Help with understanding coordinate formats

[+traildad]

I have been emailing the Mendocino National Forest about the coordinates they posted on their web site. First they had them listed as Longitude 123 which was not correct. They corrected that. I suggested that not using the W or the minus sign for Longitude coordinates could be confusing to beginners. They said it is in the US so everyone should know that it is supposed to be a W or a minus.

http://www.fs.fed.us/r5/mendocino/recreati...mping/se/#cedar

I will use this listing as an example. They posted the coordinates in this format.

Lat - 39° 15' 51.5592", Long - 122° 41' 55.086"

My MapSend Topo program says the camp is located at N39°15.863' W122°41.940'. http://boulter.com/gps/ translates them as N39 15 51, W122 41 56. This seems pretty close to what the NF shows except the numbers after the decimal point. When I told them that the coordinates didn't seem to fit any format that I had seen, they replied

Ken,

There are several different types of datum utilized in mapping within the US in conjunction with many different types of projections. Each of these carries their own standards but the common mapping datum for "most" federal land agencies is the NAD27 (Western US) which is what most of the Forest Service maps are compiled in or the NAD83 (CONUS). The projections seem to be what you are discussing and these can be very confusing for even an experienced GIS person as the GPS companies are not standardized on which projection their units utilize. UTM seems to be in the forefront for becoming the standard these days.

 

Since they are on the web site to help people to find the campgrounds I am wondering if these are in a usable format. Do these coordinates work in regular GPSr's or does it require a higher end device? I was trying to suggest to them that maybe this is not the best format to use if the intent is to help the general public. They seem to think everything is fine. Any opinions?

[+IndyJpr]

Their format is fine - not sure what you think is incorrect? They are using deg min sec - is it the decimal seconds that are throwing you off? If so, just round the sec to:

 

39 15 52 and 122 41 55

 

Your program is using deg min - the seconds are the decimal part of the min. I don't know that software but most will let you pick which format you want the coordinates displayed in. The decimal difference between your converted program numbers and their numbers could be due to accuracy of the original measurement, rounding, or a datum mismatch.

 

The coordinates can be expressed as:

39.2643		   -122.6986		 (dd.dd)
39 15.8593		-122 41.9181	  (dd mm.mm)
39 15 51.5592	 -122 41 55.086	(dd mm ss.ss)

 

All of these represent that same point and are considered "normal"... Assuming the negative for US longitudes is not standard but I see it quite often.

 

Hope that helps,

dfg

[Suscrofa]

Same as: 3 yards 1 feet 7.045 inches. You could give that all in yards and decimal yards or feet decimal inches or decimal inches or fractional feet and fractional inches or or what this is crazy isn't !!!

Why not invent something easy, decimal based with only one unit

 

Some reading: http://www.dbartlett.com/

Edited May 25, 2007 by Suscrofa

[+traildad]

Their format is fine - not sure what you think is incorrect? They are using deg min sec - is it the decimal seconds that are throwing you off? If so, just round the sec to:

 

39 15 52 and 122 41 55

 

I had entered the coordinates into a web page that converts them to several formats. http://boulter.com/gps/

I got locations that were in error when I included the part after the decimal point. When I put them into MapSend Topo 3d I get a location that is in error. There is no option on my GPSr to use this format. Microsoft Streets and Trips does seem to handle them correctly though. I wonder how much position change there is by adding the fraction. After all they are only marking a campground.

[andylphoto]

 

I had entered the coordinates into a web page that converts them to several formats. http://boulter.com/gps/

I got locations that were in error when I included the part after the decimal point. When I put them into MapSend Topo 3d I get a location that is in error. There is no option on my GPSr to use this format. Microsoft Streets and Trips does seem to handle them correctly though. I wonder how much position change there is by adding the fraction. After all they are only marking a campground.

 

It looks like you're all on the same page--just that they're using DD MM SS format for their coordinates, while you're using DD MM.MMM. You should be able to set your mapsend program (I assume) to use DMS if desired for entering coordinates.

 

Your GPSr should also have the option to use the DD MM SS format. How many decimal places, if any, your particular unit or an online calculator can support is the only thing in question. As IndyJpr suggested, if can't use decimal places, just round to the nearest second. If my calculations are correct, the distance of a whole second on average would be only around 80-100 feet. So even an error of a whole second (your biggest error would be about a half second) would still get you close enough that you'd be able to find the campground.

[+apersson850]

Why not invent something easy, decimal based with only one unit

That does exit. It's called "meter".

[+Cardinal Red]

This is one coordinate copied directly from the Mendocino web site:

 

Plaskett Meadows

Lat - 39° 43' 40.97317", Long - 122° 50' 42.62681"

 

First, I can't believe they are publishing seconds coords to 5 decimal places.

.1 (1/10th) second of Latitude = 10 feet

.01 (1/100) second of Latitude = 1 foot

.001 (1/100) second of Latitude = 1.2 inches

.0001 (1/1000) second of Latitude = .12 inches

.00001 (1/10000) second of Latitude = .012 inches

So we're looking at about 1/83rd of an inch of Latitude. Right.

 

But after that incredible feat of accuracy, notice that it shows MINUS 39°.

And they said about Longitude, "it is in the US so everyone should know that it is supposed to be a W or a minus"?. Then I guess they should know that minus 39 (SOUTH Latitude) is in the Pacific Ocean somewhere between South America and Australia. I'd love to hear them explain away that 5467 mile error.

[andylphoto]

Actually, I figured (eventually) in looking at the page that it was intended all as positive with a dash as a separator, and the assumption that people could figure out that the Lat. was North and the Long. was West. I would think that if it was intended as a minus, there wouldn't be a space between the minus and the degrees.

 

But on the other hand, I would also think they might use a colon as a separator so it couldn't be confused with a minus sign.

 

My experience with government has been that there are some good employees who know their job and do it well, but many others who really couldn't find their way out of a paper bag--even with a GPS and coordinates in a clear and unmistakable format.

 

Edit to correct an inexcusable grammatical error.

Edited May 25, 2007 by andylphoto

[+Sputnik 57]

The reference in their reply to datums show that that know just enough to be dangerous. There are two distinct issues. One is format. GC.com uses dd mm.mmm instead of dd mm ss, but as noted above, it is easy to convert. Your GPSr will do if for you if you'd like. Change the format to dd mm ss, enter the coordinates, and then change back to dd mm.mmm. The unit will do the math.

 

Datums are entirely different. Especially with their laughable level of accuracy, you have to know what datum they are using. USGS survey maps mostly use the old North American 1927 datum (NAD27). GC.com uses World Geodetic Survey 1984 datum (WGS84). In many places in the US, these two can be 200 feet off or more.

 

All GPS manufacturer will let the user select the format and datum. AFAIK, they all use dd mm.mmm and WGS84 "out of the box."

[+traildad]

 

It looks like you're all on the same page--just that they're using DD MM SS format for their coordinates, while you're using DD MM.MMM. You should be able to set your mapsend program (I assume) to use DMS if desired for entering coordinates.

 

I had seen decimal degrees and the DD MM.MMM both using decimal fractions. I had not seen DD MM SS using the decimal fractions and I thought maybe they had mixed two different formats together.

 

First, I can't believe they are publishing seconds coords to 5 decimal places.

.1 (1/10th) second of Latitude = 10 feet

.01 (1/100) second of Latitude = 1 foot

.001 (1/100) second of Latitude = 1.2 inches

.0001 (1/1000) second of Latitude = .12 inches

.00001 (1/10000) second of Latitude = .012 inches

So we're looking at about 1/83rd of an inch of Latitude. Right.

This just added to my assumption that it was a bad format. Why would anyone need to locate a campground down to 1/83rd of an inch.

Now that I know what format they are using it is simple to understand rounding down. I was trying to convince them to use a format that is common so beginners will not be confused. I may try one more time but his last response did not leave me hopeful.

[+apersson850]

All Garmin units that allows the user to enter coordinates at all (not all automotive units do), allow lat/long as

hddd.ddddd

hddd mm.mmm

hddd mm ss.s

So fractions of seconds are used there too, although only one decimal digit.

[+NightPilot]

Just because they use ' and " doesn't mean it's really degrees and seconds. It could easily be decimal minutes, poorly and incorrectly formatted. Just because there is a website, you can't be sure anyone connected with it has any clue about anything. The only requirement for a website is a minimal amount of money.