 pm
When a lot of survey work was going on in Wookey, some years back, Willie Stanton set up a calibration leg in the  car park - only to be told some time later that a bloody great conduit stuffed with magnetic ooh nasties went just underneath!

What this reminds me, however, is that calibration should be carried out both very close to the site being surveyed in both space and time.

Now, unless the Wexses intend re-surveying North Hill Swallet on a regular basis ...
Title: Re: Shetland Attack Pony
Post by: graham on May 29, 2008, 08:50:35 pm
So nobody's actually read the calibration method from the maker of the device then.....guys you only need to read the 1st 4 points and all your questions/thoughts/ponderings will be answered...

Aha, OK. There are two different things going on here. The calibration that Menacer is talking about will ensure that the instrument is recording consistently. The calibration that I, and I suspect others, am talking about is calibrating the instrument with respect to a known bearing. Given that the instrument is reading a magnetic bearing, the one form of calibration (Menacer's/the manual's) does not mean that the other form (mine/every magnetic instrument's) can be forgotten.
Title: Re: Shetland Attack Pony
Post by: graham on June 26, 2008, 05:41:42 pm
How accurate is a SAP.

OK, I have the data, recorded using a SAP.

I know the mag var on the day.

But what other correction is needed?

Or to put it another way:

How accurate is a Shetland Attack Pony?
Title: Re: Shetland Attack Pony
Post by: menacer on June 26, 2008, 06:31:39 pm
They are calibrated to a normal magnetic compass or clino.....is that what you mean.....(my pony says 100 degrees so does my tandem) so it depends on how accurate my compass clino is really...
The manual states tolerances of +/- 1 degree...

That small loop we did in sidcot down the lobster and up lobster pot bypass with an error of 0.61 gives it a fairly good field test, but mine in particular is reliant on the tandem I set it up with...as will be every individuals once they calibrate theirs after the first "factory setup" from Phil...
I did a manual pony test before and after use in sidcot and nothing changed.....(checked the left right up and downs)

Maybe email Phil at furbrain dot org dot uk to get "the info from the ponies mouth"

Title: Re: Shetland Attack Pony
Post by: graham on June 26, 2008, 07:17:35 pm
So your pony has the same systematic error as your tandem.

I wonder what it is ...
Title: Re: Shetland Attack Pony
Post by: menacer on June 26, 2008, 07:42:02 pm
My tandem is 1 degree different to the factory settings supplied on the pony.
The pony is accurate to +/- 1 degree.
My tandem could be spot on or it could be 2 degrees out.

Either way it would be consistent.

Im guessing the surface calibration thing against a fixed point(as done with normal compasses) would equally apply here.
Title: Re: Shetland Attack Pony
Post by: graham on June 26, 2008, 08:12:27 pm
Im guessing the surface calibration thing against a fixed point(as done with normal compasses) would equally apply here.


YES!!!
Title: Re: Shetland Attack Pony
Post by: jguarro on July 22, 2008, 07:09:33 pm
Hi everybody,

I'm a caver from Barcelona (Catalonia), and I've recently received a new unit of the SAP, and now I would like to download the Windows Driver and the PonyTrainer software, but it looks like their website www.shetlandattackpony.co.uk (http://www.shetlandattackpony.co.uk)  is down and Phil is bouncing the emails I sent him. Does anybody know an alternative place to download the software and driver?

Maybe Phil manages to put his website running in short, but I'm going to a caving expedition and we won't have internet connection available.

Thank's in advance,
Josep Guarro
Title: Re: Shetland Attack Pony
Post by: menacer on July 22, 2008, 08:48:19 pm
I have a copy of the drivers i could mail you if you send me your email address..
Tried ringing phil but no answer  :'(
Title: Re: Shetland Attack Pony
Post by: jguarro on July 23, 2008, 08:56:30 am
I have a copy of the drivers i could mail you if you send me your email address..
Tried ringing phil but no answer  :'(

Many Thank's! I've just sent you a PM.

Josep
Title: Re: Shetland Attack Pony
Post by: furbrain on July 23, 2008, 11:16:04 am
Sorry for not answering your calls - I've been in surgery this morning! The website is down, but I am hoping it should be up and running soon. If its not running by the end of today, I'll sort out an alternative site for hosting drivers etc.

For the meantime, you can contact me on: beardydoc <at> googlemail <dit> com
Title: Re: Shetland Attack Pony
Post by: furbrain on July 23, 2008, 03:19:02 pm
Ok, website is back up now.

Phil

PS if anyone has any questions about the SAP, please email me; I don't check this forum terribly often.

PPS if you were worrying that I was unwell - I am a doctor; I was working in a GP surgery today!
Title: Re: Shetland Attack Pony
Post by: jguarro on July 25, 2008, 11:57:11 am
OK, thanks to Carmen, and Phil I have been able to download the Ponytrainer software and diver, it's now running on my PC, and I can have access to the stored data etc.

I see that in the "File" menu there is the option to "Reset Leg Count", but in my unit it does nothing at all (no reset of the leg count), Am I doing anything wrong?

Thank's
Josep
Title: Re: Shetland Attack Pony
Post by: furbrain on July 25, 2008, 03:33:53 pm
No, you're not doing anything wrong. That menu item doesn't currently do anything - that should be fixed with the upcoming
firmware and PonyTrainer upgrade (which will include support for 400g surveying).

Going back to an earlier question. If you have calibrated a pony with a compass that points to magnetic north, the pony will read magnetic north. If you calibrate it with a compass values with respect to true or grid north, then your pony will read true north or grid north only for that place and time - it will drift off true/grid north as the magnetic pole moves. It is therefore simpler to calibrate it with respect to magnetic north.

Phil