I've got a couple admitedly dumb compass nav questions...

I come from an aviation (lat./long. background) so UTM is a bit wierd to me...

1) Is the UTM grid (for US latitudes) oriented to true north? Specifically, can you plot off it to get an accurate bearing?

2) why does the UTM grid not match up to the orientation of a USGS 7.5 minute map?

3) Why does UTM not match up (as in run parallel) to the 1mile section lines on the same map?

Typically I've drawn in the UTM lines on my quad maps and taken true bearings off those. I manage to find my way home...

thanks

The UTM grid breaks the world up into 6 degree slices of longitude and uses a mercator projection based on that. The further away you go from the center of those slices, (123w, 117w, 111w, 105w, etc) the more the grid's projection differs from the map's. West of the center line grid north will drift to the west, east of the center line it will drift to the east. So at -123 GN points straight up, at -120.1 it's leaning east and at -119.9 it's leaning west.

One of the many consequences of warping a 3D world to a 2D map.

So for the western continental US is it off enough to throw off bearings? I'm guessing that it is simply based on how out of kilter it looks on my map.

If you can't plot bearings off a UTM grid it seems only marginally useful, no?

edit: I think I understand regarding the skewing of the grid off the centerline of each 6 degree section...so, rather than "western US" then, does the UTM grid get thrown off enough at the edges of each 6 degree "slice" to skew bearings significantly?

For the Mount Wilson quad, west side 108 degrees, which is the west side of UTM zone 13, Gird North is 1 degree 48’ west. When using a compass you need to consider magnetic north. For this location it is 10 degrees 3’ east.

You can use the UTM grid for bearings. I do. You have to correct for magnetic north, so add/subtract the correction for grid north also. Also consider you’re hand holding a compass with 2 degree increments in a place where you don’t get to walk a straight line.

Section Lines - Public Land Survey System is another square grid on a round world. UTM was far in the future when the PLSS was being laid out. But like with UTM, something has to give. Unlike UTM, the PLSS was physically surveyed on the ground and was used to divide land. These surveys were not perfect (sometimes by fraud). So in addition to squares on a sphere realities, there are historical reasons why section lines are often goofy.

http://nationalatlas.gov/articles/bound ... _plss.html

http://en.wikipedia.org/wiki/Public_Land_Survey_System

http://geology.isu.edu/geostac/Field_Ex ... s/plss.htm

logsden wrote:If you can't plot bearings off a UTM grid it seems only marginally useful, no?

Spoken like someone with an aviation background. We use UTM a lot for SAR, but the helicopter pilots are all in lat/long land, because UTM is great over small scales but breaks down at larger ones.

The great thing about UTM is that its in meters. If you're at 39x-121 and you want to get to 39.1x-120.9, what direction do you head? It's not 45 degrees, because the ground distance taken up by 1 degree of lng shrinks once you move away from the equator while 1 degree of lat stays the same. The only way to figure it out is to plot it on a map.

Now, if you're at 744500E 4358000N and you want to go to 745000E 4358500N, you instantly know that you need to go 500 meters east and 500 meters north (grid bearing, possibly 1-2 degrees from true north/east). If you know A^2+B^2=C^2, you also know that you have to travel .7km to get there. When you're getting positions over a radio and trying to plot them on a map and figure out how close people are, this stuff is super useful.

Not to shill for my own site, but if you go to http://caltopo.com/map.html and click on the red UTM in the top right, it will bring up a UTM grid; you need to zoom in a little as I stop showing it once the map spans more than 2 zones. Pan around at a nice wide zoom level like 7 or 8 and you get a good visualization of how the UTM grid changes with longitude. USGS quads should list GN alongside MN at the bottom of the map; I don't know what it is exactly but 1-2 degrees sounds right for the edges of a UTM zone. The grid lines can look really skewed but it only takes a degree or two for things to look very non-parallel to humans.

mattyj wrote:Spoken like someone with an aviation background.

MountainHikerCO wrote:For the Mount Wilson quad, west side 108 degrees, which is the west side of UTM zone 13, Gird North is 1 degree 48’ west. When using a compass you need to consider magnetic north. For this location it is 10 degrees 3’ east.

You can use the UTM grid for bearings. I do. You have to correct for magnetic north, so add/subtract the correction for grid north also. Also consider you’re hand holding a compass with 2 degree increments in a place where you don’t get to walk a straight line.

So...correct me if I'm wrong...but for a 090* true bearing, with an 8 degrees 15' east net dec. correction based on your stats (?), this works out to (approx.) 082 degrees magnetic bearing? I'll be honest, I've never even noticed the GN references before.

With the understanding that life never really gets that precise out there...

thanks.

When taking a map bearing using the UTM grid, we no longer need concern ourselves with true north. We want the correction to be between grid north and magnetic. With the Mt Wilson quad example we would add the grid correction (from true) to the magnetic correction (from true). So the net correction is 11 degrees 51’ east. So if you measured 90 degrees using the UTM grid, you then spin the compass dial clockwise to about 78 degrees to then follow a magnetic bearing.

I avoid doing mental math in the field. I use the compass to take a bearing off the map. In the western US, I then turn the dial clockwise the correction amount. (Because Magnetic north is east – or clockwise from true north) I guess that would be visual math.

Also part of the mix, magnetic north is decreasing in the western US. Depending on the age of the map and location, this difference is often a couple degrees. While it is good to know the exact corrections, the UTM grid north and decreasing magnetic north might cancel each other (if you are in the west side of a UTM zone like much of mountain Colorado)

http://www.ngdc.noaa.gov/geomag-web/#declination

thanks a lot MH. that clarifies things.

thanks to all who replied.

It’s not the easiest thing to put into words. It’s easier for me to show someone with a map & compass on a mountain. I suggest practicing when you’re able to take your time on a hike where you already know the terrain. That way you know if it’s working.

mattyj wrote:Not to shill for my own site, but if you go to http://caltopo.com/map.html and click on the red UTM in the top right, it will bring up a UTM grid;

That is an incredibly helpful site, thanks!