----- Original Message ----- From: "Olin Lathrop" > Robert B. wrote: > > DMS is your friend if you're ever humping around in the woods with > > nothing but a compass and a map. > > You must have a pretty fancy compass and an amazing ability to follow a > bearing if less than a degree makes any difference to you in the woods. Well yes, thank you. A degree or two actually makes quite a bit of difference. > I'm a hike leader with the Appalachian Mountain club, have used map and compass > in the woods on a number of occasions, and have even taught it to beginners > in formal courses. Plus or minus 1 degree would be amazingly accurate. > Most real hiking compasses aren't even marked that accurately, except the > silly fancy models some stores try to sell to those who don't know any > better. My compass is marked in 5 degree increments, and that's just fine. I have an ordinary US military issue navigation compass which is marked in 1 degree increments. It works quite well, and I use it often. > > > Dead reckoning (on foot, at least) > > is based in part around the DMS system, and knowing how many "paces > > per second" your stride is. > > Maybe if you live on the plains or stick to pavement. Even on regular > trails, you usually just guess your speed in terms of minutes/mile, then use > a watch to get a rough idea of how far you probably traveled. No, I do not. Obviously if you're on the trails then it's pointless to count paces, but if you're tracking a deer in the back woods of Wyoming, trails are not too easy to come by, and estimating average speed is nearly impossible. I do most of my compass navigation in the deep woods, faithfully counting paces. > Of course > this is all out the window when bushwhacking thru sporadically dense > vegitation. Thats where my method actually works, and it is amazingly accurate, I'd recommend you try it some time. In hilly or dense terrain especially, pace counting will still get you amazingly close. And thanks to the DMS system, all you need to know is how many paces it takes you to cover 100 feet, since the maps are marked in minutes or seconds of arc (DMS). > > > IIRC my pace count is right around 45 > > paces/second of latitude (also conveniently 45 paces/100 feet). > > So you're honestly telling me you count paces to measure arc distance > travelled!!? Usually I measure distance, and it is mostly by coincidence that the maps are marked in DMS, which happens to be ~100 feet. So yes, I honestly count paces when I'm navigating by compass. I don't know of any other way to do it that is nearly as accurate (excluding GPS). I learned it in the boy-scouts, and from my uncle who learned it in the US military. > > > I > > suppose you could do it using decimal degrees as well, but trying to > > compute that in my head would be a little tough. > > Even if you did this, it would only work when going north or south, unless > you want to do the mental correction for the shortening of longitude > depending on latitude. I guess if I decide to go hike in Alaska I'll have to modify my system. But I don't usually hike more than a couple hundred miles from whatever latitude I start at. The accuracy is more than good enough. > > > This system translates nicely to GPS, and allows you to go on week-long > > expeditions on a single set of GPS batteries, by just using the GPS > > for checking up on your position. > > OK, but you started out saying DMS was useful with "nothing but a compass > and a map". It is useful with "nothing but a map and compass", but it is also useful when complemented with GPS. Is that a problem? > > Admittedly, the dead reckoning system also uses standard distances, > > such as miles and km. But these also transfer much better to the DMS > > system (and human memory). For example: if you're 1 mile off > > course, this is roughly one minute. > > But what does that tell you, when you've got just a map and compass? "One > mile off course" is already what you want. What you really want to know is > where you are and where you want to be on the map. Thats the point of pace-counting. You know where you start, so if you know your vector, then you know where you are. I don't know how else to explain it. Maybe a dense mathematical formula is what you want, but it's just not that complicated. Start at A, travel in B direction for C distance, and mark the map as point D. Now travel in direction E for F distance, and mark the map as point G. Now, even without modern technology, you can compute your position fairly easily as the simple addition of vectors. If you want, you could even put it in a neat matrix and use Kramer's rule, but usually a little common sense and 3rd grade math works for me. > From that you set your > compass to the bearing to follow. The compass is set by using the lines on > the compass and the lines on the map. The units the compass is marked with > don't enter in at all except when correcting for magnetic declanation. I don't think I'd like to follow you in the woods if that's the way you navigate. The way *I* navigate is by taking a bearing in the direction I want to go, counting the paces, marking the map, triangulating to make sure I'm where I want to be, and correcting if I'm not. Then repeat the process until I get where I want to be. It works well. It is complimented by GPS on particularly long excursions, and saves the time of hiking to the top of something to triangulate, and only requires a 2-3 minute on-time for the GPS unit with the appropriate maps. This will give about 2 weeks of backcountry navigation on a single set of AA's. If you'd like to try this method, all you have to do is go to your local football field and count the paces it takes to walk it on flat land. Divide by 3 for a 100-yard field, and commit that number to memory. In hilly terrain your step sizes average out to very near this number, and in bushy terrain it is sometimes necessary to compensate. I'd also recommend a simple decade-bead counter to help keep track of your steps, just flip a bead every 100 steps (1-s of arc). _______________________________________________ http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist