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Position averaging how to question

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I'm 1 day away from placing my first cache. I visited the site last night to begin getting the coordinates. I set 11 waypoints that are up to 46 feet from each other. How should I set about averaging and listing the location of my cache? I have a GPSMap76.

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Guest napgravy

I would just stand at the location you want to place your cache. Mark a waypoint using your GPSMAP 76... BUT before you save it, hit menu and choose the "average" feature. I'd let it average for 5-10 minutes tops.

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Guest arffer

Okay, lets assume you've used the standard DD MM.mmm nominclature.

 

Assuming that you are not on a degree boundary (all your Lat DDs are the same, and all your Long DDs are the same), you will actually only be averaging the MM.mmm parts.

 

Look at your 11 waypoints, if possible, try and identify 1 or two that are WAY out of line, and discard them.

 

Now, add up the remaining MM.mmm of Lat, and divide by how many you added together (lets say you kept 9 of your original 11). That's your averaged Lat.

 

Now do the same for Long.

 

You're done. Reagrding napgravy's comment, if your GPSR has averaging, it should indeed be used when you took your 11 waypoints (IMHO, 2 minutes averaging is enough). But even with a GPSR that has averaging, I suggest taking multiple readings as you did. My method is to let my GPSR average for 2 minutes, note the results, turn it off, back on, average 2 minutes etc. I do this for 5 readings (5 two minute averages, each seperated with a power cycle of the GPSR). Then I average the 5 resulting waypoints using the above math.

 

Hope this helps!

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Guest Jors

Another way would be to plot all the points on a comfortable scale, so the outlyers can easily be identified, as was mentioned before, then choose a point in the middle of the densest grouping of points, and read off the co-ordinates. This can be done using only the last, lets say, two decimals of the minutes, meaning the decimals that changes.

Effectively this is a sort of weighted mean, or a graphical method for a least squares solution, which should give very good results.

 

[This message has been edited by Jors (edited 30 August 2001).]

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Guest arffer

I agree with Jors graphical means. But I couldn't think of a way to describe an easy way for you to do the first step, plot all the waypoints you recorded.

 

I personally use ExpertGPS to do this exact procedure, but was hesitant to answer your question by suggesting you buy a $59 program.

 

Some GPSRs have this exact function built in. They collect all the points, allow you to zoom way into the resulting fuzz, and view the co-ordinates of a point in the middle. Cool!

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Guest Pote

Thanks for the feedback. I had been scouring the ******* GPSMap76 manual for the averaging instructions and couldn't find it. Tonite I will take about 10 averages and plot those with the 11 I have already (non-averaged) and see where it puts me. I do use Mapsource and plotted the 11 I took the other day and 4 of the waypoints are on top of each other by about a foot or so, but I need to determine that those 4 are really where the stash is! :^)

 

Thanks for all the advice.

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Guest Geoffrey

I have a Garmin 3plus, but was thinking of getting the Magellan 315. If I averaged with a Magellan and a Garmin, then averaged those, then nobody should complain about a cache being way off.

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