Jeff,

Rarely get to work with the military which is too bad. I am retired Navy and miss all the great people I worked with.

I mainly teach classes to hikers,hunters, and SAR team members.

The few SAR teams that I work with rely on the baseplate type heavily.

Blake
www.outdoorquest.biz

@MDinana
Doing that, you are actually adjusting for declination

The method you've described relies on the assumption that the Mag North and True North on the map doesn't vary through time. You wouldn't want to rely on this method using an old map for the Alaskan region for example as the magnetic pole will have shifted quite considerably depending on the age of the map.

With Grid mapping systems it is even easier. You work out the magnetic declination (time varying) to true north which will also have a true north to grid north adjustment angle as well (this being fixed and not subject to time change). Add or subtract to give the Magnetic variation with respect to the map grid then use the magnetic variation with respect to the grid north lines to compensate for measured azimuths from the compass and visa versa. This ensures more accuracy and when using a baseplate compass very little pencil and protractor work is required i.e. line up the edge of the baseplate from where you are on the map to where you want to go, rotate the inner bezel until the internal lines within the compass bezel are running parallel with the map grid northing, read of the compass bezel angle and adjust for the Mag to Grid variation. When the compass needle is pointing north on the compass bezel you move off in the direction the baseplate is pointing.

You can usually get this whole process done within a few seconds with practice.


BTW depending on the scale of the map with the grid system. The variation of Grid North to True North may be slightly different depending on which part of the map you are working on. The map should indicate this so that you can accurately determine the Mag North to Grid North Variation.

And if using a GPS system always using the appropriate GPS datum system conversion to match the map grid system you are using. i.e OSGB, WGS-84 etc. Getting the wrong datum can lead to significant errors in navigation. Also the GPS needs to know the Grid to Mag variation as well. Some GPSs do not calculate this variation automatically for you.

Even with a GPS to tell you where you are (very accurate), GPS systems are not really to great at telling you which direction you should head of in (not very accurate). A map and compass is more accurate in this respect.

The only time I see this as having any practical implication would be covering vast distances in landscapes where the terrain does not dictate where you go. Flat plains and such, where being slightly off course may mean many miles extra traveled.


My world isn't a flat plain, and I am quite happy with the fact that if I move 10 meters my GPS will tell me the difference between my line of movement and my target. Both visually (at the screen) and with numbers, if you prefer that. Time/distance wasted due to zigzagging around the "perfect" course doesn't matter because the terrain dictates that you can't move in a perfect line anyway.


How accurate do you need your course to be? At my GPS I can at any time see the difference between the shortest available path from A to B, my track and the direction from my current location to my target. Heading off in the general direction of my target and then adjusting my course according to the GPS display is certainly good enough for me.


The only thing a GPS isn't doing very well is aiming at the horizon and finding a good visual target in the direction you want to go. The way I see it, there is less need to do this when you use GPS as primary navigation.


But GPS skills does not render compass obsolete! If you use GPS and really want to pick a visual target that is EXACTLY in the direction you want to go - then take the numeric course value from the GPS, add it to your compass (with adjustment for true v.s. magnetic north) and aim at the horizon ... Problem solved.

Map, compass and GPS complement each other nicely.

I guess it depends on the GPS in question and the precision of the navigation required.

http://www.youtube.com/watch?v=CJyt3RFycsE

Although mapping GPS devices are constantly improving, ensuring that the correct GPS Datum and Grid Mag variation are programmed properly, even the most expensive mapping GPS systems don't generate a truly accurate bearing to follow unless they have an electronic compass built in (again these are only usually accurate to +- 2 degrees. GPS firmware can give a reasonable indication once you are on the move using velocity filtering but if you are under cover or the number of satellites begins to vary (affect HDOP) this can effect your positioning making your position jump around (i.e. even up to +- 100m) giving inaccurate compass bearings to a nearby waypoint when standing still. I basically don't trust GPS compass information to say with confidence as to the direction to move off in to a way point (unless a built in electronic compass is in use rather than pure GPS)- there is just to many things that need to be right to point you in the correct direction not including the fact that GPS information is slow as indicated in the video.

An experienced orienter would have finished the course using map and compass by the time the waypoints had been programmed into some of these GPSs.

EDIT;


As a little experiment I took my GPS (a Garmin Camo) and set my position as a waypoint called 001 (I was using the GPS indoors so had no WAAS PRN and could only find 4 PRNS to generate a fix, which simulated heavy foilage coverage. I then programmed the GPS for another waypoint 002 directly North which was 200 metres away then turned off the GPS for 5-10 minutes.

Switching the GPS back on and allowing the GPS to again aquire a fix showed that the waypoint 002 was now showing a bearing of 332 degrees and was 182 metres away rather than being directly North @ 0 degrees 200 metres away i.e. a 28 degree error. A significant azimuth error.

GPS receivers also are subject to errors just as a map and compass work are for the the unaware, the problem is that the technology is seen by many as being 'fool proof' and leads folks into a false sense of security leading to potentially disastrous results.

[/quote]
Off topic for a moment, but is declination *really* needed? Most of the topo maps I've seen have a Mag North and a True North imprinted. I just tend to use the Mag north as my reference and ignore declination. I get a yardstick, continue the MN arrow until is hits the end of the map, and draw parallel lines from there. That way my compass can be placed on those lines, orient the map, and away I go.

MDiana,

Do NOT use this unless you have checked the angle with current. The USGS specifically states that the arrows are NOT drawn to match the declination. There can be significant error this way. If you want to use this technique then use a protractor or baseplate compass to draw the angle from a True North line on the map.

All,

I have used a Brunton Pocket Transit for about 45 years and use it for everything but orienteering. A baseplate is designed for that. It (like the lensatic) is not ideal for lots of things and overkill for most, but I regularly use all the features and at work need the accuracy so I use it even when other types would be better.

The best,

JerryF

Almost 30 meters deviation over 200 meters is basically unacceptable. You'd totally miss a tent/shelter in whiteoout conditions with a lousy reading like that. Another reason why map and compass will continue to be a necessary skill.

Way back when, when I was a Scout - my dad constructed a compass course in the 6 acres in our backyard. The course started and ended at the same spot. It was hilly and forested. The scouts were taken to the start point, aimed at the first Check Point, took an azimuth and paced out the distance. There were 15 checkpoints over a 2k course, more or less. The scouts then would plot the points using azimuths and distance to see if they could come close to the starting point. IIRC, most were within 20 meters, with some less than 5m off.

I found that using a military compass with mil reading helped a bit, cut my error down to 2m. Never really did get much better than that.

Most everyone I knew who didn't have a need for a mil scale compass voluntarily handed in the USGI lensatic compass and bought a Silva. USGI compasses are bulky and heavy. Losing one, it happens, I've lost gear in a swamp (kerplunk ... gone forever), can be problematic. I've been told that they can charge you several hundred dollars for replacement. Lose the Silva and Uncle Sugar doesn't say much. You are out $17.

Tip - If you use a Silva style compass around water it can pay to tie a bit of bright foam, sold in sporting-goods stores as fishing floats, to the lanyard so they float and are easier to find if dropped.

USGI lensatic compasses, and other gear that is too heavy to save with a small float, like a SAK, benefit from a 'dummy cord' that attaches the equipment to your belt.

Wow, that's interesting. I always was taught that their little MN/TN diagrams were accurate (at the time of publication). You're saying that it was likely just standard diagram, with the declination correction put in for the appropriate area??? That's pretty scary if true, since I don't recall EVER seeing that on the map itself.

AMLF, most of the maps I used were within 10-20 years. Fortunately, that's usually not too long for the pole to move. Plus, as someone said, my world isn't flat, so I checked pretty often. I also tend to stick with trails. When off trail (base camp, usually), I'm pretty good with landmarks for my goofing around.

Sounds like I've got the gist of it, but certainly potential for error.

Oh, and I still don't have a GPS... one less thing to fail

Lately, the magnetic north pole has been shifting west at dozens of miles per annum.