Longitude & Latitude Basics

[roadtripper]

Hey Everyone,

I’m struggling a bit trying to apply the basics of longitude & latitude to New England and I have a couple questions:

(1) Does anybody have a favorite way to determine longitude & latitude, besides for GPS? I’ve been using Google Maps (which works OK by selecting a point and hitting “directions from here”, which then shows you the coordinates for that point. I’ve also tried the 2006 version of Microsoft Streets & Trips (which has a great locator feature that does long/lat), but this program (at least this edition) is missing far too many natural features when compared to Google Maps (i.e. rivers, streams, etc.).
(2) I realize there are minor differences based on where you are in New England, but can anybody approximate the difference in feet for:
a. Latitude: a change in the 2nd decimal point (i.e. 42.11 vs. 42.12)
b. Latitude: a change in the 3rd decimal point (i.e. 42.114 vs. 42.115)
c. Latitude: a change in the 4th decimal point (i.e. 42.1142 vs. 42.1143)
d. Longitude: a change in the 2nd decimal point (i.e. 73.44 vs. 73.45)
e. Longitude: a change in the 3rd decimal point (i.e. 73.446 vs. 73.447)
f. Longitude: a change in the 4th decimal point (i.e. 73.4463 vs. 73.4464)
(3) Based on websites & books you’ve seen, what appears to be the most common way to display long/lat? So far, I like the decimal system with four places, but do you recommend three or five places instead for trail features (summits, waterfalls, etc.)? (i.e. 42.114 versus 41.1138 vs. 41.11376)

I think I may need to watch some sort of basic online tutorial on this stuff, so if anybody has any recommendations for that, I'll take that as well.

Thanks!

 

[RoySwkr]

(1) Does anybody have a favorite way to determine longitude & latitude, besides for GPS?
(3) Based on websites & books you’ve seen, what appears to be the most common way to display long/lat? So far, I like the decimal system with four places, but do you recommend three or five places instead for trail features (summits, waterfalls, etc.)? (i.e. 42.114 versus 41.1138 vs. 41.11376)

Acme Mapper (mapper.acme.com) lets you double-click on a point from Google map, topo map, or satellite photo to get coordinate in a number of formats

Geocaching mostly uses degrees and decimal minutes DD MM.MMM, not sure why

GNIS database uses degrees/minutes/seconds

Army uses UTM which is easiest to use if your map has a UTM grid WITH THE SAME DATUM YOUR POINT IS GIVEN IN, note that older USGS maps do not use the WGS84 datum which most GPS points are in

 

[Stan]

I highly recommend a navigation or map and compass course. Learning to navigate without GPS can help you utilize, and appreciate, the powers of GPS and may assure your survival when it inevitably fails you at the most inopportune time.

Learning a few GPS procedures by rote is no substitute to understanding map and compass.

You can do the math but one minute of latitude is one nautical mile, anywhere on earth. A nautical mile is roughly 2000 yards ... close enough to keep a sailor off the rocks and sink submarines ... or 6076 ft. A mile on our trail maps is 5,280 ft.

(For Doug Paul and other purists, there is actually a slight variation in a nautical mile between equator and poles, the planet being less than a perfect sphere. The difference is roughly + and - 30 ft. in each direction.)

There is no easy correlation between longitude and miles or feet. Longitude lines converge at the poles so a degree of longitude at the equator is a far greater distance than it is at the poles. I'm sure there are easier and more accurate ways to estimate distance than messing with rules of thumb on longitude at a given latitude.

Sorry if I sound like a navigation snob but I'd become so accustomed to old fashioned means of estimating position and courses that when lent a GPS I found it a bit of a distraction from my enjoyment and oneness with the more basic and primitive aspects of the wilds.

 

[RoySwkr]

There is no easy correlation between longitude and miles or feet. Longitude lines converge at the poles so a degree of longitude at the equator is a far greater distance than it is at the poles.

If you have a paper USGS map, the ratio between Long & Lat is the same as the aspect ratio for the map, say 2:3 for NH as rough approx. Measure the map if you want it closer

 

[DougPaul]

Army uses UTM which is easiest to use if your map has a UTM grid WITH THE SAME DATUM YOUR POINT IS GIVEN IN, note that older USGS maps do not use the WGS84 datum which most GPS points are in

Just want to add that matching the datum is essential whether you use UTM or lat-lon cordinates. The most common datums of interest to US hikers are NAD27 and WGS84. (Both are used on USGS topos.) The difference in the White Mtns is ~230 meters. GPSes use WGS84 internally but can be set to input and output any of a wide variety of datums.


All formats of lat and lon are used, it just depends upon the convention chosen by the group (or whatever you prefer if you are working alone). Digital mapping programs and GPSes can generally be set to any of the standard formats: ddd.dddd, ddd mm.mmm, ddd mm ss.ss. (The number of figures to the right of the decimal point can vary.)

One minute of latitude (N-S) = 1 nautical mi = 6076 ft
One minute of longitude (E-W) = 1 * cosine(lat) nautical mi

For North Woodstock, NH (44.030 deg N), cos(44.03 deg) = .719

Doug

 

[sardog1]

Hoo boy, a chance to pitch UTM. OK, chillens, lissen up. Determining distance with lat/lon is a pain in the posterior and never worth the effort for a ground-based navigator. Accurately determining distance with UTM is a snap -- anyone with an overlay and/or a calculator can do it in no time. (Your USGS topo sheets most likely have the UTM grid marks, as will many other topo maps these days.)

Go to MapTools.com "Using UTM/MGRS Map Coordinate System." (No, I'm not involved and I'm not getting any remuneration.) Read the stuff there, as much of it as you need or find useful for your own circumstances. Then either buy a UTM grid overlay from the site provider, or use his all-too-altruistic Free Tools.

 

[AndyF]

Google Earth is an easy way to get latitude and longitude values. Just move your cursor around and they are displayed in the status bar.

 

[SAR-EMT40]

Army uses UTM which is easiest to use if your map has a UTM grid WITH THE SAME DATUM YOUR POINT IS GIVEN IN, note that older USGS maps do not use the WGS84 datum which most GPS points are in

Just a small correction I'd make. The Army actually uses MGRS (Military Grid Reference Sytem) which is identical to the UTM between 80deg S and 85deg N except it uses a different labeling system. Both systems have the ability to change the length of the coordinate representation depending on the resolution that is needed i.e. 1000 meter grid square or 100 resolution or 10 meter resolution even 1 meter resolution can be represented but probably has little meaning and I have never seen used. This is very handy.

I couldn't agree more with SARdog1. The Military has been using MGRS for many, at least 45 years that I am aware of. MGRS, UTM, USNG are all based on meters or kilometers if you prefer. Use of them couldn't be easier. UTM is the civilian equivalent of MGRS and unless you say things like avast ye mates and Arrrhhh or you fly around then the coordinate system for you is UTM.

There is also the USNG (United States National Grid) which is being used by federal/state/local agencies to geocode most things now. Again, this is very similar to UTM.

Regards,
Keith

 

[Nessmuk]

Sorry if I sound like a navigation snob but I'd become so accustomed to old fashioned means of estimating position and courses that when lent a GPS I found it a bit of a distraction from my enjoyment and oneness with the more basic and primitive aspects of the wilds.

My feelings exactly, Stan. While certainly a handy and sometimes necessary "tool" to get a particular job done (SAR for example), I feel the GPS becomes an unnecessary distraction to my backcountry navigation enjoyment of practiced simpler methods that get me all the backcountry precision I need to find any place on the map I choose in this part of the world.

On the topic of a grid system, in the old days of SAC when flying over the polar regions we'd "go into grid" to avoid the problem of unreliable magnetic compass operation and rapidly changing compass headings due to both changing declination and longitude convergence. It was an inertial compass based system, which maintained accuracy with frequent checks using a sextant and star shots by the navigator to correct for gyro precession. I practiced many training missions using the USAF Grid system over the lower 48 as well as up north and other parts of the world. But it could not have been the same as UTM grid because to enter and exit I recall turning the compass dial by many tens of degrees from true or magnetic.

 

[sardog1]

Since I've helped contribute to thread drift here just a little, I decided to make recompense by starting a new VFTT social group dedicated to backcountry navigation: Pathfinders. All are welcome.

 

[nartreb]

Strange discussion. Why would you need to estimate distance based on latitude & longitude in the field? Your topo map is to scale, and your compass has a ruler along the edge.

 

[sardog1]

Lots of maps do not use the scales that are available on some compasses (a notorious example being the AMC trail maps.) Some of these do not have a scale bar, or a usable scale bar, as well. Without a calculator, it's a little tough to do the math conversions in the field from the actual map scale to the edge of the compass. (With a customized grid overlay or ruler from MapTools, it's very easy.)

 

[Nessmuk]

Strange discussion. Why would you need to estimate distance based on latitude & longitude in the field? Your topo map is to scale, and your compass has a ruler along the edge.

If you understand your map and have done some study before heading out, then it doesn't (shouldn't) take very long to become comfortable with estimating distances on a map once you get the scale in your head. I can hold up my index finger and thumb to pretty accurately measure out a mile. Any fraction thereof is just as easy. When using newer topos with UTM grid overlay there's no excuse for not using the km squares as a reference. Experience and understanding my travel speed over the observed terrain converts distance to time. Accuracy greater than I get from that in the field is generally unnecessary unless you are calling in a military strike force.

 

[nartreb]

Calculator? The AMC trail maps currently advertised on their website are 1:95,000 scale. In other words, if you measure in cm, you get the number of km (with 5% error, which is less than I commonly introduce when trying to measure a map over my knee). Equivalently, 1 inch represents 1.5 mi, so measure in inches and add half to get miles. No calculator needed, whichever units you prefer.
But I wouldn't use AMC trail maps for off-trail navigation (you get about five times more detail from USGS topo maps), and if I'm hiking on trail, the trail distances (in miles) are already labeled.

 

[sardog1]

Calculator? The AMC trail maps currently advertised on their website are 1:95,000 scale. In other words, if you measure in cm, you get the number of km (with 5% error, which is less than I commonly introduce when trying to measure a map over my knee). Equivalently, 1 inch represents 1.5 mi, so measure in inches and add half to get miles. No calculator needed, whichever units you prefer.
But I wouldn't use AMC trail maps for off-trail navigation (you get about five times more detail from USGS topo maps), and if I'm hiking on trail, the trail distances (in miles) are already labeled.

The AMC's Map 1 of the Presidential Range is at 1:47,500. Its inset of the Mt. Washington summit area is at 1:24,000. On the reverse side, Map 2 of Franconia/Pemi is at 1:95,000.

(I do use both the AMC maps and the USGS topo maps for bushwhacking, the former for the trail details and the latter for the finer landscape details.)

 

[Stan]

Lots of maps do not use the scales that are available on some compasses (a notorious example being the AMC trail maps.) Some of these do not have a scale bar, or a usable scale bar, as well. Without a calculator, it's a little tough to do the math conversions in the field from the actual map scale to the edge of the compass. (With a customized grid overlay or ruler from MapTools, it's very easy.)

I find that if I keep my fingers really rigid I can come within 1/4 of a fingernail accuracy ... width, not thickness ... and, so far, that has been close enough for most hikes. I don't try that at home, however, at which time I use dividers and really get down to the nitty gritty, which roughly approximates 1/4 of a fingernail. Of course, you should keep your paws pruned for best accuracy and, speaking of dogs, if worse comes to worse and you get lost in Maine, they have some wonderful tracking dogs.

 

[cushetunk]

Well, if we're all going to one-up each other on our inaccuracy, I just want to say now that I usually just use a binary system. Distances are either:

"I think I can walk that" or "Probably can't walk it"

One plus of this system is that you don't need to remember the various scales of the AMC maps.

 

[RoySwkr]

Why would you need to estimate distance based on latitude & longitude in the field? Your topo map is to scale, and your compass has a ruler along the edge.

If you know only the coordinates of a point and don't know where it is on the map

For instance on a summit in ME I once found a note that a good view could be had at XXX YYY (UTM). I didn't have a GPS and I didn't know the coordinates of where I was so I didn't get to see it. If I'd known the UTM coordinates where I was, it would have been trivial to go to that point even with no map.