Discussion: Grid North vs True North

If a declination angle is defined as the difference between true north and magnetic north, what use is this when plotting points in grid north? If all of my azimuths are made using grid north then i can't use the declination angle to convert it to magnetic (declination angle uses true north).

Or... are true north and grid north generally the same thing when using small USGS topo maps?

The declination on most (all?) topos is the declination measured from grid north to magnetic north.

Some (most?) will also include the deviation of grid north from true north, but that is mostly just a curiousity information piece and not really useful data.

Or you could just come to St. Louis where all three norths are so close you don't really have to worry about it.

Sticklers for small deviations should note that, since magnetic north is a function of time, any printed value of declination may not be accurate depending on where mag north is in it's cycle - it can drift by several degrees at some locations.

Thanks Seagull, I think you nailed it. Looking at my map now I see that it gives declination angles for both magnetic and true north. Most online definitions said that a declination angle was always from true north to magnetic, they didn't mention that it could be from grid north also. My USGS topo gives both.

Buckley: I'm staying in Ohio where magnetic north is nice and constant, though I was surprised to see how messed up it is in other places. Some spots in the world have magnetic north pointing directly south!

Look it up before the meet:
http://www.ngdc.noaa.gov/geomagmodels/Declination....

The Wikipedia page has a cool animated graphic on the changes in declination through the years:
http://en.wikipedia.org/wiki/Magnetic_declination

Why didn't they set up the grid so that grid north and true north are the same? The Wikipedia articles don't explain this.

Welcome to the wonderful world of datums, coordinate systems, and projections!
True north always is toward the geographic north pole. Grid north is north along the coordinate system that has been defined in relation to an earth datum, and a projection.
The datum is a known point, that the coordinate system references from. Normally this is near the center of the earth.
Since the earth is not round, and is hard to model, we define local projections that approximate the surface shape in an area, as you go farther from the center of that local projection, the errors become larger. It is frequently more convenient to define a local coordinate system that has a difference between grid and true north, so that your local grid is accurate.
It is more obvious when we get to global coordinate systems, where we define a singe grid over the entire planet. Since the grid has to approximate the shape of the earth, and we generally want it to be more correct where people are, not at the north pole, the poles of the grid do not line up with the geographic poles.

That makes sense, thanks for explaining!

A very good explanation, Ed. Projections make my head hurt. :)

I can't help but ask this hypothetical question...

If your on a pole, be it magnetic or true. Which way is west using the respective magnetic or true methods? Once on a pole isn't every direction the opposite pole? How did the polar explorers overcome this?

Which way is west

"West" has no relevance at the pole, every direction is North/South. I assume the polar explorers overcame it by not trying to go West. :)

I always find it amusing and perplexing when I read that Peary never actually reached the north pole because he was too far west.

If you are standing on the north pole, rotating counter-clockwise, your toes, shoulders, and nose are "going" west.

Perhaps polar coordinates is the answer.