18 forum posts
Adjusting for decliation,how ?
18 forum posts
5,881 forum posts
Your post name gave a bit of a start - "Wrongway" was an alternative nickname for a very dear departed friend, John Miksits, aka Zippo, well known to many on this and the MtnCommunity boards back when both were under Views From The Top.
Anyway, your post actually got me rather confused. But let's start from first principles and see if we can straighten things out. Actually, the whole thing is very simple, even though often explained in a very confusing way.
Your primary means of navigation should be your map, which you should be orienting primarily by the land and landmarks. But sometimes you will want to orient the map to north when you don't recognize the landmarks.
There are, as you found, 3 kinds of "North" often found on maps. Unless you insist on using UTM or MGRS coordinates on your GPS receiver (or are calling in the artillery or a chopper for an air strike), forget about "grid north". GN, which means "Grid North" is the alignment of the Universal Transverse Mercator (UTM) grid. As you might guess from the "Mercator" in the name, the grid is lined up with true north in the middle of a zone, but quite offset as you go toward the edges of a zone.
So now we are down to 2 kinds of north (see how fast things simplify?). True north is the direction to the geographic north pole, and is the alignment of the vast majority of government-produced maps around the world, and the derivative maps produced by MyTopo, National Geographic's Topo!, MapTech, and other computerized map programs.
So, your map is printed with the borders being true north-south lines (the longitude lines). Getting simpler, isn't it? You can align the map by pointing the right or left edge toward the North Star.
For the most part, though, you will be navigating and orienting yourself with the magnetic compass, which points to Magnetic North (no surprise here). By your numbers, your magnetic declination is about 10 deg West (the magnetic declination website shows that the middle part of PA is about 9 deg, but no matter since few people have either the compass or skill to read a compass more accurately than the nearest one or two degrees).
Ok, so you have several choices for using a compass to orient your map, depending on what kind of compass you have. I will recommend that you get yourself a basic base-plate compass. These are the easiest to use and understand, and besides they are cheap and the preferred compass for elite orienteers. The "lensatic" compasses are geared to military use, when you want to call in the artillery, and the ones without a baseplate are little better than toys (even though some cost over a hundred dollars):
1. OFFSET THE NEEDLE method - First align the bezel (that is the rotating dial) so the "N" (might also be "0 deg" or "360 deg") is lined up with the direction of travel arrow on the base plate (it is often marked as "Direction of Travel"). Now place the edge of the compass on the longitude line of the map (put the map on the ground or a table, just so it is not next to steel nails or bolts in the wooden table, and not on the hood of the car - yes I have seen people do this). Now rotate the map and compass until the red end of the compass needle is lined up toward the N and direction arrow. Hmm, now the "true north" map is aligned to magnetic north. Remember your diagram where the magnetic north line was to the left of the true north line? continue to rotate the map/compass combination until the magnetic needle is to the left of the N or direction arrow, just like the diagram. The amount should be 10 degrees for your area, which means the needle will be pointing to the 350 deg mark on the bezel. (for folks who are in the East Declination part of the world, out here in the Sierra, for example, offset to the right instead).
2. OFFSET THE BEZEL method - A variation on this is to set the bezel so it is lined up with the "N" (or "0" or "360") to the left of the "Direction of Travel" arrow by 10 deg. Again set the compass with the baseplate edge on the longitude line bordering the map. But this time, just rotate the map/compass until the red end of the needle is aligned with the "N". In other words, just offset the bezel and align the needle.
3. "PERMANENT DECLINATION" method - If you have a compass that can have the declination set in it, then just offset the capsule in the bezel. In some of the Brunton compasses, you pinch the capsule between thumb and forefinger and twist the bezel ring. In your case, twist it so the arrow inside the capsule is to the left of the N by 10 deg. In many of the more expensive Brunton (the real Silva Sweden compasses), Suunto, and Silva (the Johnson Worldwide compasses made by Suunto, but with a Silva logo) compasses, use the tiny brass screwdriver to rotate the screw located on the bezel (top or bottom, depending on the model) to offset the interior arrow by 10 deg to the left of the "N". In either case, align the bezel so the "N" is lined up with the direction arrow, just as in the first case, set the compass on the longitude line, and again, rotate the map/compass so the needle is lined up with the arrow inside the capsule.
4. DOING THE MATH - This is confusing to many people, since it actually involves math. But if you just remember which side of True North that Magnetic North is on for your area, the math becomes simple. In your case, the magnetic declination is West (left of true, or counterclockwise). This means that any magnetic bearing is going to be larger numerically than the true bearing. So if you are given the true bearing of an object, you add your 10 deg West declination to that true bearing to get the magnetic one. Out here in the Sierra, where we have an East declination, we subtract the declination from the true bearing instead.
If I write it as a formula, Magnetic Bearing = True Bearing + West Declination (subtract if it is East Declination).
SETTING THE BEARING IN THE COMPASS - Once you have your map lined up to the land and True North by any of the methods above, then just use your compass as a protractor. Draw a line between where you are and where you want to be, set the compass with its edge on that line, and rotate the bezel until the magnetic needle and arrow in the capsule are aligned. You are now ready to follow your compass. Warning, though - trying to go in the straight line shown by the compass may direct you through ravines, wide rivers, and up steep hills and cliffs. The better way is to just orient the map then use the map as your primary navigation tool.
There are several excellent books on land navigation that illustrate these methods with pictures. I recommend you get one of the ones from Mountaineers Press or from NOLS.
Thankyou very much for explaining the procedure of adjusting the declination to me in a way that I can understand.You had my mind wondering back to a place long ago with artillery and chopper gunships.Although I was a ground pounder with the 173rd and the 82nd,I owe a lot to those men that supported us with their heavy fire power.Take care Bill and God's speed........Ray
Bill,or whoever can help,
I have yet another question in regards to " setting the bearing in the compass" that was in Bill's reply to me.I think that I am making this harder then it is but I have to ask.
Ok,I have two different compasses,a Silva explorer base plate compass with a delination scale that doesn't lock in for a permenant setting allowing for my 10 degree declination.I can turn it to 10 degrees declination in order to orient my topo but here is the question : when I lay the compass on the map to go from my present position to a destination, after orienting the map,do I need to, again, add the 10 degrees to the bearing after boxing in my magnetic needle with my orienteering arrow?,keep in mind that this is with the silva compass without the permanent declination adjustment.I didn't think that I had to add the 10 degrees declination every time that I took a bearing to a destination point since my map was already oriented with the declination in consideration.But then,I picked up,"Brunton's A,B,C's of compass and map" VCR tape with their baseplate compass included,which has a permenant declination adjustment feature built in.While watching their movie,I picked up on something in it that sparked me to ask this question.Once they set their compass for there declination ,locking it in pemanently,they did not change the compass at all for taking a bearing from their position to a destination point after orienting their map to the lay of the land.
See what I mean? If I do it one way with the silva compass,minus the lock in feature as opposed to the Brunton's lock in feature,I will have a 10 degree difference in my bearing.
Which way is the correct way,in obtaining a bearing after orienting the map?
Thanks again for your help.I have a feeling that I am getting closer to having a handle on this delination thing.
Maybe this will help. I was just checking on some stuff when I came across your question. I'm an army scout observer/controller at the Joint Readiness Training Center in Ft. Polk, LA. I have been navigating for years mostly with lensatic compasses but I have a little experience with these orienteering type compasses and general map reading stuff. I think you are reading into it a little too much. Simply put, if you can not lock in your declination permanently with your type of compass, then you must factor in the 10 degrees of declination each time you take a bearing from point A to point B on your map. It doesn't matter if your map is oriented or not. I think that should answer your question. Good Luck
I knew that I was making this harder then it was but between you and Bill,you have gone a long way to making this very clear to me.I never had much map training,just what I picked up on my own.I worked with lensatic compasses in the paratroops but the CO would pass along the bearing for me to take through the boonies when I was a pointman.If I got lost,I was sure not to let the other 100 soldiers behind me know.lol
Topic: Adjusting for decliation,how ?
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