# Accuracy Question...

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Hey everyone,

OK, question. Let's say that I've got a waypoint set, and I'm aiming for it.

With the coordinates format set as xx° xx.xxx', I believe that means that I've got a square approximately 6 foot (north and south) by 5 foot (east and west) I'm being aimed at. (I forget where I got the info from, but the 5'x6' is based on my lattitude of 33 degrees. If I'm wrong, please speak up.)

So, when I'm aiming to get to this 5'x6' box, is the GPS taking me to "anywhere" inside that box or is it aiming at the center of the box, and how does the accuracy of the GPS unit tie into all this?

To state the obvious, I know that the accuracy of the unit isn't going to be high enough to worry about where I am inside a 5'x6' box, but has anyone ever played with this to see how close it is?

Sorry if this is confusing, it is to me too!

AKA Old Bill

You are right about the 5x6 at 33°. You may have gotten that from Markwell's great site.

My analogy would be trying to put the point of a pencil on the intersecting lines of some graph paper. The pencil tip and the lines are about the same diameter. Put the pencil tip to paper, and then zoom in with a microscope until the lines appear to be 5 or 6 feet wide. There's a huge pencil point, 6 feet in diameter on the lines. The center of the pencil lead may be on the center of the lines, but it isn't likely. They likely overlap by less than half, but not be exactly lined up.

Reading back over this, then analogy is sort of lame, but hope it helps. There is only so much precision when dealing with blunt instruments.

Old Bill Posted on Jun 24 2005, 11:04 AM

To state the obvious, I know that the accuracy of the unit isn't going to be high enough to worry about where I am inside a 5'x6' box

Hi Bill (Thad) whatever. Starting from the premise you have already acknowledged above, this is a theoretical discussion that doesn't mean carp. But let's discuss it anyway.

Your rough estimate of .001 Minutes of Latitude = 6' anywhere on the Planet is VERY close. Good enough.

The Cosine of 33° = .83867

.83867 times 6' = 5.03202'

So your rough estimate that .001 Minutes of Longitude in Phoenix = 5' is also VERY close. Good enough.

For me in Pennsylvania at a Latitude of about 41° the .001 Minutes of Longitude spacing = about 4.5'

Cosine of 41° = .7547

.7547 times 6' = 4.5282' ( close enough to 4.5' ).

Use these examples for any Latitude anywhere, to determine your local Longitude spacing.

Yesterday I was alternating between 11 and 12 Satellites with an EPE of about 6.5 in an open space (parking lot). This is exceptional for Pennsylvania. I suspect much more common in Phoenix. One of the birds was the east coast WAAS #35.

If you have a lock close to this good, you can hope to view a stable location on the display. What part of the box are you in? You might not even be in that box. The error tolerance can easily have you 3 or 4 or 5 boxes off of ABSOLUTE accuracy.

That's all you ever hear. The inherent error of ABSOLUTE accuracy. But what about RELATIVE accuracy. The accuracy of one location RELATIVE to another during a short time window. That will be a much smaller, more manageable amount of error. If the Latitude is bouncing between .004 and .005, the indication is you are in the Southern half of the block. You'll never know for sure, but you do have an answer.

Walk North till you lock on to .006

Walk South till you lock on to .004

Stand in the center of those two points. Are you locked on to the center of .005

Possibly, probably not.

Change the GPS display to DD.ddddd

Your box dimensions are now only 60% of what they were.

In Phoenix about 3.5' by 3'

Your box is now about 3' by 3' (1 meter by 1 meter) anywhere on the Planet except the Polar Regions.

So just because this really doesn't telly you anything, you can get an answer if you want one. Just don't bet your life on it.

And don't bet on the Cardinals either.

OK, question. Let's say that I've got a waypoint set, and I'm aiming for it.

With the coordinates format set as xx° xx.xxx', I believe that means that I've got a square approximately 6 foot (north and south) by 5 foot (east and west) I'm being aimed at.

I don't think you have a box. You have an exact point, an exact location in the world for a particular set of coordinates. The "box" only exists when you extend the coordinates another 1/1000 of a minute to the next integer. While its' true that the instrument can't get you to that exact pooint because of its inherent inacuracy, mathematically, a coordinate set is an exact spot.

I'm not sure I totally understand which thing you are asking, but let me take it from a couple different direction.

Probably not what you are asking: The accuracy reading or EPE on your GPS Is only a programed guesstimate of how your unit is doing and varies from brand to brand and model to model. If you have an accuracy reading of say 12', there is a 50% chance that you are within 12' of the coords. Maybe.

Are you asking me?: I have tested my GPS a couple of times on top of a Suvey monument (a known position). Without WAAS on I wondered around about 10' to 15' off the mark as I recall, and with a WAAS fix about 0' - 7' from the mark.

Other than that, what Alan2 said.

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