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-   -   Two questions about map and compass (http://www.adkforum.com/showthread.php?t=17584)

wiiawiwb 08-30-2012 08:56 PM

Two questions about map and compass
 
I have two questions about how you use you map and compass.

1) I do almost all of my hiking in the Adirondack Park and have a Silva ranger. I can permanently set the declination to 14 degrees. I've never done so.

Do you use the offsetting mechanism and permanently set the declination on your compass?

2) Do you draw magnetic north lines on your maps?

Wldrns 08-30-2012 09:41 PM

Quote:

Originally Posted by wiiawiwb (Post 190886)
I have two questions about how you use you map and compass.

1) I do almost all of my hiking in the Adirondack Park and have a Silva ranger. I can permanently set the declination to 14 degrees. I've never done so.

Do you use the offsetting mechanism and permanently set the declination on your compass?

2) Do you draw magnetic north lines on your maps?

As you will see from several older threads, this has been discussed before. But I never tire of talking about land navigation... :rolleyes: Short answer is I use and teach both methods, plus the third no lines method of aligning the map to earth.

I prefer to draw mag north lines on my maps if I have the time. I will do it on traditional USGS paper topographic maps, and also digitally on mapping software before I print them. I prefer this method of using MN reference, leaving the declination set to zero on the compass, and instead compensating via map mag north line reference. While drawing the lines it is an excellent time to do some map study and get familiar with the expected terrain features. I do have several hundred USGS paper maps in my collection that I purchased and others that a library gave to me, but for the most part only the most often used (Adirondacks coverage mostly) are lined.

However, since I am frequently called on very short notice for SAR, I don't always have time to draw the lines, or for any map study for that matter. In that case I will grab a map from the chest (previously lined or not), or quickly print several in the expected search area, or accept the maps the rangers give me when accepting my search assignment... and then use the declination offset on the compass. Rangers generally use this method also, as they don't have time to draw or explain to others how to use mag north lines.

If you don't draw MN lines, then you must either align the map to earth (with no declination offset on your compass), or you must have (or draw) true north lines on your map. Most recently printed maps will have UTM grid lines pre-drawn on them, which for most practical purposes GN is close enough to TN. However, at the edges of a zone (every 6 degrees of longitude), GN declination can be as much as 3 degrees in error from TN. (Note - Many topos have not been updated or reprinted in several years and still do not have the UTM overlay.)

John H Swanson 08-30-2012 10:08 PM

no, because I hike in a variety of areas with different declinations

no, because I determine bearings using map true north and adjust by declination to mag north. I use the edge of the map and fold the map precisely and use the edge (the fold) for a NS reference. Sometimes I draw a few NS lines in the area I'm using. This assumes there is no UTM grid. If there is a UTM grid, I'll use it ignoring the slight difference vs. true north as I find the error inconsequential.

Justin 08-30-2012 10:23 PM

I draw multiple magnetic north lines on my maps, especially in areas where I'll be bushwhacking.
That's how I learned how to use map and compass together, and for me it takes away all of the confusion.

DuctTape 08-30-2012 10:35 PM

I too draw mag north lines on the map.

geogymn 08-31-2012 07:37 AM

I set the declination on the compass and use map edge for NS reference.
"GN declination can be as much as 3 degrees in error from TN." I didn't realize that there could be that much difference! Thanks!

DuctTape 08-31-2012 08:22 AM

Another consideration is magnetic anomalies. A few years ago, while researching some out of use trails I cam across a USGS map for part of the the 5 ponds WA.The map had iso lines but they didn't correspond to elevation, but to magnetic error due to large deposits of magnetite (if I remember correctly). These errors were not insignificant either. Of course proximity to the magnetite outcroppings would be the controlling variable. After reading about these anomalies, I wondered if any of my off trail routes, while appearing to be relatively straight as per the compass, actually took a bend due to the ore much like a space object hurling through space will bend towards a massive object. I wish I saved the link to that old map. some google-fu might turn it up if one was interested.

edit: found the map. it was within a book page 160, searcheable on google. http://books.google.com/books?id=U-0qAQAAIAAJ&pg=PA160 I think the description is on the next page. Fascinating reading for navigation, history, or geology geeks.

Wldrns 08-31-2012 09:04 AM

1 Attachment(s)
Quote:

Originally Posted by geogymn (Post 190899)
I set the declination on the compass and use map edge for NS reference.
"GN declination can be as much as 3 degrees in error from TN." I didn't realize that there could be that much difference! Thanks!

Here's a training slide example I use to show a section of map in New Hampshire near the UTM zone border (72 degrees longitude). GN and TN are only 2 degrees different here, but that difference increases the farther north you go. Note that the GN/TN angle is is quite noticeable by eye at the edge of the map.

For most recreational navigation purposes a 2-3 degree error will not get you lost (you are primarily navigating by using terrain observation with M&C, right?).

But if you do not consider magnetic declination at all, a 16 degree error here could be extremely confusing. Do the math wrong if you are making a TN/MN conversion and you get a 32 degree error (all too easy even for the experienced when tired and partially confused already), then you are sunk. That's another advantage of pre-drawing the MN lines on the map - there is no need to do the mental math conversion in the field.

Wldrns 08-31-2012 09:15 AM

Quote:

Originally Posted by DuctTape (Post 190901)
Another consideration is magnetic anomalies. ...

edit: found the map. it was within a book page 160, searcheable on google. http://books.google.com/books?id=U-0qAQAAIAAJ&pg=PA160 I think the description is on the next page. Fascinating reading for navigation, history, or geology geeks.

Thanks for the reference, I'll have to investigate it further.

I was working with a ranger to establish a "bump line" to define a SAR search block a couple of years ago in St Lawrence County. We were both using a compass to flag a straight line when suddenly the line wanted to make a 30 degree jog. it didn't last very long, maybe 50-75 yards at most before our compasses returned to the normally expected heading, but it was quite evident that both of our compasses swung noticeably to one direction just as we crossed a small stream. We attributed it to a magnetic anomaly. I've long heard they occurred in certain areas, but that was the first time I actually experienced one.

All the more reason to constantly pay attention to all the navigation clues available, particularly the ability to observe and interpret terrain. When something doesn't make sense with everything else (such as an erratic compass), figure out why before you get yourself into trouble.

wiiawiwb 08-31-2012 11:12 AM

Quote:

Originally Posted by Wldrns (Post 190887)
I prefer to draw mag north lines on my maps if I have the time. I will do it on traditional USGS paper topographic maps, and also digitally on mapping software before I print them. I prefer this method of using MN reference, leaving the declination set to zero on the compass, and instead compensating via map mag north line reference.

I've looked at Paint and Windows 7 snipping tool but there is no reference to north and no way I could find to create mag north lines. I've used a protractor to manual draw them after I print them from on my mapping software (Garmin BaseCamp with NYTopo11. Garmin BC has no ability to digitally prepare those lines.

How do you digitally prepare your mag north lines?

Wldrns 08-31-2012 11:39 AM

Quote:

Originally Posted by wiiawiwb (Post 190914)
I've looked at Paint and Windows 7 snipping tool but there is no reference to north and no way I could find to create mag north lines. I've used a protractor to manual draw them after I print them from on my mapping software (Garmin BaseCamp with NYTopo11. Garmin BC has no ability to digitally prepare those lines.

How do you digitally prepare your mag north lines?

The easiest and fastest way for me is to save the selected map area in pdf format from NGTopo or some other map display application, then open with a drawing tool application that will let you draw lines at measured angles. Just draw one line and copy/paste to duplicate it at appropriate intervals across the page. I use Photoshop to get the job done, but there are many cheaper alternatives. Since I use a Mac I cannot advise what you would use on windows, but I'm sure there must be inexpensive graphic/drawing tools available.

The old fashioned way of using a protractor and ruler or yardstick works just as well. Just draw lines the width of the yardstick replicated across the area of interest. You can be very accurate by eye if you are careful from just the first protractor measurement - you don't have to physically measure each drawn line with the protractor, but you can check your work part way through to be sure.

Use the largest protractor you can find for the initial line. Cheap plastic school protractors are fine. In a pinch your compass and any long straight edge will also work as a protractor.

DO NOT simply extend the little short line from the declination diagram in the legend of the map, for two reasons - it is far too short to be accurate for extension; and it is only representative of the declination angle, not guaranteed to be at the actual declination angle, particularly for small angles.

Huginn og Muninn 08-31-2012 01:33 PM

I usually draw a magnetic north reference line somewhere near the edge of the map to aid in orienting it.

TCD 08-31-2012 05:42 PM

I have the same compass, a Silva Ranger. I set the declination on the compasss - set it and forget it; north on my map is now north on my compass. I only think about it when I travel to another area with a different declination.

I know other folks use it, but I've never seen an advantage in drawing lines on the map. I think that's only useful if you have a compass without a declination adjustment. Not sure why it's popular...maybe one of you could enlighten me to advantages I had not thought of?

Wldrns 08-31-2012 07:57 PM

Quote:

Originally Posted by TCD (Post 190928)
Not sure why it's popular...maybe one of you could enlighten me to advantages I had not thought of?

Sure. As with many such things, there is no one general "best" method of using map with compass. It is mostly a matter of preference, sometimes a matter of what you have that works the best.

So... if you are to use your compass on a topo map, it is a given that you must account for magnetic declination. A bare compass reads angles relative to magnetic north, the map is printed relative to true north.

In using a compass directly with the map, how would you use the compass to measure a course from one location to another (compass as a protractor)? You need a baseline reference for your angle measurement. Some kind of lines must be drawn on the map. You can adjust the compass for area declination, but you must now use a true north oriented base line. The left and right edges of the map work, but unless you have a compass that is around 3 feet long, you need some additional baselines on the face of the map. Most people will use the UTM grid overlay (if the map is so printed), assuming that the grid is oriented to true N/S. Not a bad assumption, but not totally correct either.

What if you are using a USGS topo map that does not have the UTM overlay? There are many areas of the country, NY included, where the most updated topo map does not have any N/S grid of any kind printed on it. So you have a choice, you can draw your own N/S lines, oriented either to true north or to magnetic north. Or you can put the map on the ground and rotate the map itself to TN using your compass every time you want to measure a course.

I learned to draw lines on my map many years ago in a class in which it was not assumed that every student had an adjustable compass. Today I am the instructor, and find that in a comprehensive land navigation course there are still people with nonadjustable compasses. But more than that, the process of WHY navigation works by drawing MN lines helps to better explain and give a greater understanding of what can otherwise be a difficult concept for some people to comprehend.

Furthermore, I find the time I take to draw the lines is a great time to do some map study of my planned course. I prefer to not have to worry about whether my compass is properly adjusted for the region I am in.

Do I have to draw the lines? No, of course not.
Do I always draw lines? No, not always.
I don't restrict myself to any one method, I just prefer using drawing the MN lines method.

wiiawiwb 08-31-2012 10:05 PM

I took a wilderness survival course many years ago and everything related to map and compass was geared toward orienting the map to to MN. It all made sense to me. The notion of declination and adding one way and subtracting the other just seemed foreign.

Obviously, we all probably feel most comfortable using whatever system we were taught....for better or for worse. Drawing MN grid lines on a map and planning from there just feels comfortable to me.

I recently brought a date hiking and tried to explain declination to her. Forget it. Most people aren't into the math of hiking. I can't blame her. She just wanted to see a vista and trusted I would get her out safe and sound.

I really want to thank all of you for your patience and understanding. It's truly quite something to "rub elbows" with people like yourselves who not only have a complete knowledge of things (and are willing to share it) but, most importantly, you have a thirst for it as well.

To bat around strategies with those of you on this forum is a real treat.

Kudos to all of you.

geogymn 09-01-2012 09:33 AM

Wldrns, Drawing MN lines on a map is fine but what happens when the declination changes? Only kidding! Although I have drawn a grid on a topo , I used TN lines. As usual I am impressed by the way that you explain things and although I am quite comfortable with my technique I will need to acquaint myself with the techniques that you present. If for nothing else it will aid me in teaching the compass to another if the opportunity arises, after all, teaching is both self rewarding and self educational. My hat is off to you for taking the time and effort to promote learning.

DSettahr 09-10-2012 12:58 PM

Quote:

Originally Posted by wiiawiwb (Post 190886)
1) I do almost all of my hiking in the Adirondack Park and have a Silva ranger. I can permanently set the declination to 14 degrees. I've never done so.

Do you use the offsetting mechanism and permanently set the declination on your compass?

Yes, I do. I think that it makes life a lot easier when navigating by compass, and makes your navigation a little bit less prone to error- provided that you really and truly understand what you are doing when you set the declination.

IMO, you need to learn and be confident with working with declination without adjusting it on your compass first. The reason being- if you do the declination calculation every time, and you make a mistake, that mistake won't follow through on subsequent recalculations, and is less likely to cause a serious mishap.

If you set the declination on your compass, though, that's one single calculation that you rely on for maybe even your entire hike. If you make a mistake and don't catch it, you could end up in serious trouble. You'd better be sure that one single calculation is correct... hence the need to really and truly understand what you are doing when you set the declination on your compass.

Quote:

Originally Posted by wiiawiwb (Post 190886)
2) Do you draw magnetic north lines on your maps?

Nope. I can understand why this might be beneficial to some people. I've never found it to be necessary though.

Wldrns 09-10-2012 01:20 PM

Quote:

Originally Posted by geogymn (Post 190944)
Wldrns, Drawing MN lines on a map is fine but what happens when the declination changes? Only kidding! Although I have drawn a grid on a topo , I used TN lines.

You would be using a very old map to have the slowly changing declination make much of any real difference in most places and situations.... :eek:

May I ask what TN lines you use? The edges of a topo map are aligned to TN, and there are TN tick marks on the top and bottom margins, but otherwise there are no TN lines drawn across a topo map. I suspect you are using GN lines, which as mentioned in a previous post, are not aligned to TN but are usually close enough to use with other navigation aids (e.g. terrain observation).

Quote:

Originally Posted by DSettahr (Post 191276)
If you make a mistake and don't catch it, you could end up in serious trouble. You'd better be sure that one single calculation is correct... hence the need to really and truly understand what you are doing when you set the declination on your compass.

As I often say, you are allowed one navigation mistake. If you are truly navigating (i.e. not relying on a single point of old information), soon enough that mistake will make itself known and be relatively easy to recover from. But compound that first mistake with a second mistake and your recovery may become exponentially more difficult. If, as DS says, you really and truly understand what you are doing, that won't happen.

Which ever compass/declination method you use, completely understanding the relationship with your compass of that little declination graphic diagram in the legend of the map will solve many problems and answer just about all questions. If your map does not have the graphic printed (newer metric series maps may not), draw one from the margin text that tells you what the declination is. Understand it and use it.

My flight instructor years ago told me something that has proven true over and over again and I have never forgotten... "All navigators make mistakes. The difference between a new navigator and and an experienced nav is how quickly the mistaken is caught and corrected..." Best done before the pilot even notices. :rolleyes:

geogymn 09-10-2012 06:13 PM

Quote:

Originally Posted by Wldrns (Post 191279)
You would be using a very old map to have the slowly changing declination make much of any real difference in most places and situations.... :eek:

May I ask what TN lines you use? The edges of a topo map are aligned to TN, and there are TN tick marks on the top and bottom margins, but otherwise there are no TN lines drawn across a topo map. I suspect you are using GN lines, which as mentioned in a previous post, are not aligned to TN but are usually close enough to use with other navigation aids (e.g. terrain observation).

. :rolleyes:

Your suspicions are correct, I stand corrected.

John H Swanson 09-11-2012 01:21 PM

Quote:

Originally Posted by wiiawiwb (Post 190935)
I recently brought a date hiking and tried to explain declination to her. Forget it. Most people aren't into the math of hiking. .

When teaching map and compass...the best way to divide the group into biginners and others is to ask about declination. If you know don't what it is - and can't explain it, you go into the beginner group.


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