True North or Magnetic North?

[Hardman]

Hi there, I'm a newbie to GPS.

 

I have recently purchased a Garmin Etrex and have been studing GPS, mapping as well as Geacaching. Today I found my 1st Geocache which was great and most enjoyable.

 

I have a question though which hopefully someone can answer.

 

When using my GPS with UTM topo maps, should it be set for "true north" and by the same token, when using it alone in the field without any maps, should it set to "Magnetic north"?

 

Thanks

The Hardman

[QuigleyJones]

I'd say grid north if its available.

And then magnetic if you were with other people with magnetic compasses, other wise its personal preference.

[raybonz]

I'd say grid north if its available.

And then magnetic if you were with other people with magnetic compasses, other wise its personal preference.

I just got a gps compass and it has a declination scale on it to adjust it to true north based on location. From the literature that comes with it I think they said most maps are drawn using true north. Here is the link to the compass I got: http://www.sierratradingpost.com/p/,98634_...se-Compass.html . I plan on learning how to navigate with it as a back up. When you think about the early explorers it's amazing what they accomplished using so little..

 

Ray

[+Red90]

With UTM topo maps, grid north as it will align with the grids on the map and is best for aligning the map in the field.

[raybonz]

With UTM topo maps, grid north as it will align with the grids on the map and is best for aligning the map in the field.

Guess I have lots to learn eh?

 

Ray

[John E Cache]

Hi there, I'm a newbie to GPS.

 

I have recently purchased a Garmin Etrex and have been studing GPS, mapping as well as Geacaching. Today I found my 1st Geocache which was great and most enjoyable.

 

I have a question though which hopefully someone can answer.

 

When using my GPS with UTM topo maps, should it be set for "true north" and by the same token, when using it alone in the field without any maps, should it set to "Magnetic north"?

That is a common question. The short answer is the true vs. magnetic setting doesn't affect what coordinates the GPS gives you. It only affects directions like bearing, course etc.

 

The problem is when you are at point A and want to get to point B what direction do you go to get to B. If the GPS gives you a magnetic bearing you just sight on your compass. The only obscure reason I can think of for using a true north setting is if you mark your location A on the map and want to draw an arrow towards B using a protractor. There must be a better use, but I can't think of one.

[DogFleazJR]

[The problem is when you are at point A and want to get to point B what direction do you go to get to B. If the GPS gives you a magnetic bearing you just sight on your compass. The only obscure reason I can think of for using a true north setting is if you mark your location A on the map and want to draw an arrow towards B using a protractor. There must be a better use, but I can't think of one.

 

John,

 

There are only a few instances where the choice between mag and true north matters. Since nearly all maps are drawn to true north, this is almost always the better choice as your GPS and paper map will align correctly in the field. That is why the GPSr ships with true north as the default.

 

The only time you need to change this is when you are using the GPS in place of (or in conjunction with) a compass. For example, you have compass bearnings (or an orienteering course) that you are trying to replicate.

 

When following a bearing to a waypoint with known coordinates (such as a geocache) the choice does not affect the GPS function in any way. The bearing pointer on the GPSr will point you directly to the waypoint regardless of the true/mag choice. That is why for 95% of GPS users, the choice doesn't matter.

[+kenk]

My suggestion is to:

 

(1) Use a compass that has adjustable declination, so the compass can be set to true north.

 

The Suunto Leader M2 w/o the Global Needle is a very high quality compass with adjustable declination, but only costs about $20-25. Brunton also has some very nice compasses with adjustable declination for about the same price. Don't confuse "adjustable declination" with a "declination scale".

 

(2) Set the GPS to true north.

 

(3) Set the GPS to the same datum as displayed on the map, usually NAD27 for USGS 7.5 minute topo maps (on Garmin's use NAD27CONUS).

 

My Garmin GPS defaulted to WGS 84, so make sure you make the change. This is needed to make sure the coordinates on your GPS match your map. If you're not using a paper map, stick to the default for the GPS.

 

(4) Align the paper map to true north before taking or using bearings. The edge of the map should provide a north edge.

[John E Cache]

The bearing pointer on the GPSr will point you directly to the waypoint regardless of the true/mag choice. That is why for 95% of GPS users, the choice doesn't matter.

[nit pick]Only if your GPS has a built in compass or you are moving and then it isn't very accurate.. When I stand still and want to know where the arrow is pointing, I sight within a half a degree with my bearing compass. I do this because my GPS gives me a magnetic bearing. I use this method for triangulating on a cache in poor reception areas. Bearing from a clear area seems to be more accurate than location in cover.

[John E Cache]

(2) Set the GPS to true north.

OK. Now we are getting somewhere. I have wondered why you would do this. What info does the GPS provide when you do this? Are you projecting waypoints with map bearing and distance?

[+Red90]

The OP said quite clearly that he is using the GPS WITH UTM TOPO MAPS...... If you wan tto use them with THOSE maps then grid north make the most sense as the GPS compass display will align with the map grid. This allows for easy alignment of the map in the field which is the normal way to navigate using a map.

[+macroderma]

Get real guys

 

Who can navigate by magnetic compass so accurately that the variation is important?

[GreatCanadian]

Get real guys

 

Who can navigate by magnetic compass so accurately that the variation is important?

 

I am not sure I understand your comment, but if you are saying what I think your are saying, then my answer would be lots of people. I worked with the forestry and used a magnetic compass. My bearings had to be accurate. Sight in on a distant point using a slot in the compass, and a mirror which aligns a sightline through the center of the compass bezel. Get to that point, sight again using the same bearing. With experience, it is incredibly accurate. It had to be. That's what I was paid for. And if you are talking about the variation between true and magnetic north, then you are most definitely way off course here (pardon the pun). Magnetic declination here is 21 degrees west at the moment. If you don't adjust for that difference then you cannot navigate accurately. Experience compass users in this case would set there declination on their compass to 21 degrees. Not 20 or 22. That 1 degree will make a difference to a good compass user. If you don't have a magnetic declinationadjustment screw on your compass, the user would just add 21 degrees to the given true bearing. Yes, they are that accurate in the hands of a competent user.

Edited February 12, 2007 by GreatCanadian

[John E Cache]

The OP said quite clearly that he is using the GPS WITH UTM TOPO MAPS...... If you wan tto use them with THOSE maps then grid north make the most sense as the GPS compass display will align with the map grid. This allows for easy alignment of the map in the field which is the normal way to navigate using a map.

OK. I don't understand the question, I guess.

 

I can see how you navigate by map. I have done that with topo maps. You constantly keep track of where you are by looking for features.

 

I have navigated with GPS and the GPS tells you where you are. Much easier. It tells you where you are going by a bearing.

 

I just don't see how you would use both map and GPS. And if you did why would you need to line them up? North at the top would be good enough, right. I see this question all the time when people switch from maps to GPS and I don't get it. Please explain(without capital letter yelling, please)

[+Red90]

When the map is aligned correctly, you can easily and quickly located landmarks for deciding on your route. If there is large declincation and the map is ariented incorrectly, you may place landmarks incorrectly.

[+kenk]

(In this reply, when I refer to a "compass" I'm not speaking of the on in the compass. I'm talking about a magnetic compass)

 

A GPS and paper map go together very well, especially if your GPS doesn't have on-board mapping OR doesn't have on-board maps that as much detail as the paper maps.

 

Typically I get my current UTM coordinate from my GPS and then a UTM grid tool (see http://www.maptools.com ) to find my location on the paper map. For this to work accurately the datum on GPS has to be changed to match that of the paper map.

 

For the most part I let the GPS calculate bearings for me (rather than taking them from the paper map). The utiity of the map is to determine the coordinates of the destination, and to identify features which might help/interfere with my travel (trails, roads, hills, rivers, wetlands, etc).

 

If I have destination on a paper map, I can determine the destination's UTM coordinate on the map using the UTM grid tool and then enter that coordinate as a waypoint in the GPS. I'll usually try to pre-load destination waypoints using ExpertGPS software, but inevitably I don't get them all in ahead of time.

 

With the destination entered into my GPS as a waypoint, the GPS will give a bearing from my current location to the destination. I shut off the GPS to save power and use the magnetic compass (not the one on the GPS) to follow the bearing. As mentioned before, I myself prefer to have both GPS and compass using true north (my compass' adjustable declination set to match the declination in my area).

 

It really doesn't matter whether you set your GPS with true north or mag north, so long as your compass is "set" to match the GPS setting. If your GPS is set for true north, you'll need to make sure your compass' adjustable declination is set for the area of use, so the compass uses true north (when pointing north, the arrow points to 0 degrees). If the GPS is set for mag north, then you'll need to make sure your compass' adjustable declination is NOT used (set to 0 degrees, so the north arrow does not points to 0 degrees - unless declination is 0 degrees for your area of use).

 

The reason I like using true north on both GPS and my compass is that I align my maps to true north before taking bearings off of the map. I place the edge of the compass (with declination set) on the edge of the map, and rotate my body until the N end of the arrow is "boxed". Now the landmarks on the map should match those in real life, and I can get map bearings quite easily.

 

You could do the same thing by not adjusting the compass for declination and then using the true north arrow on the map to orient the map, but I find the true north arrow on topo maps to be kind of small for easy use.

 

In short, I like to work in a true north world. I map sure the compass is adjusted for my area's declination, make sure the GPS' datum matches the map's, and set my GPS to true north.

[Hardman]

Many thanks for all the comments.

 

I've read kenk's explanation and it does match that of the book I'm reading using GPS with topo maps.

 

It takes a bit to obsorb the information, but I do now understand when I need to set GPS to "true north" or "magnetic north".

 

Thanks again

 

The Hardman