Discussion: AR folks: do you care about true north vs. grid north?

Question for the Adventure Racers & Rogainers, who typically navigate over longer distances with gridded maps: do you bother with worrying about grid north vs. true north? Either way, to what line do you orient your compass' capsule north/south lines: the (possibly slanted) UTM grid, a meridian of longitude, or just eyeball it relative to the edges of the map?

I keep reading that the difference between UTM grid north and true north is not enough to worry about, but when I look at my UTM gridded maps (from both NatGeo TOPO and MyTopo.com), there sure is a strong slant to that grid...

Well, if you're using a compass, neither true north nor grid north is what you want...

Grid north would be for UTM (or MGRS---which is really the same thing just written differently). This page (scroll to the very bottom) says the maximum true-to-grid error in UTM or MGRS is 3 degrees.

The bigger problem is the difference between magnetic and the other three: True, UTM, or MGRS, especially in places where that difference is large. Where I live in the middle of the US, magnetic and true are 5 or 6 degrees different, but other places, it can get very large. (Example from 2000)

The biggest UTM dec I've seen is about 2.5 deg, so yeah, 3 seems about right. Here's a plot I made of the UTM zone 18 declination vs longitude across the state of PA, and an empirical function of lon that is a good approximation. Zone 17-18 boundary is -78 lon, and the max dec there at this latitude is 1.9 deg. Also shown is the PA State Plane South dec vs longitude. Magnetic Declination in central PA is 11 deg. W. Boston is 15 deg W (of true).

At the bottom of the page in the first link I posted, it says the 3deg error only exists at the poles, so everywhere else it is less than 3. At the equator, it's zero.

Since UTM switches to a different system called UPS near the poles (84-90N and 80-90S), the 3deg error isn't available in actual practice.

(Sorry for the geekiness---I've been on a GPS binge lately. I bought a 1991 GPS off ebay and I've been playing with it. It won't give the right date, but it seems to find the position just fine. Eats 6AAs in 4 hours. Any modern GPS works better. This thing barely works on the dash of my car---it wants to be out in the open air. Plus it's the size of a man's shoe and weighs maybe 2 pounds.)

I bought a 1991 GPS off ebay

I have a question....why? :)

Does it even talk NMEA? Can it output GPX files? *prods old tech with a long stick* It smells funny, too.

It does NMEA, apparently doesn't do UTM (!). It originally sold for over $3000. I got it for less than 30 including shipping. The real reason I wanted it was to show people how far GPS has come. That and an office near mine in 1992 had this exact model.

The real reason I wanted it was to show people how far GPS has come.

Fair enough. :)

It is indeed pretty impressive that the end user gear has come so far while still using a 20-year-old platform, and that the 20yo gear still works.

Getting back to the original question: for AR and Rogaines in the midwest, I always just use whatever grid is on the map figuring I can't stay within a few degrees over any distance anyway (UTM, True, and mag are all within a couple degrees here). However, in times when I've done races in the Pac NW, I find out what the declination is and then draw mag north lines on the map. Since USGS (the standard source for AR/Rogaine maps) prints text in all directions, I also put arrows on the lines so I can tell which way is north when the map is folded. I do this whether or not I'm adding my own lines.

Drawing in magnetic north lines, as Eric says, is the right thing to do. Rogaine maps are issued with magnetic north lines on them already, though I wouldn't count on that with an AR map.

Going back to the original question, why would you really care about the difference between magnetic/grid North when most AR controls are bingos, and the favoured technique seems to be to hunt around in a mob until one team finds it?

I think he's not talking about doing an AR per se, but just navigating with a topo map over longer distances, and figuring that AR and Rogaine people would have the most experience with that, and with similar types of maps.

I understand the error induced between grid and true north will be small to nonexistent (usually), I just want to truly understand all the factors. I realize that inaccuracies from consumer-grade compasses, less-than-excellent sighting technique, small cartography errors, and the realities of moving over varied terrain are all more significant.

"I think he's not talking about doing an AR per se, but just navigating with a topo map over longer distances, and figuring that AR and Rogaine people would have the most experience with that, and with similar types of maps."

Exact-a-mundo. I am not an ARer or Rogainer but I figured those folks would know. People who only do Orienteering sometimes seem to think everyone who ventures outdoors is running around with a 99.9% accurate 1:10,000 map with a 5m contour interval that was updated last week and has no grid considerations. I think they get accustomed to the calculus of

(great map) + (short distances) + (easily readable terrain) = (compass details don't matter)

Comparing some of the UTM-gridded maps I have from MyTopo.com and NatGeo TOPO, I made a discovery:

- the MyTopo maps have been 'tilted' so that the grid is squared to the paper, thus the top of the map is aligned to grid north. There is still an occasional longitudinal meridian line shown, and it is skewed relative to the top edge of the map, confirming this observation. Per the MyTopo declination diagram, the difference between grid north and true north on this map is only 1 degree. Further confirmation is that the declination diagram has grid north normal to the top of the map, with magnetic north to one side and true north to the other side.

- the NatGeo printout's grid is skewed to the paper, thus the top of the map is aligned to true north. If I print lat/lon lines, the lon meridians are normal to the top of the map, confirming this observation. The program does not give the difference between grid and true north, only between magnetic and true north, but true north is drawn normal to the top of the map.

Assume that you're using the classic Silva 1-2-3 method.

It seems to me that if you align the compass capsule's N/S lines to the grid (instead of a true N/S meridian like a longitude line), then you are taking a grid azimuth and to accurately translate that to the field, you must adjust for both magnetic declination and the difference between grid and true north.

Again, it all works out in the real world because the grid/true factor is small (usually). But I am not entirely convinced that everyone who does this stuff is knowledgable about why it works out. When I bring it up I often get the standard advice about magnetic declination and that's when I realize they don't understand the question.

From an orienteering/Rogaining perspective, grid north and true north are both irrelevant, arbitrary things and simply get ignored (except when in a location where they're close enough to magnetic north that it doesn't make a difference). In any case, an angle difference of a degree or two is small enough that it's not a factor in compass navigation, it's in the noise. For any navigational leg that's long enough that that sort of error will grown to a significant offset, you should be using features on the map to correct your path anyway.

Echoing above with a couple of things from my Rogaine/AR memory banks:
1. Drawing the mag lines on the map takes the gridN / MN delta out of the compass direction equation (TN is irrelevant)- one less calculation/adjustment to mess with on the move. (NB it's wise to recognize the inherent human limitation of following a bearing over longer distances- map reading trumps!)
2.AR course setters and racers tend to forget the precision or lack thereof associated with UTM coordinates, unless also identified as a definitive geographical feature, the UTM coordinate identifies the SW corner of a square with size as follows: 10 digit UTM = 1x 1m, 8 digit =10x10m, 6 digit 100x100m - the latter really does make for a crap shoot/ bingo CP.

A specific AR question: what is the general nature of the checkpoints? Do they require precision like you'd use for of a high level orienteering control? Or are they typically more easily recognizable like a major ridge, peak, or gully? I.e. if you can make your way over a few clicks or miles to that major drainage or river and use it as a handrail, you'll find the CP?

Looking at a 1:15k map vs. a USGS 1:24k map, it would seem the course setter would have to use a more easily identified checkpoint rather than something that requires a fine azimuth and pacecount (after bagging a good attack point, of course).

BTW, what does the term "bingo" mean here?

Many compasses have the ability to skew the compass circle on the compass bezel with respect to the lines on the bottom of the compass fluid capsule. (Most pure orienteering compasses don't---but look for an arc of angle markings on the bottom of the capsule centered around N and going maybe 40 degrees to either side.)

The nicest compasses with this feature have a little geared screw visible from the bottom or top of the compass and a tiny screwdriver on the compass lanyard to turn it. At the other extreme (cheap compasses), the adjustment is a pure friction fit and you have to hold the center of the capsule with your thumb on top and two fingers underneath and then turn the bezel to achieve the declination as shown on the map.

When someone shows up with one of these for orienteering, I usually check it for them and show them how to adjust it so the two north indications (top of the bezel v. the bottom of the capsule) are coincident. Even our loaner compasses, this one, have a friction adjustment for this, and they are often loose and prone to slipping in use. We try to identify and separate the loose ones so they don't go out very often.

My understanding of the way these compasses are meant to be used is to set the bottom of the capsule to magnetic while the top of the bezel is aligned to the true (or grid---your choice). Then you take bearings off the map by using the top of the bezel (true/grid), and you run that bearing in the woods using the lines on the bottom of the capsule (magnetic, like the needle).

The trick is to use the "three arrows" indicator on a USGS map (or to draw the magnetic north lines in) then lay the compass over it and see that the true or grid lines up with the top of the bezel, and the magnetic lines up with the markings on the bottom of the fluid capsule.

Finally, since most of my compasses are pure orienteering with no provision for declination, I just draw in the magnetic north lines and completely ignore the true or grid. The only time I set the declination is when I don't want to (or can't) mark magnetic lines all over the map, and when I'm not racing. You don't want to be tired and sucking air and trying to remember whether to use the north from the top or the bottom of the compass.

re precision of CPs. My experience (now a few years old) is that it depends - but I suspect is generally better now. Some low key events were unnecessarily vague (inexperience) while major events provided precision (and CP descriptions) to level the playing field ie recognizable on the ground and on the map. "Technical" nav/orienteering stages tend to use "center of the circle marks the location".
Bingo = an English /Antipodean term to denote by chance or luck as in "Strewth mate, that was a bingo control- howsabout a fair suck of the sauce bottle then" (I could go on but wont :-)

Thank you Cedar! I use a Suunto M3G (global) both on my own and for O-meets. I threaded the little declination adjustment key on the keeper cord. Before a meet I adjust the declination back to zero. Man those good O maps are such a wonderful luxury.

This is crazy: Compasses with Adjustable Declination

A specific AR question: what is the general nature of the checkpoints? Do they require precision like you'd use for of a high level orienteering control? Or are they typically more easily recognizable like a major ridge, peak, or gully?

It depends entirely on the course setter and race director. It is a gross (but oft repeated) generalization that all adventure races involve crappy maps and bingo controls. Sure, some of them do, but experienced adventure racers do not consider that to be acceptable in a quality event.

Many adventure races around here are designed by strong navigators, some of whom can kick butt in orienteering events and are as meticulous as any orienteering course setter. They may have to choose their checkpoint (control) locations differently to suit whatever map is available for a given area. One of the skills in AR navigation - a skill where orienteering experience helps immensely - is understanding how to get maximum value from different types of maps, e.g. older maps or maps with less detail. This is not a negative aspect of the sport. We get to travel through beautiful, remote wilderness areas and this means we have to use whatever maps exist for those areas. Good navigators can work with that.

As in orienteering, we expect the CP locations and descriptions to be fair and well-defined in the context of those maps and the terrain.

The adventure race I'm doing tomorrow is using GHO's orienteering maps for portions of the course. Many Ontario adventure racers also participate in Dontgetlost.ca adventure runs and other orienteering events, so they are sophisticated enough to know and complain if AR maps are inexcusably bad or if a course setter does a poor job. The majority of AR events around here these days have appropriate maps and correctly placed CPs.

As per Bash. And to add - Canadian maps include the UTM grid ( grid north). A quick declination adjustment and you're set. My math skills disappear on Day 3, so no thumb compass for me in these events. It needs to be brain muscle memory.

In the US, if drawing a grid, it makes sense to make this magnetic north.

CP placement " should" be on a map feature and as accurate as the map allows.

What Bash says about AR checkpoints also applies to Rogaine controls. You won't often see one on a 1.5 m boulder or a pit, because those things aren't mapped. So instead you'll have features that are, like top of hill, N tip of lake, or stream junction. Sometimes you'll see something like a reentrant, but it will necessarily be a large one, and the control needs to be hung high and visible (actually, that's true for most Rogaine controls). But I have also been to events where the control locations were... unsuitable,)

Excellent info fellas, thank you.

Bash: "...this means we have to use whatever maps exist for those areas. Good navigators can work with that."

This is something I'd like to get better at. After working with good O maps I suddenly feel hamstrung by older 1:24k maps. It takes a little woods time for me to recalibrate.

For night time events, do AR control points have reflective tape on them as a matter of course, or is that a sometime/sometimes not thing? Somewhere on the web I read an "intro to AR navigation" essay that said reflective markers are common and the tactic is to bring a superbright flashlight to hunt'em down from relatively far away.

In rogaining, the rules state that controls in overnight events must have reflectors - and yes, it can be easier to find controls at night than in daytime now that many participants have super bright lights. Understandably, not everyone is pleased with this development as it takes away some of the advantage of more skilled navigators.

Adventure racing doesn't have standard international rules so it depends. In many of the longer adventure races I do, the number of controls (CPs) is relatively small because the challenge is supposed to be in choosing a route over a fairly long distance, not in finding the flag. A number of the CPs have staff at them with radios and first aid kits.

As an example, I'm assistant race director for the Wilderness Traverse 24-30 hr adventure race in Ontario this August. We have a 30+ km off-road trek that will probably have about 3 CPs between the two end points. As JJ says, the CPs will be at obvious places such as a cabin along an ATV trail or a point on a big lake. You wouldn't need a reflector to find these flags once you've found the features - but that's not intended to be the hard part. We think it will be one of the most challenging navigation sections in any North American AR this year.

Having said that, there have been adventure races where flags were hidden under tree branches with no reflectors and only got found by a few of the teams. Racers should complain when that happens since these races should be designed to test skill, strategy and fitness, not luck.

Re your comments about map quality. Great idea to practise. It's fun to grab whatever topo map exists for an area and head out to find a few hilltops or stream junctions or whatever seems obvious on the map. As orienteers accustomed to good maps, we've built up an arsenal of navigation tools. Part of the fun and challenge of nav - even on detailed maps - is figuring out when to rely on which nav tools. An older, incomplete or less detailed map is an excellent way to fine tune your skills and your judgement.

This discussion is bizarre. Anyone who thinks the difference between true and grid north is distinguishable on foot with a hand-held compass should stay indoors.

Funny you say that gruver because I failed a "navigation test" at an AR event here in Ontario because when I was asked to determine declination based on the date of the published map and the mag north drift my answer was something like 8 degrees give or take a degree or two. "sorry sir you need to be more exact when setting your declination on your compass"... "I don't own a compass with declination and also know I cant follow a bearing that precisely anyway", I responded. "well I am going to have to flag your team and talk to the organizer" the navigation expert tester told me.

Next day my team and I waited 55 minutes for our bikes (only 2.5 hours into the race) after a running section because the organizer didn't anticipate teams could get there so fast and the bikes weren't delivered.

I've always draw my own mag north lines, still have never owned a compass that has declination on it and have never worn a compass around my neck. ;-)

and have never worn a compass around my neck. ;-)

And that's why they call you [MC] Hammer and not Flavor Flav.

I constantly fool myself into thinking that I can be certain to at least hit the control circle from 400m away, if I'm careful with a bearing. But no, so I'm probably missing by 4 degrees, more often than I would like.

Never wore one around my neck that I can recall, but I did used to tie it to the zipper pull of my windbreaker.

Less surprising perhaps is the widespread confusion in this thread between the true/grid difference (bugger all) and magnetic declination (the angle between one or other of the above - it hardly matters which - and magnetic north).

Around here magnetic north is 23 degrees east of true. But FCS 20 or 25 is good enough. Given a topo map I just imagine the magnetic lines rotated a bit.

Australian rogaining rules forbid reflectors and there is currently a debate/discussion afoot re control placement etc within the IRF including reflectors. Interesting input re northern hemisphere forests and reflectors in autumn; an experience I have had in the southern hemisphere on a limited basis with introduced trees.

Enjoyed this discussion.

A strong rogainer I know had a bad day and drove his team mates to distraction arguing that they were wrong and he was right about declination apparently they ignored him after a while and when he was leading just kept an eye on whether he was heading in a odd direction. He was able to laugh about it later.

Bash you describe what any navigation sport to me should be about whether that is a micro event, a classic O, a rogaine or an adventure race "One of the skills in AR navigation - a skill where orienteering experience helps immensely - is understanding how to get maximum value from different types of maps, e.g. older maps or maps with less detail. This is not a negative aspect of the sport. We get to travel through beautiful, remote wilderness areas and this means we have to use whatever maps exist for those areas. Good navigators can work with that.

As in orienteering, we expect the CP locations and descriptions to be fair and well-defined in the context of those maps and the terrain." Exactly.

Some of the old-timers may remember when control descriptions could read "A Boulder", meaining the control is on an unmapped boulder in the center of the circle versus "Boulder", meaning the boulder indicated on the map. I have no problem with hanging the control on an unmapped feature provided the feature is described, relatively unique within the circle, and the map gives enough context to navigate to that feature. I've seen AR controls on unmapped point features like sheds, large boulders, and small ponds that have been quite fair. As has been stated, it's all about getting the most out of the map you have.