# smallest unit of measure on GPS coords?

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What is the smallest unit of measure in feet for a change of one unit on coords of a waypoint. example: .001 vs .002 ?? I think it is 6 feet...right or wrong?

"The more original a discovery,the more obvious it seems afterwards"

Although it can be approximated, technically you have to know where you are because the space between longitude lines vary as you move away from the equator.

Plus, it may depend on how many significant digits your GPS display....

The other answer is: I don't know.

DustyJacket

...If life was fair, a banana split would cure cancer.

Yes, it is roughly 6 feet for 0.001' of latitude, and about 4 feet for 0.001' of longitude at my latitude (N 47°).

An easy way to figure it for latitude is to take 25,000 miles as the circumference of the earth, then divide by 360 to get the distance in miles per degree. Divide that by 60,000 to get distance per 0.001', then multiply by 5280 to convert the units to feet. That comes out to about 6 feet.

For longitude, the math is a little trickier since the lines of longitude converge toward the poles. At the equator, you can use the above steps; at other latitudes, you need to involve some trigonometry. For these instances, you need to figure the circumference of the earth at that latitude using the cosine of the latitude. For my latitude, that circumference is 25,000 x cos(47°) = 25,000 x 0.68 = 17,050. Now, take that figure and use it instead of 25,000 in the equations above. The result comes out to about 4 feet.

Would COS(my latitude) X 6ft. give a useful approximation?

One UTM is one meter (3.2808... feet)

State Plane coordinates are commonly in feet.

No matter which system you use the smallest unit is well under the level of accuracy of your device so you end up with the old precision versus accuracy problem.

========================================

Friends don't let Friends geocache drunk.

quote:
Originally posted by happycycler:

Would COS(my latitude) X 6ft. give a useful approximation?

Yes. Once you know the value for one (either lat. or long.), it is easy to approximate the other. My equations above assume you are starting from scratch.

OK, Smarty-pants mount10biker,

How many angstroms would that be?

--majicman

My new book available now!: (http://www.mcwj.com )

First you must convert chains per hour to furlongs per fortnight...

========================================

Friends don't let Friends geocache drunk.

quote:
Originally posted by Moun10Bike:

But if you had it in meters from the beginning, you would simply multiply by 10^10. Which one can do a lot easier.

Anders

quote:
Originally posted by Dave54:

One UTM is one meter (3.2808... feet)

State Plane coordinates are commonly in feet.

No matter which system you use the smallest unit is well under the level of accuracy of your device so you end up with the old precision versus accuracy problem.

And that's the whole point as it really doesn't matter as whatever system is way way less than the "actual" accuracy the system provides.

Some of the units can now spit out 0.0001' but it really doesn't mean much unless put in context.

anyway that's 3.28084'

Cheers, Kerry.

I never get lost everybody keeps telling me where to go

quote:
Originally posted by Kerry:

Some of the units can now spit out 0.0001' but it really doesn't mean much unless put in context.

anyway that's 3.28084'

Cheers, Kerry.

I never get lost everybody keeps telling me where to go

Actually, .0001' of latitude would be 7.2912 inches

[This message was edited by Poindexter on February 14, 2003 at 04:15 AM.]

quote:
Originally posted by Dave54:

One UTM is one meter (3.2808... feet)

State Plane coordinates are commonly in feet.

That's not entirely true. Most - if not all - states have dumped their old state plane coordinate systems based on NAD27 and feet and started using new definitions based on NAD83 and meters. One advantage to this is that it's virtually impossible to confuse coordinates from one system with coordinates from the other, because they occupy different ranges of numbers (because of the "false northing" and "false easting" that are common in state plane systems.)

I was interested because, a friend and I got different coords on a cache he was placing. I wondered if, one of us was right, how many feet from being correct, was the other reading, if it was like .009 different. I will use 5 or 6 ft as my figure from now on, per .001. That's close enough to figure the error in feet, I think.So at that rate, a difference of .009 would be about 50 feet of difference.

"The more original a discovery,the more obvious it seems afterwards"

Poindexter,

" anyway that's 3.28084'" was in reply to this "One UTM is one meter (3.2808... feet)" but I might see how you interpreted things as referring to 0.0001'

However since you've raised the "length" of 0.0001' (latitude) now would 7.2912 inches be ellipsoid or plane distance and also sounds like around the 45 lat deg mark.

At the equator 0.0001' lat is "only" round-about 7.2552 inches (on the ellipsoid).

Cheers, Kerry.

I never get lost everybody keeps telling me where to go

Kerry: Ok, I should have looked a little closer and I would have seen that. Actually, I just thought that you had taken 1/2 of .001 (~6') instead of 1/10th. As far as what is .0001 I'm fine with "around 7 inches"

A short time ago I created a MS-Excel spreadsheet to compute the lat/lon of a third location from two known locations and the courses to the third location from each known location.

The MS-Excel spreadsheet can be used to determine how far a distance is between two waypoints. You can use the "Solver" function to determine a waypoint given a course and distance.

The spreadsheet can be copied and modified as desired... (It is virus and macro free.)

Enjoy!

paul_stratton

And to think that I once had trouble finding my own "@@@" with both hands...

[This message was edited by paul_stratton on February 16, 2003 at 07:35 PM.]

quote:
anyway that's 3.28084'

Cheers, Kerry.

I hope we are not going to get into a quibble over the fifth decimal place of meter, a distance smaller than a period at the end of a sentence in the geocache logbook.

You may have overlooked the ellipsis (...) at the end of the number, indicating it went on.

Yes, worrying about that level of precision is counterproductive. A freshly sharpened pencil has a point somewhere around 0.5 millimeters diameter. A line drawn with that pencil on a 1:24,000 scale map is approximately 40 feet wide on the ground. The paper map itself distorts more than that with changes in humidity or how it was stored in your home. If the map was printed on your home PC who knows what the scale is? Most home printers will distort 5% or more in one dimension than the other. Being concerned with this level of precision for recreational geocaching borders on anal retentiveness. If we were doing legal property line surveying this may be a valid discussion, but geocaching?

========================================

Friends don't let Friends geocache drunk.

quote:
Originally posted by Dave54:

Being concerned with this level of precision for recreational geocaching borders on anal retentiveness.

Aw, you're no fun.

quote:
Originally posted by Dave54:

I hope we are not going to get into a quibble over the fifth decimal place of meter, a distance smaller than a period at the end of a sentence in the geocache logbook.

You may have overlooked the ellipsis (...) at the end of the number, indicating it went on.

Yes, worrying about that level of precision is counterproductive .....

No, only start quibbling when it becomes critical and that's a little more then the fifth decimal place (on most days)

But if surveyors thought relative (in accuracy terms) as some do with the actual v perceived accuracy of recreational receivers then we'd really have to split hairs.

Ah, don't worry it wasn't a serious comment, I know where your coming from.

BUT how about we scrap those flintstone feet and just stick with metres , that's easy 1 metre = 1 metre (or the distance travelled by light in a vaccum in 1/299792458 of a second )

Cheers, Kerry.

I never get lost everybody keeps telling me where to go

or 3.2808398950131233595800524934383 feet

or 1650763.73 wavelengths of the radiation in vacuum of the transition between 2p10 and 5d5 levels of the atom of Krypton 86. (Official definition in the Handbook of Physics).

I looked those up, I don't have them memorized.

========================================

Friends don't let Friends geocache drunk.

quote:
Originally posted by Dave54:

.... or 1650763.73 wavelengths of the radiation in vacuum of the transition between 2p10 and 5d5 levels of the atom of Krypton 86. (Official definition in the Handbook of Physics).

Dave, the Krypton 86 one was superceded (replaced) in 1983 by the light in a vaccum one.

You'd think they'd make up their mind wouldn't ya

Cheers, Kerry.

I never get lost everybody keeps telling me where to go

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