Confused about coordinates

[Guest Buddie Holly]

I left my first cache last weekend, but spent at least an hour trying to figure out the coordinates to use for locating it. (I have a Magellan GPS315)

I stood at the location of the cache and made note of the coordinates, then SEVERAL times walked away far enough for them to change and then walked back to the original point. Problem is...each time I came back the last three numbers were different. For example, 1st it read 34*03.048N, then 34*03.42, then 34*03.44, then...41, then...34, etc. Will this make a difference when trying to find the cache?

 

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Go Indians!

>^..^< Ozzy&Buddie Holly >^..^<

[Guest peter]

It's not surprising to see some variation in the position if you repeatedly measure the same spot. A difference of 0.001 minute of latitude is about 6 feet, so some fluctuation in the last digit is normal, but the variation from 3.034' to 3.048' is a little high (I presume you meant that all your readings were N34 deg, 3.0xx').

The 315 goes into averaging mode when stationary, so the best thing to do is place it near the cache site and leave it alone for awhile to let it average many readings before you record the coordinates.

Some other GPSs, like my Garmin eMap, also let you average the position by selecting that as a menu choice. With GPSs that don't support averaging, like the eTrex, you can take multiple readings like you described above and then average them yourself.

Another factor is the satellite visibility from the site. Readings with only 3 satellites are very suspect in accuracy and it's best to have 5 or more spread out over most of the sky.

[Guest Buddie Holly]

es for others to find? My GPS does do averaging and this feature WAS used while trying to fix in on the coordinates and there were 5 satellites used. Each time I came back to the cache location I sat still for about ten minutes to get the average and I still got so many different readings for the last three numbers (XX*xx.???)

 

This hike was pretty easy and short and I was there for at least three hours trying to get it right!!? (Very beautiful hike too, as it has a nice waterfall)

 

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Go Indians!

>^..^< Ozzy&Buddie Holly >^..^<

[Guest cache_ninja]

well, i know some etrex users(etrex does not average) take multiple waypoints and then average them themselves. so i suppose if you wanted to you could do what you did, go back each time, have your unit eaverage, take what you got each time, average that yourself, and post it on the url..seems kinda redundant though, but thats a simple solution and the most accurate reading i would guess, unless something was wacky on one approach etc.

 

c/n

[Guest kurt]

Another point to make is that the cache SEEKER would also need to do some form of location averaging in order to get really close to the cache site.

Actually there are 3 types of averages: Mean, Median, and Mode. Mean is the normal average as we think about it. Median is the center or middle value of an ordered data set (ordered means all the values are listed from lowest to highest). Mode is the most typical value that comes up in a data set, but because location averaging usually gives all unique values, mode would not be best in this situation since it requires a number to occur more than once. I think median would be the best way to determine location averaging since it is affected less than the mean by extremely high or low values. A recent post gave several location values and I noticed that one of those values was much lower than the the other values hence the median of those values would have been better than the mean (or average) of those values. For all I know, Garmin and Magellan probably already do this in their location averaging subroutines, but for those of us HIDERS who have to do this manually, try taking at least 5 to 10 readings and using the median (not mean) value as the actual coordinates. I feel it is very important for the hiders to go to all this trouble, because the seekers can't do any better than the coordinates given to them. If those coordinates are slightly off, it could mean an extra 30 minutes easy to find the cache. One cache I found recently had different coordinates listed on the container lid than what was posted on the website! And yes, those coordinates were more accurate. Just my 2 (and a half) cents.

 

Kurt