True North on MyTopo Maps

I use topographic maps from MyTopo.com with UTM grids and Lat/Long tics printed on them.

I know the difference between true north (TN) and magnetic north (MN) shown on the maps. For me living in the United States I tend to imagine true north being somewhere north of the Mississippi River.

Via email communication with the folks at MyTopo.com I have confirmation that on my maps the grid north (GN) is based upon the UTM grid at the center of the map.

Now, I have two questions about true north references on topo maps. The answer might be specific to how MyTopo maps are printed - I'm not sure.

Question #1: Do longitude lines (north-south) always lie on true north? Or might they be slightly off? Maybe something to do the datum model?? These are hard to work with on my maps since I only have longitude tics printed, but I'd still like to know.

Question #2: Are the left and right edges of the MyTopo maps always true north? I think these have the highest chance of being true north, but I'm not 100% sure of that. I'm waiting - hoping - to hear back from MyTopo on this question.

I've seen some people recommend orienting the map to true north using the TN symbol lines, but they are kind of short for easy use.

Actual lines of longitude run from pole to pole, so kinda by definition are running true north/south. I have no idea how the edge of your topo map is determined.

There is True North, Magnetic North and Grid North, and they generally do not match up. You must consult the declination diagram which is normally at the bottom of your map. It will give you the difference between all of the "Norths" in degrees. To convert from compass to map, and visa versa, you must do the adjustment for the declination.

Take a ruler and connect the tics for Longitude. Those lines will be true north.

Yes, by definition, Longitude lines run exactly North-South. See above regarding connecting the tics.

Don't know. MyTopo will need to answer that one. If there is a Longitude tic at the corners, compare at the top and bottom. If on one side Longitude is the same on the top edge and bottom edge, then that edge is true N-S.

I'd be careful with that one. I think sometimes the little symbol declination diagram is just that...diagrammatic? Especially when for example TN & MN are only a couple of degrees apart it could be tough to show on the diagram without exaggeration?

anecdotal...after the cease fire in 1973, and the air war slowed down a bit, I had an opportunity to update some of the tattered working aids, and rebuild some of the unit's plotting boards... the 1:250000 UTM aeronautical charts were spliced together and covered with plexiglass so they could be written on with grease pencils... guess where the cartographers decided to have divergence in the charts?...zulu +8

I have seen many large gaps in various places in the world. In some areas the only existing maps are very old and were of questionable sources. In the Amazon River basin the maps are mostly reflecting the contours of the tree tops instead of the terrain underneath. There are so many grid systems, projections and datums worldwide. I usually end up right where the lack of good data is. The introduction of GPS has helped tremendously, but you still don't know what the terrain will be like based on the existing maps in some areas.

Speaking of the magnetic declination, it does change. So the map you may have from 30 or 40 years ago will not reflect the current declination. There are places on line to check current declination for your area.

The differences in datums and grid systems is more relevant when you are sending coordinates to someone else. If you are working on your own map you just need to ensure you know what you are working with and that your GPS agrees with your map. There are ways to convert, but it is a pain.

I totally agree with all of Montanero's comments.

Regarding out of date maps, USGS topos for much of Alaska date from the 1950's. For many parts of the state, the first order survey data used to tie into datums was not good. Hence in some areas there are still significant "busts" between adjacent map sheets. Mapping up here is slowly being updated, starting with the populated areas. But it will be a long time, if ever, before Alaska topos are anywhere near as good as in most of the lower 48. (And I know there are still some areas in the 48 states with issues.)

Regarding declination, I strongly second Montanero's comments. Declination changes with time. In some areas it changes quite rapidly. If the info on your map margin is more than a couple of years old, I would check before relying on it. One place to check is at: NOAA's Magnetic Declination page

When taking kids of the untrustworthy kind on forced excursions, we would cut the edges off the maps including any roads or dwellings. Eliminated a lot of run-aways.

I like to draw in the UTM lines on frequently used maps to make it easy to find coordinates when the GPS map is questionable or effected by overhead obstacles.

I found that easier to measure than using lat lon coordinates. Also to communicate easily over the radio sometimes we just used 3 north /south and 3 west/east numbers from the UTM.

I wish they still sold USGS maps locally.

Longitude lines (also called meridians) run north and south. On a USGS map the borders of the map are true north and south.

MyTopo maps can provide true north and south information. Make sure your basic default settings are adjusted to degrees true.

Another mapping software to take a look at is www.caltopo.com.

The point of this thread escapes me. In reasonable conditions and weather, northerly direction is readily apparent and one can orient a map in interesting country by terrain association - "that big pointy peak on the horizon is our objective;we follow this canyon until we begin to climb, etc. etc."

Don't forget about local variation, caused by anything from local ore deposits to your favorite neck knife, etc. These can sometimes throw a compass reading off considerably. Declination changes, although usually the changes are insignificant for most users.

I carry a compass always and use it about once a decade (on land). Use it but don't rely on it completely. Pay attention to your surroundings. Even GPS receives can be off a bit in some situations.

Depends on where you are. In my neck of the woods there are many times impossible to see the horizon. It might look like this:
http://www.luonnonperintosaatio.fi/sites...G?itok=OOmt_gPc

Point well taken. Thank heaven for the sweeping vistas of the American Southwest, dry though they be....

Learn to read terrain, which is much more important than precise azimuths.