Averaging

[+Team Hobbs 1]

Maybe somebody could help me understand "averaging". I have a Magellan Sport Trak Pro, and I believe it does have the averaging function, but I am not sure how to do it, or even it's purpose...

 

Anybody?

[+CompuCash]

the ST pro (same one I have) does averaging when you are standing still. This is likely the function you are talking about

 

There is a manual way of averging where you take several reading over time and/or by walking away and approaching from a different direction and making the waypoint - then averaging the reading manually with standard mean math.

[+briansnat]

Your GPS is not 100 percent accurate. Even while standing still you will notice the coordinates changing. What averaging does is take a number of readings over a period of time and average them to arrive at what hopefully is the most accurate reading. In practice it works some of the time.

 

To be totally accurate, readings should be averaged over a period of days. In the real world the value of averaging is debatable. With a unit that does auto averaging, its not a major production to take an average when hiding a cache, so if you feel better if you average, great. But to average with a unit that doesn't have the auto averaging feature, it can take some time and really isn't worth the effort. First, under some conditions averaging can make your reading worse and even under perfect conditions where averaging can improve the accuracy of the coordinates, is it really worth the effort to make your coordinates more accurate by a few feet?

[+Team Hobbs 1]

The sudden clarity is dazzeling, thank you for your help!

[+blindleader]

It’s been many years since I took probability and Statistics for engineers, but the basics should never leak out.

 

The short version of the answer about averaging is that auto averaging as used by our consumer units is pretty much a waste of time. If your location is changing a bit over a few minutes of averaging while standing still, that’s just an indication that a significant number of the averaged fixes are bad.

 

In order for any average to be representative of a population (all possible good satellite readings) you need at a minimum 10 samples (the more the better) and they must be truly randomly selected from the population. The only way to meet those to criteria is to average over about a six hour period and throw out all readings with a calculated DOP (dilution of position) worse than some number (GDOP less than 6, maybe). If you want to take the six hours, more power to you. But if you aren’t using a surveyor’s GPS equipment and you aren’t writing down the fixes manually and recording the received satellites for each (must do), then you can’t know which fixes to throw out. That means you’re averaging an unknown number of bad fixes with an unknown number of good fixes.

 

The best you can do on the spot is to pick a time when satellite geometry is best, go to the site and record one fix (or a dozen if that makes you feel better) when your EPE reaches a minimum. This is more than most cache owners do and is plenty good enough for geocaching For my Meridian there is the added caveat that certain satellite configurations that are obviously crappy (lots of satellites but all strung across the sky close to a single line) will result in a low calculated EPE and astoundingly bad fixes.

[+Thot]

The best you can do on the spot is to pick a time when satellite geometry is best, . . .

Is there a way (such as a website) to tell when the sacred orbs are in the best alignment (It is the age of aquarius, uh querr re us)

[+blindleader]

Is there a way (such as a website) to tell when the sacred orbs are in the best alignment

Verily there is a program, grasshopper. - Leica Geosystems.

 

There are others but this one is simple, if a bit clumsy (legacy from the days of DOS).

 

You need to check and update the almanac every week or so at the same URL. Use GDOP not PDOP. This program calculates PDOP in some way (wrong) that never matches any other PDOP I've seen.

 

Here's a site where you can get the data for one particular location and time.

[team_tar]

The best you can do on the spot is to pick a time when satellite geometry is best, . . .

Is there a way (such as a website) to tell when the sacred orbs are in the best alignment (It is the age of aquarius, uh querr re us)

Yes, there is

On the Trimble site you can download their

Planning Software that, together with the last ephemeris file (you can download that too from the same page) is going to tell you when in a given place the diluition of precision is lower, and your measuerments better

Is a pretty powerful software, and is free!

 

Ciao

 

Acaro of Team TAR

[+Thot]

This game is like checkers. It’s very easy to learn the basic moves and start playing, but there’s a lot to learn after that.

[CoyoteRed]

All thoeries aside. I have real-world experience with a STPro.

 

I picked this up from another regular here in the forums and it works very well for me.

 

Find your spot and put your unit down so it is near verticle with a good view of the sky. After 5 minutes move it just enough to break averaging and put in back in the same spot. With WAAS, decent bird geometry, and a clear sky in about 15 minutes you should have .001" repeatability. In the woods, wait a while longer.

 

Sometimes you will have poor satellite geometry, but they are moving fairly fast. Give it an hour and the geometry will have changed completely.

 

I've tested the above in my backyard and have gotten .001" (that's minutes, not inches) repeatability. (Note, that's repeatability, not accurancy which is really something different, because I don't know my fence post is at N33° 01.792 W080° 07.688. I only know that each I check with the above method I get pretty much the same numbers.)

 

Hope this helps.

[+crash331]

Sometimes you will have poor satellite geometry, but they are moving fairly fast.  Give it an hour and the geometry will have changed completely.

I thought the locations of the satellites never changed (geosynchronous).

Edited September 10, 2004 by crash331

[CoyoteRed]

The WAAS satellites are relatively stationary. (Most, if not all geostationary satellites make a small figure 8 in the sky. You can't tell though unless you're tracking it with a large high gain dish. I'm a former satellite communications tech.)

 

The main satellites from which your unit gets the primary data are constantly moving across the sky.

[+WeightMan]

Sometimes you will have poor satellite geometry, but they are moving fairly fast.  Give it an hour and the geometry will have changed completely.

I thought the locations of the satellites never changed (geosynchronous).

If you stop and think about it for a few minutes you will understand why the satellites should not be in geosynchronous orbit. If they were then we would always have a straight line of sats and for those at latitudes much removed from the equator would have that line of sats near the horizon. To get a good reading you need to have a constellation of sats scattered around the sky. I don't recall which is which, but between location and altitude, one needs the sats well scattered and the other would like them in a line. I may have that screwed up, but that seems to be what I remember from forum posts some time ago.

[+TotemLake]

To expand on WeightMan's comments...

 

The wider the geometry of the satellites, with a scatter shot appearance being preferred over a straightline, the better the receiver can calculate your fix.

 

Having satellites orbit rather than being geostationary also means fewer satellites are needed to do the job effectively.

 

As CoyoteRed pointed out, even the geostationary are moving. All satellites have to fall fast enough to beat the effects of gravity enough to stay up.

[+Sputnik 57]

I am sometimes amazed at how long a thread can go before someone Markwells it (this time, to Markwell's own site, which, If I'm not mistaken, is Markwell squared!)

[+blindleader]

I am sometimes amazed at how long a thread can go before someone Markwells it (this time, to Markwell's own site, which, If I'm not mistaken, is Markwell squared!)

Of course bogus information Markwelled is still just bogus information.

eg:

Therefore the longer the unit sits in one spot, the more accurate the latitude and longitude are. So when you're hiding a cache, just leave your unit on the cache for about 10 minutes and you're pretty well set.