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gaffman
10-07-2005, 09:59 AM
I could use a bit of assistance. I have a longitude of 71 23 499 W. Can someone tell me how far away (in feet) the longitude of 71 23 498 is? Thank you.

phiil
10-07-2005, 10:05 AM
About six feet.
At the equator, that is. Nothing at all at the poles.

[ 10-07-2005, 10:07 AM: Message edited by: phiil ]

P.I. Stazzer-Newt
10-07-2005, 10:05 AM
Not without the lat.

6 feet times the cosine of the latitute.

Assuming that the last digit of the position is accurate - BIG ASSUMPTION.

[ 10-07-2005, 10:16 AM: Message edited by: P.I. Stazzer-Newt ]

Billy Bones
10-07-2005, 10:22 AM
I <strike>figgered</strike> guessed about 8 feet at a guess of N35deg. Too lazy to get a map. So much for my navigation.

You can figure it out yourself at this website (with lots of other neat calculators as well)...

http://deepcreekyachtclub.com/WebPage/calc.html

Hope you didn't just run aground!

[ 10-07-2005, 10:35 AM: Message edited by: Billy Bones ]

Bruce Hooke
10-07-2005, 10:31 AM
Assuming the latitude is the same for both locations, as P.I. noted, you really need more digits to get a very meaningful value. If I am thinking about this correctly, and if 6' is right, then I think we should actually be saying 6' plus or minus 6'. By way of example, 499 could, in fact, be anywhere between 4985 and 4994 and 498 could be anywhere between 4975 and 4984, which means the actual difference could be anwhere between 0001 and 0019...

gaffman
10-07-2005, 10:45 AM
Thank you for your replies. After looking at the calculator site, which is interesting, I suspect everyone is correct including Bruce who makes sense.
Thankfully I am not wearing my shirt that says "you haven't been around if you haven't been aground". I made the shirt after I went aground some years ago and now leave it in the boat for the next time I find bottom.
I'm working through a mooring placement issue.
thanks again.

Bruce Hooke
10-07-2005, 03:10 PM
This brings up another point, which is what you are using to measure these locations with. Using the GPS units I'm familiar with it is pretty questionable whether you could even measure a distance of 6' with enough accuracy to be at all meaningful. From my experience I can stand on exactly the same spot on two subsequent days and hit the waypoint button and get two waypoints that are commonly 15' apart according to the GPS.

Billy Bones
10-07-2005, 05:57 PM
Well, it can be done. Through my wife's work we took on a mapping project with NOAA which required a benchmarked location from which to broadcast a differential beacon. Not having a benchmark handy on our island open to aircraft overflights, they put one in for us---in our front lawn. The process involved a fancy antenna and a logging GPS unit which tracked the usually 22 satellites in the sky at any given moment. Three logging sessions of 6 hours each (logging continuously) yielded data which, after running through a program to average the results and correct for known problems yielded a point which they called "B-order accuracy" which meant +/- 2cm in each of three axes.

But that's a lot of logging.

Interestingly, they told us not to expect that point to show up on most maps, which aren't accurate to anywhere near that degree. They warned us not to be surprised if that point was in the ocean on many maps.

Hey. Waterfront property. Cool.

Bruce Hooke
10-07-2005, 06:40 PM
Wow! That's cool to hear how it's done when you need to REALLY know where something is located! I have had heard that there were ways to get that sort of accuracy because I know scientists who use GPS to measure quite small movements in things like glaciers...

Wild Wassa
10-07-2005, 08:28 PM
Having recently gone through a process of learning how to transfer GPS co-ordinates to maps accurately; recordings taken, need to be compatible to the map geocentric datum, for the results to be accurate. Even then allow an error of 20m when transferring data to a map.

In Oz we have three datum still in use (as many maps published are still behind the times).

The particular map datum that the map was writen in, will be on the map, as the orientation of the co-ordinates are not in the same orientation as the grid lines on the map. I couldn't figure why my readings were continually 5-6 seconds S out (about 190m), until I rang NSW Land and Property Information and they put me straight. LPI have an excellent phone-in service, for GPS users over here.

Warren.

[ 10-07-2005, 08:45 PM: Message edited by: Wild Wassa ]