Mag north... Why?

[bsignorelli]

Several of us are having a discussion about why magnetic north needs to be shown on a cave map. The consensus is that it's a piece of info that (generally) isn't helpful if the map is drawn to true north and I was wondering what your opinion was.

This conversation was started because the declination in north central Arkansas is currently ~1.5 degrees according to the calculator in WALLS.
It's hard to show a magnetic north line that's only (or less than in some places) ~1.5 degrees off of the true north line... It tends to look
cluttered.

A few other reasons for worthlessness that were mentioned...

* The survey was corrected to true north when the data was reduced.
What value is magnetic north?

* It's only valid for the date it was determined. If you want to use the map 20 years from now you'll need to know the declination THEN not
now.

[Jeff Bartlett]

also:

* one of the surveys in question has not only been corrected to true north on a trip by trip basis, but it's been going on for 17 years.

on a timeline that large, nothing is gained by randomly selecting a date for the Nm declination or trying to work out some kind of average. i can see where it would be significant to note the declination for a particular survey portion on the original sketches and line plots (even though it's just as easy to look up after the fact anyway), but not on the final map.

[ian mckenzie]

IMHO, the principal reason for using Nmag plus the date is that the survey can be drawn up without having to calculate Nt (hmm, is that to the left or right of the N arrow?). We did lots of little surveys like that when I was younger, i.e. pre-internet when finding the current declination for your particular locale was not so easy. Recording Nmag and date allows future cavers to understand your data better, and to correct (if necessary) your declination which might have come off a 30 year old topo sheet or something. Besides, Nt or Nmag doesn't really matter if its a single survey of a small cave.

As for larger, ongoing surveys, I assume it is now common to let the survey software do the declination calculations for you, and that a calculated Ntrue automatically appears on the survey. Then there's Ngrid...

[bsignorelli]

one of the surveys in question

Jeff,

This isn't a problem. When entering the data into Walls you enter the date of each survey trip. The software then calculates the declination at the caves location on those particular dates.

So each individual survey is corrected to Ntrue by it's own declination and that corrected value is used for all future calculations done by the software.

And in the end, the lineplot is correct to Ntrue.

[Jeff Bartlett]

one of the surveys in question

Jeff,

This isn't a problem. When entering the data into Walls you enter the date of each survey trip. The software then calculates the declination at the caves location on those particular dates.

So each individual survey is corrected to Ntrue by it's own declination and that corrected value is used for all future calculations done by the software.

And in the end, the lineplot is correct to Ntrue.

i know that. my point is, in this instance, there's no way to put an accurate or reasonably useful Nmag even if you wanted to.

[Teresa]

i know that. my point is, in this instance, there's no way to put an accurate or reasonably useful Nmag even if you wanted to.

But think of it as generally useful to non-surveyors who want to know (in general) which direction the cave runs. They are extremely unlikely to calculate Nt, if they even know how to. In some caves (I can think of one large one not yet finished which has been mapped over the last *40* years) and which the surveyors only put down Nmag. We have no real clue what the cave actually looks like, because the drafters both old and new (using paper) only drew a pencil North arrow --presumably Nmag.

However if some farmer wants to know if the cave goes under their property, the Nmag is generally good enough. I understand local declination have varied about 5 degrees over that time span.

Or, maybe it is just one of those caves that move around underground.

[ian mckenzie]

But think of it as generally useful to non-surveyors who want to know (in general) which direction the cave runs.

Think you're missing the original point. Where several surveys over a long period of time have contributed to a map, there is no way of even indicating Nmag, because there will have been several different Nmags over time... so Ntrue (or Ngrid) will be the only possible arrow.

It is, I suppose, possible to calculate and add the current Nmag to a finished cave map. Perhaps that's what you meant. But that Nmag will of course become obsolete over time.

Only if all of the surveying occurred in a single year, then I agree that an Nmag with the date could be of use or interest, as a means of checking the original decination calculations for errors.

[Teresa]

But think of it as generally useful to non-surveyors who want to know (in general) which direction the cave runs.

Think you're missing the original point. Where several surveys over a long period of time have contributed to a map, there is no way of even indicating Nmag, because there will have been several different Nmags over time... so Ntrue (or Ngrid) will be the only possible arrow.

Not missing the original point at all, Ian. Most of the earlier year surveys ONLY had Nmag. Everyone around here worked in Nmag back then, and few people even attempted to account for declination. This is why data complied at differing times may be quite inaccurate numerically, but is still 'generally useful." Because of the change over time, (and the fact no one person has all 40 years of time stamped field notes) there is some question if the cave map is accurate at all, since the 'joins' to the older map may actually be at the incorrect angles. But it would probably take another 40 years to fix the original, so we've got what we've got.

What I meant by "non-surveyors who want to know (in general) which direction the cave runs" was in very general and intended for ridgewalkers -- presumably folks bumbling out out with a non-calibrating compass and a topo looking for the cave or expressions of the cave. Also, in the US Midwest, the difference in declination from absolute has varied, (we're not in one of those wide swing regions) but not that greatly since most cave surveying began.

So, no, I wasn't missing the point, just addressing the use of the maps by a different audience of map users than the accuracy-obsessed cartographers.

[ian mckenzie]

Sorry Teresa; i meant xcathodex's point. I actually agree with what you've said.

[Spike]

If you have reduced the data and corrected for declination. I see no reason to include a magnetic north arrow.

Spike

[Tom Gilleland]

I just put true north and magnetic north on my north arrow. Here's an example: Offering Cave, Belize It's just more information for the map user.

Tom

[driggs]

I agree with putting Nm on a map (alongside true N), but it doesn't necessarily have anything to do with the survey at all. The Nm declination should be as recent as possible, and is there for users of the map, not surveyors.

Seriously, do you check the present magnetic declination of the area that you're going caving in before using a cave map, every time you use the map? Have you ever "blundered" and added the declination to the West instead of the East (or vice versa)?

By putting a date on the Nm arrow, the user of the map has an idea of how accurate the declination on the map is. By putting both Nm and true N on the map, the user gets the best of both worlds.

Besides... I always have such a hard time finding the North Star from inside a cave...

[bsignorelli]

I'm glad to see that others think the Nm arrow is redundent if the survey has been corrected to true North.

The Nm declination should be as recent as possible, and is there for users of the map, not surveyors.

Hard to make it more recent than when the map was printed. If any time goes by that declination will be wrong.

Seriously, do you check the present magnetic declination of the area that you're going caving in before using a cave map, every time you use the map?

Um... no. I've never checked the declination of the area before going caving. I dunno anyone who uses a compass (surveying not included) while in a cave.

By putting a date on the Nm arrow, the user of the map has an idea of how accurate the declination on the map is.

Not how accurate but rather how current it is. But thats the point... if 5, 10, 20 years go by ... that declination isn't good for anything anymore. So the user would need to know the CURRENT declination ... not the declination when the map was made.

And if users want to add anything to the map then they will correct their data to true north and everything will synch up properly.

Anyways...

[tropicalbats]

Seems that most are agreed to kill the use of the mag arrow. Not to pick on Bryan, but he posted last so I quote:

Not how accurate but rather how current it is. But thats the point... if 5, 10, 20 years go by ... that declination isn't good for anything anymore. So the user would need to know the CURRENT declination ... not the declination when the map was made.

end quote: (sorry, don't know how to do those spiffy box things)

This seems to sum up the argument that including magnetic north arrows on all those cave maps, and all those topo maps we used back in the old days, was a waste of ink.

However, just because we know that the declination will change over time, do we not also know that the cave will change over time? Should we not include that bit of open floor because in the future it is obvious that the great slab on the ceiling is peeling off and when if falls our map will no longer be, um, current?

Just making hay in the forest with this example, to show that a map is a presentation of all things about the structure shown on the map, at the time of publication. Otherwise, why would anyone ever publish a map of forest cover, as this often changes more quickly than declination. We publish what is, when it is and as it is. I see no reason not to include a description of where magnetic north is at publication, especially since we use magnetic north in mapping it.

That is, other than the original problem, where the declination is so close to true north that the arrow(s) become unclear. Seems a bit of creativity could make an arrow thingy that was clear, but it would be a bother unless on a significant map.

Cheers,

Keith

[Martin Sluka]

This seems to sum up the argument that including magnetic north arrows on all those cave maps, and all those topo maps we used back in the old days, was a waste of ink.

Keith

NO in anyway. Any map of cave is better than no map. After time the maps are ONLY source of informations about caves - mostly those which are not in permanent interest.

For practical point of view - to visit cave - it doesn't matter. 2-3 degrees difference is not important.

But there is quite important to write on map the datum when it was made. When the declination was calculated. Only because we are able to discuss hours and hours about methods of mapping, precision of loop closure and so on.

And there is one solution today - to use a computer generated maps and if you need the super actual map with correct declination to generate new one and print it out.