Lat/Lon Coordinates from 7.5 min topos

[le Voyageur]

For this, you'll need a 7.5 min quad - any will do - a calculator (just basic math functions will do fine), a 12 inch ruler graduated in 16ths, and a pencil with a good eraser.

 

First, every topo is divided into 9 grids of 2'30" on all sides. Converting 2'30" into seconds gives 150 seconds.

 

Now the topo that I'll be using is a Moyock Quad, and my PoI (position of interest) is Panther Landing on the Northwest River.

 

As I look at the map, I note that the PoI is in the first grid (bottom right). And, at the bottom right corner on the margin are intersecting lat/lon's. The latitude here is N36 30' 00", and the longitude, W76 07' 30".

 

Moving north along the margin 2'30" will be another number - 32' 30". From here, traverse west to the first tic mark, it'll look like this +, and lay out a faint east/west line connecting these points. From this tic mark now lay out a north/south line to the base to complete the grid.

 

Next, run two faint pencil lines through the PoI, one north/south, the other east/west.

On the ruler, mark a line at 9 3/8 inches (150/16).

To find latitude, place the ruler's zero mark at the base, and the 9 3/8 starting near or at the tic mark. Slide the ruler, and adjust until you get a perfect fit. Note where the PoI's east/west line crosses at the ruler. On my map, that intersection is a little over 7 5/8. which becomes 7(16) + 5(2) = 112 + 10 = 122 seconds. Adding 122 seconds to N3630'00" becomes N3632'02". Adjusting a bit for the wee extra past the 5/8 mark on the ruler, the latitude becomes with a bit of guessing about N3632'03.27"

 

To find Longitude, just repeat the above. The only differance is to use the vertical north/south line on the right side of the grid.....for the Moyock grid it's 5 3/8. Converted to seconds

5(16) + 3(2) = 80 + 6 = 86 seconds. Adding to W76 07' 30" the longitude is W76 08' 56".

 

Thus, our lat/long for Panther Landing is N3632'03.27" W76 08' 56". We'll be off a wee bit, but fairly close to where we can eyeball ourselves in when working with GPS.

 

Should you need to convert to Decimal degrees use - DD + (mm/60) + (ss/3600)(This message has been edited by le Voyageur)

[Twocubdad]

I'm guessing you are leading up to a discussion of why UTM is easier to use than long/lat?

[le Voyageur]

Nope...Just sharing a technique to pull lat/lons from topos. Always good to know other ways of navigating in the back country(This message has been edited by le Voyageur)

[qwazse]

Good 'nuff for government work. Keep in mind the further north, the closer the longitude lines will be. That "wee extra" can amount several seconds. Usually not a problem for orienteering courses. For some geocaches, 40 feet off can yield a "did not find". For settling land disputes or staying out of Iran, it won't hold up in court.

 

If you need to ensure that level of precision, there are plastic orienteering templates for various scales (including 1:2400) that include a degree longitude conversion symbol (it look's sort of like a curved funnel) for every degree lattitude. First place I'd look for them is an military surplus store.

 

Lacking a template, my approach: with a yard/meter stick or tape measure, I would take the measurment from the edge (the one with the lowest longitude) of the map to my PoI, divide it by the measurment between the map edges at that lattitude, multiply by 7.5', and add the product to the lowest longitude on the map. Repeat for lattitude.

 

What's nice about this method, is it works well if you are using a scaled down map that you downloaded from USGS.gov. And, it doesn't matter what measurment system your stick has marked on it (as long as the ticks are evenly spaced!).

 

Needless to say, the finer your stick, the more accurate you will be. Obviously, because keeping things "square" can be a hassle, you can grid things out at the 2.5' hashes, I just don't like marking maps. I do write my calculations in the margin. This includes a couple of ticks representing my estimated error of measurement. (E.g. 1/8 inch, 1/16th inch, 1 millimeter.) It's really handy to know how far off you can be, when you hit that mark with the GPS.

 

P.S. (from the school of hard knocks) - Once you write your calcs, have your buddy check them for errors!(This message has been edited by qwazse)