Andy,

This is a multi-part problem:
1. Understand what degrees minutes seconds are
2. Understand how they locate you on the earth.
3. Understand how to find those coordinates on a green trails map.


What format are your latitude and longitude in? Degrees Minutes Seconds (DMS), Decimal Degrees (DD), etc?

1 Degree = 60 minutes
1 Minute = 60 seconds

E.g. N45 30' 00" = North of the equator 45 degrees, 30 minutes, and 0 seconds

or in Decimal Degrees
N45 30' 00" = North of the equator 45.5 degrees.
Why .5? Because 30 minutes is 1/2 of 60 minutes (and there are 60 minutes in a degree).

If it was N45 30' 45" it would be N45.575

Lat long coordinates locate you on a globe rather than a map and are unique to a particular datum. You need to know that the globe/map you are using is based on the same datum as your GPS, or change your GPS to display coordinates in the same datum as your Map (don't forget to change it back).

Latitude starts at 0 degrees at the equator, and goes to 90 degrees at the north pole. Similarly it goes to -90 degrees at the south pole. Instead of positive /negative numbers you will also see latitude written as N45 30 30 where the N indicates that it is North or a positive value.

On most US maps Longitude starts at 0 degrees at the prime meridian, but if you are travelling in other parts of the world you need to read the map legend to determine where their prime meridian is (Many countries define their own prime meridian and datum). Assuming 0 is at the prime meridian then values go from 0 to 180 going East of the prime meridian and from 0 to -180 going West of the prime meridian. That second fact makes longitude in the US a little bit tricky since all of our values are negative and many people/GPSs replace the "-" sign with a W or "West" designation, people get confused when a larger value preceeded by a W moves them to the left instead of the right. I like to just change the W with a - sign when I see it, so my brain knows where it is at on the number line.

Now the part you are interested in, locating your position on a green trails map. Your green trails map will have several grids drawn on it, but one of them will have labels near the edge that are in degrees minutes seconds. Typically, the other grid is for UTM coordinates and you can ignore it for this application. If you have mentally converted your coordinates to decimal degrees, your map is in the same datum as your coordinates, then you just use this grid to locate yourself. I'd start with Latitude (North and South) and find a grid label on the map collar (white edge around map). Find another label that your value falls between and interopolate (estimate 1/2 of the way between or whatever portion of the interval your coordinate is between the two. Note that the grids are not usually at 1 degree intervals since that is a huge amount of area, so you have to be careful. Once you've figured out where you are, then you can use a ruler to pencil in a mark or just hold your thumb at that spot on the left hand margin while you repeat the process for longitude. Just make sure that when you do the longitude you realize that you are in Quadrant 2 so -123 is to the left of -122, etc.

I've attached a map with some of the map features highlighted.

Very good, technical information in this thread but the quickest way to locate gps coords on a map is to buy maps that already have them overlayed.  I really like the Delorme topo atlas books, they are a bit large in scale (1:160,000) but it does cover the entire State.

Craig, lol, it isn't.  I just had to read your answer 3 or 4 times before I realized there wasn't going to be a math test involved  ;)

Seriously though, I couldn't agree more that terrain navigation is a worthwhile skill to develop since even the best gps is only as good as the map paired with it.

Craig,

I would be very interested in your Navigation 101 class.

Dustin

Yeah - everything after "David -" was in response to David's post.  He seemed pretty familiar with mapping concepts, so I indulged myself a little.  All of these concepts are a lot easier to understand in person where I can use props and graphics to explain myself in detail.  Your comment has me thinking though, that when I write-up the navigation section in the handbook I'm going to need a basic "Here's how you do it in the US" section that doesn't provide all the details necssary to do it anywhere in the world.

I'll start another thread with additional details where I can gauge interest.

Craig

Yeah - everything after "David -" was in response to David's post.  He seemed pretty familiar with mapping concepts, so I indulged myself a little. 

I’m a cartographer actually (and recently a GIS Technician as well, blech), so I’m relatively familiar with mapping concepts ; )   I appreciate the in depth replies!

I don’t, however, have a ton of field experience with map and compass navigation so I’d be interested in your class as well. I’m not sure when I’d be able to make it up to your neck of the woods though. 

As you mentioned geographic coordinates are the standard for cross platform sharing of locations. But that has more to do with sharing between systems and not personal use. I can see how a string of vehicles with different map users would benefit from lat/long in the field though. I wonder what the VDEG says about this? Anybody here have a copy?