Another Lat/Lon question

[Diver1111]

What is the distance between two objects that have these numbers for the 2nd, west set of coordinates? They are real numbers I need help with.

XX YY.319

XX YY.321

XX YY.359

I need a handle on the increments that the last set of 3 digits represents-a helpful answer would tell me how far one foot is, using the decimal system above, focusing on the last digit set of 3.

For example how far is from 319 to 321? Miniscule? 6 feet? 30 feet?

The answer will be useful to me as I sometimes pick up apparently new objects on side-scan in an area I have been before. I have so many that I now check my spreadsheet of finds and cross check Lat/Lon on the side-scan to my sheet right on the boat. If they are for the most part the same numbers I don't gear up and dive it as I assume it's the same thing but targets are getting so close together now I need to determine exactly how far apart a known target is from what may be an unknown one.

If the water was super clear and I could see 40 feet or more this wouldn't be nearly so important but not only is there very "dark" water in Winni (parts of Alton, Lees Mills, Greens etc.) even when it's not dark water, the deeper you go of course in "clear" water the lower the visibility. Last year I gave some trusted divers the Lat/Lon of a nifty wreck and they landed only 15 feet from it-and almost couldn't see it at all because of the severely lowered viz..

Thank you all.

[Rattlesnake Guy]

Diver.

Like most of us, the earth is a bit larger around the waist. (25 miles in diameter)

Each Degree is about 69.1 miles.

Each 00.001 of a Minute is about 0.001154 miles
So each 00 00.001 is about 6.091 feet.

Example 1:
XX YY.234 ----> XX YY.235
If your boat is at the first point, the second point is due North 6 feet.




Example 2:
XX YY.123 ------> XX YY.023
If your boat is at the first point, the second point is at a point...
609 feet South

That was fun.

After reading Slickcraft's response, I realized that I answered the wrong question. My answer is only valid for North South direction.

[Slickcraft]

I assume you provided only the west longitude change data in degrees, decimal minutes.

I picked an arbitrary point
Point 1
43 deg 32.583 min N
-71 deg 15.319 min W

And then assumed westerly movement to:
Point 2
43 32.583
-71 15.321

And then more west to:
Point 3
43 32.583
-71 15.359

Using the coordinate transformation to get the UTM easterly:
http://www.rcn.montana.edu/resources...ordinates.aspx

The easterlies are:
Point 1: 317,798 meters
Point 2: 317,795 meters or 3 meters west of point 1
Point 3: 317,774 meters or 24 meters west of point 1

Again assuming no keyboard errors on my part and remembering that the conversion to UTM is rounded to nearest meter.

Of course the distance per minute of longitude is a function of how far north of the equator you are:
http://www.hypernews.org/HyperNews/g...SAR/291/1.html

[NoBozo]

All this stuff may work out on paper. It's fun to do the math. The problem is.. Non-Military GPS is not as accurate as you would like to think. To prove this...Go to the waypoint of your choice and "MARK" the position on your GPS.

Now: Go back tomorrow to the same waypoint you "marked" yesterday and see if the multiple digits match up. Civilian GPS Repeatability is maybe good for +/- 100 feet. ..Maybe.

GPS is a GIFT from the military. They still regulate the results on your civilain GPS unit. Just Sayin... NB

[Rattlesnake Guy]

I found this on the Internet so it must be true. I thought I remembered that the military no longer took away the accuracy of GPS. My son works summers for a survey company who use GPS to achieve 1/2 inch accuracy with their equipment.


copied from : http://www.streetdirectory.com/trave..._tracking.html

GPS tracking is one of the most interesting technological advances of our time. GPS stands for Global Positioning System, and is used just for that. GPS can be used to find anyone with a tracker anywhere in the world. This is because the whole system makes use of the satellites that occupy the space above the earth. With special signals, and a little triangulation, GPS tracking can help anyone figure out where they are and plan out where they should go next.

GPS tracking began as a project especially for the United States military. In the 1980s, the system was being developed so that the military would have better capabilities on the ground to keep on target. It could also be used to help find lost soldiers and to more quickly locate the wounded. GPS provided a powerful tool for the United States and its allies. And it is still used by the military today.

Even though civilians could use GPS tracking in the late 1980s and 1990s, it was not the same system that the military used. The GPS used by civilians was less accurate and quick. The military scrambled its signals so that only it had access to the highly precise GPS calculations used in operations. This is why GPS was slow to catch on among “regular" folks. However, that all changed in 2000.

In the year 2000, at the turn of the century and the millennium, then-president Bill Clinton ordered the military to stop scrambling its GPS tracking signals. This did not mean that all of a sudden civilians could access battle plans and find out where troops were. What it did mean was that anyone with the right equipment (which is sold just about everywhere and fairly easy to get) could have access to the military’s system. This allowed anyone, individuals and businesses, to make use of the extremely accurate military system.

Since GPS tracking has been open to everyone, the marketplace has responded by making all sorts of consumer goods that allow people to find out where they are, and even track their kids and spouses. These is because the technology is so advanced that anything can be used for GPS tracking — even a cell phone.

[Island-Ho]

While it is true that the military no longer "dithers" or degrades the accuracy of the GPS signal available to civilians, the "P" or precision code, transmitted on a slightly different frequency from the consumer GPS system is still encrypted and not available to the general public. This code, in conjunction with the civilian code, provides the military users with much greater accuracy than is available to the common GPS units we are used to. That said, we often see accuracy approaching 20 feet!

[Rattlesnake Guy]

After more checking, I agree with your 20 foot statement for most consumer hardware.

[Diver1111]

The intentional error built in to the GPS system was called Selective Availability; I saw a massive difference in my navigation (woods, water etc.) when it was removed.

Thanks for your help all. My question has been answered and it's very helpful.