Bad coords <> clever hide

[+AbMagFab]

To me, this imposes two requirements:

1. You (personally) must visit the site; and
   
2. You must use a GPS to at least verify the co-ordinates.
   

Note that the requirements don't call for you to obtain the co-ordinates using GPS alone and, pragmatically, I suspect that the requirement is there to ensure that the cache can be found by GPS so, for example, it would rule out placing a cache deep within a cave system where there's no GPS signal.

 

In my case, I'm suggesting that I go out to the site on more than one visit and use my (known to be inaccurate) GPSr to obtain several fixes per visit. I'll then plot those on Google Maps. If they surround the co-ords obtained from GM then I'll consider the GM co-ords to be the best estimate even if it doesn't coincide with the average from my GPS fixes. IMO I'll have followed both the letter and the spirit of this requirement because I'll have used a GPS during the process of obtaining the co-ordinates and I'll have demonstrated that it is possible to navigate to the cache using GPS alone.

 

The alternative IMO is that I don't make any hides until I've obtained a better GPSr, which might not be for a year or two (if ever), and somehow it feels wrong not to give something back for the wonderful places other cachers have taken me.

 

Geoff

If only all hiders were like you... At least at this point, in my area, I know which hiders are almost always 30-feet off. One hider just seems to have a bad GPSr as she's always 30-feet NE off of the posted coords.

 

However another local hider seems to get great pleasure in posting coords that are randomly 30-50 feet off. Yuck.

[+popokiiti]

For my one (and so far only) cache hide, we took two GPSrs and took readings on both. We then averaged the reading between the two. Worked fine until the cache went AWOL.

[+edscott]

Don't know where this data is coming from, but if it were true I think I'd have a few less finds.

Well, I did mean seconds when I said minutes, so my bad there... But if you're using GPS to record the coordinates, and GPS to find the coordinates, then it wouldn't be an issue with finding something -- that problem should only arise if you're posting coords from Google Maps and trying to find with your GPSr, or vice versa.

 

In the link I posted, take a look at the pic of the GPSr in the 'IERS Reference Meridian' section, and you'll see that it's off by a little over 5". Try going to http://maps.google.com and enter "N 51.477222 E 0"; you'll see it places the marker somewhere along a row of parking spaces. Now try entering "N 51.477875 W 0.001490", and you'll see the marker moves a bit north and to the west to where the Prime Meridian is marked at the Royal Observatory. *That's* the discrepancy to which I was referring...

 

I often see errors in coordinates the GPS user has supplied, but there is no pattern to the errors except they are usually in a cardinal direction.

[+fizzymagic]

In the link I posted, take a look at the pic of the GPSr in the 'IERS Reference Meridian' section, and you'll see that it's off by a little over 5". Try going to http://maps.google.com and enter "N 51.477222 E 0"; you'll see it places the marker somewhere along a row of parking spaces. Now try entering "N 51.477875 W 0.001490", and you'll see the marker moves a bit north and to the west to where the Prime Meridian is marked at the Royal Observatory. *That's* the discrepancy to which I was referring...

I often see errors in coordinates the GPS user has supplied, but there is no pattern to the errors except they are usually in a cardinal direction.

I'm confused here. Is somebody claiming that Google Maps is off because it doesn't show the Prime Meridian marker at E 0?

 

Everybody knows that the Prime Meridian marker is off by ~100 m from the WGS-84 prime meridian, right?

[+DENelson83]

The best way to get coordinates for a geocache is to first place the cache, then put the GPSr directly above the cache (opposite to the direction of gravity, maintaining as clear a view of the sky as is available for your GPSr), mark a waypoint, average the coordinates over, say, ten minutes, then save the waypoint.

 

It's basically the same thing as placing horizontal geodetic control points (i.e. benchmarks—see the GEOCAC experiment), except with benchmarks, the averaging is done over a much longer period, and other methods are used to reinforce the coordinates. What geocachers are essentially doing when placing caches is a very crude analogue of surveying.

 

If you feel a geocache you're looking for has lousy coordinates, post a "needs maintenance" log for that cache asking for the coordinates to be verified. That's what the "update coordinates" log type is for.

Edited September 23, 2009 by DENelson83

[+edscott]

In the link I posted, take a look at the pic of the GPSr in the 'IERS Reference Meridian' section, and you'll see that it's off by a little over 5". Try going to http://maps.google.com and enter "N 51.477222 E 0"; you'll see it places the marker somewhere along a row of parking spaces. Now try entering "N 51.477875 W 0.001490", and you'll see the marker moves a bit north and to the west to where the Prime Meridian is marked at the Royal Observatory. *That's* the discrepancy to which I was referring...

I often see errors in coordinates the GPS user has supplied, but there is no pattern to the errors except they are usually in a cardinal direction.

I'm confused here. Is somebody claiming that Google Maps is off because it doesn't show the Prime Meridian marker at E 0?

 

Everybody knows that the Prime Meridian marker is off by ~100 m from the WGS-84 prime meridian, right?

 

That seems to be the message I'm getting. Wait until the north pole is in Ecuador, that'll keep the forums buzzing.

[Pajaholic]

The best way to get coordinates for a geocache is to first place the cache, then put the GPSr directly above the cache (opposite to the direction of gravity, maintaining as clear a view of the sky as is available for your GPSr), mark a waypoint, average the coordinates over, say, ten minutes, then save the waypoint.

Thanks for your advice. I tried this for a point on my office desk taking 8 averages, each done over between five and ten minutes using the averaging function of BeeLineGPS. The following image shows the results of the 8 averaged co-ord sets plotted on Google Maps represented by a black dot and the actual location according to Google Maps shown as a larger, red dot. I've also included a scale (measured on the ground and transposed to the plot). Note that 5m is over fifteen feet and HDOP was between 1.5 and 5 during the test. IMO, even averaging the averages isn't good enough!

 

If I'd have used GPS alone to obtain the co-ords, the average of the averaged co-ords would have been about 5m off and if I'd just used the first obtained (i.e. the one lower-left of the image) it would have been about 10m off.

 

I feel the need to check GM accuracy in my area with a definitive reference. I thought of using an OS trig point since my area is shown on GM in enough detail to just about be able to pick out the baseplate from a GM image at maximum zoom. However, the Ordnance Survey no longer maintain these and suggest in this page that OSGB36 trig points should not be used as GPS control points as only low accuracy (WGS84) co-ordinates can be obtained from them. However, the accuracy they're talking about for GPS surveying using ETRS89 is to the nearest 5cm (two inches), so I suspect that a check of a converted 10-digit OS grid reference against GM's satellite imagery would suffice! Other than that, I guess it's a case of waiting until I have enough finds to be able to say with some surety that GM is or is not good enough to use for the primary fix, and hence whether I only need use my GPSr as a check and to demonstrate that it's possible to navigate to the cache using GPS alone.

 

Geoff

[GOF and Bacall]

The best way to get coordinates for a geocache is to first place the cache, then put the GPSr directly above the cache (opposite to the direction of gravity, maintaining as clear a view of the sky as is available for your GPSr), mark a waypoint, average the coordinates over, say, ten minutes, then save the waypoint.

Thanks for your advice. I tried this for a point on my office desk taking 8 averages, each done over between five and ten minutes using the averaging function of BeeLineGPS. The following image shows the results of the 8 averaged co-ord sets plotted on Google Maps represented by a black dot and the actual location according to Google Maps shown as a larger, red dot. I've also included a scale (measured on the ground and transposed to the plot). Note that 5m is over fifteen feet and HDOP was between 1.5 and 5 during the test. IMO, even averaging the averages isn't good enough!

 

If I'd have used GPS alone to obtain the co-ords, the average of the averaged co-ords would have been about 5m off and if I'd just used the first obtained (i.e. the one lower-left of the image) it would have been about 10m off.

 

I feel the need to check GM accuracy in my area with a definitive reference. I thought of using an OS trig point since my area is shown on GM in enough detail to just about be able to pick out the baseplate from a GM image at maximum zoom. However, the Ordnance Survey no longer maintain these and suggest in this page that OSGB36 trig points should not be used as GPS control points as only low accuracy (WGS84) co-ordinates can be obtained from them. However, the accuracy they're talking about for GPS surveying using ETRS89 is to the nearest 5cm (two inches), so I suspect that a check of a converted 10-digit OS grid reference against GM's satellite imagery would suffice! Other than that, I guess it's a case of waiting until I have enough finds to be able to say with some surety that GM is or is not good enough to use for the primary fix, and hence whether I only need use my GPSr as a check and to demonstrate that it's possible to navigate to the cache using GPS alone.

 

Geoff

 

You need to relax about it. Do the best you can averaging the coordinates when you hide the cache and don't worry about it. While I think it is wrong to intentionally "soften" the coordinates it would take a lot of the fun out of caching if they were dead on accurate. "Went to the zero and there was the cache. Saw it as soon as I stopped looking at my GPS"

[Pajaholic]

You need to relax about it. Do the best you can averaging the coordinates when you hide the cache and don't worry about it. While I think it is wrong to intentionally "soften" the coordinates it would take a lot of the fun out of caching if they were dead on accurate. "Went to the zero and there was the cache. Saw it as soon as I stopped looking at my GPS"

Thanks for that. However, the potential for the co-ords I give to be up to 10m (33ft) off worries me - especially since every cache I've found to date has been within a foot or two of where GM says it should be. Since my last post, I've found the national grid references of some of trig points near me. The site I got them from says these were obtained by conversion from WGS84 co-ords and subject to conversion error. Also, 5-digit grid references are only good to the nearest metre and my use of GSAK to convert them back to WGS84 co-ords is likely to induce further error. That said, the trig points at the corners of a quadrilateral enclosing the area in which I'm likely to hide my first few caches are all within five or six feet - as the satellite images below show!

To be that close despite the potential for error seems to be adequate confirmation that Google Maps is accurate in my area. However, I'll still wait until I've a few more finds before making my first hides.

 

Geoff

[GOF and Bacall]

Just as a point of curiosity try your experiment outdoors instead of at your desk. Say on a monument or something else stationary and visible on the maps.

 

To be honest as long as you consistently post acceptable coordinates to your cache pages I don't care how you get them. You could copy them from a fortune cookie and as long as they got me to the cache I wouldn't care.

[Pajaholic]

Just as a point of curiosity try your experiment outdoors instead of at your desk. Say on a monument or something else stationary and visible on the maps.

Strange you should suggest that - I found your post when I got back from doing such a test at the local park. I used the end of a park bench (larger red dot) that runs roughly N/S in the following plot and took fixes (smaller black dots) after approaching the park bench from 1, 2, 3, 4, 5, and 6 o'clock. Now this was under ideal conditions with the GPSr able to track 7 or 8 satellites and with HDOP less than 2:

 

 

To be honest as long as you consistently post acceptable coordinates to your cache pages I don't care how you get them. You could copy them from a fortune cookie and as long as they got me to the cache I wouldn't care.

All I'm trying to do is find a way of giving co-ordinates that that would be acceptable to me as a finder. The testing has convinced me that my GPSr isn't accurate enough to obtain co-ordinates on its own, so I need to use a hybrid method or wait until I have a better GPSr.

 

Anyway, as you suggest, it's not worth me getting het up over this and I just need to post what I believe are good co-ordinates.

 

Thanks.

 

Geoff

[+edscott]

Just as a point of curiosity try your experiment outdoors instead of at your desk. Say on a monument or something else stationary and visible on the maps.

 

To be honest as long as you consistently post acceptable coordinates to your cache pages I don't care how you get them. You could copy them from a fortune cookie and as long as they got me to the cache I wouldn't care.

 

YES!! A sensible answer.

 

Good coordinates are good coordinates no matter where they come from and crappy ones are crappy whatever their source. Those of us that read satellite images well, even if we are reading things less obvious than a building corner or a lone evergreen, can make a determination of good coordinates especially if we use a GPS as a backup system to show that a GPS user can find the cache with the supplied coordinates.

 

I have a cache where over half the sky is shielded from view by rocks on one side and a large hill obscures lots of the other horizon. A further complication is Mountain Laurel and tree cover. A GPS usually looses most of its signal about 70 feet away, but by doing the math from that point and comparing that result with the photo, I have good coordinates posted. I originally used an average of coordinates at the cache right off the GPS and had crappy coordinates. Of course the best way to do this cache is to get as close as possible with a strong signal, calculate the distance and direction to the cache, and go find it. People that follow their GPS as it bounces them around over the cliffs probably get a good workout.

[Dinoprophet]

Bad coords <> clever hide

 

Bad coords are never > clever hide.

[+DENelson83]

The best way to get coordinates for a geocache is to first place the cache, then put the GPSr directly above the cache (opposite to the direction of gravity, maintaining as clear a view of the sky as is available for your GPSr), mark a waypoint, average the coordinates over, say, ten minutes, then save the waypoint.

Thanks for your advice. I tried this for a point on my office desk taking 8 averages, each done over between five and ten minutes using the averaging function of BeeLineGPS. The following image shows the results of the 8 averaged co-ord sets plotted on Google Maps represented by a black dot and the actual location according to Google Maps shown as a larger, red dot. I've also included a scale (measured on the ground and transposed to the plot). Note that 5m is over fifteen feet and HDOP was between 1.5 and 5 during the test. IMO, even averaging the averages isn't good enough!

Well, the thing is, you were indoors. To get the best results, you should have been as far above that location as is needed to get a clear view of the sky, such as, yes, on the roof of your building.

[GOF and Bacall]

Just as a point of curiosity try your experiment outdoors instead of at your desk. Say on a monument or something else stationary and visible on the maps.

Strange you should suggest that - I found your post when I got back from doing such a test at the local park. I used the end of a park bench (larger red dot) that runs roughly N/S in the following plot and took fixes (smaller black dots) after approaching the park bench from 1, 2, 3, 4, 5, and 6 o'clock. Now this was under ideal conditions with the GPSr able to track 7 or 8 satellites and with HDOP less than 2:

 

 

To be honest as long as you consistently post acceptable coordinates to your cache pages I don't care how you get them. You could copy them from a fortune cookie and as long as they got me to the cache I wouldn't care.

All I'm trying to do is find a way of giving co-ordinates that that would be acceptable to me as a finder. The testing has convinced me that my GPSr isn't accurate enough to obtain co-ordinates on its own, so I need to use a hybrid method or wait until I have a better GPSr.

 

Anyway, as you suggest, it's not worth me getting het up over this and I just need to post what I believe are good co-ordinates.

 

Thanks.

 

Geoff

 

If you toss out the two grossly anomalous points in this test and then average the rest of the GPS readings how do the results match to your GM position?

[Pajaholic]

Well, the thing is, you were indoors. To get the best results, you should have been as far above that location as is needed to get a clear view of the sky, such as, yes, on the roof of your building.

Since you didn't use a smiley or "<FE>" (irony) tags, I have to assume that you're serious. My office is in a single-storey building and its roof has proved less of a hindrance to GPS signals than the trees in my area, I thought that doing the test in my office would be a better than fair comparison to how things would be "in the field". Put it this way, I don't intend climbing into the upper branches of the trees over a hide to get the co-ordinates!

 

If you toss out the two grossly anomalous points in this test and then average the rest of the GPS readings how do the results match to your GM position?

You're looking at that with the benefit of hindsight. You know where GM says the waypoint is because I've indicated it with a large, red dot. Remove that and it's not immediately obvious that the top-right pair are anomalous. Without knowing the actual location, I'd probably discount the bottom left pair and the top-right singleton (greyed in the image below), leaving a reasonable group of four. The average of those (guestimated by the blue dot) is almost 5m (16ft) off.

 

Of course, used with the benefit of hindsight (i.e. knowing the GM co-ords) the four closest points average to pretty close agreement with GM and thus they corroborate each other. So I guess that's the way to go.

 

Geoff

[GOF and Bacall]

Well, the thing is, you were indoors. To get the best results, you should have been as far above that location as is needed to get a clear view of the sky, such as, yes, on the roof of your building.

Since you didn't use a smiley or "<FE>" (irony) tags, I have to assume that you're serious. My office is in a single-storey building and its roof has proved less of a hindrance to GPS signals than the trees in my area, I thought that doing the test in my office would be a better than fair comparison to how things would be "in the field". Put it this way, I don't intend climbing into the upper branches of the trees over a hide to get the co-ordinates!

 

If you toss out the two grossly anomalous points in this test and then average the rest of the GPS readings how do the results match to your GM position?

You're looking at that with the benefit of hindsight. You know where GM says the waypoint is because I've indicated it with a large, red dot. Remove that and it's not immediately obvious that the top-right pair are anomalous. Without knowing the actual location, I'd probably discount the bottom left pair and the top-right singleton (greyed in the image below), leaving a reasonable group of four. The average of those (guestimated by the blue dot) is almost 5m (16ft) off.

 

Of course, used with the benefit of hindsight (i.e. knowing the GM co-ords) the four closest points average to pretty close agreement with GM and thus they corroborate each other. So I guess that's the way to go.

 

Geoff

 

Perhaps, but it looks like you have a cluster of points that are near each other with two that are out side of the cluster and one that is on the cusp so to speak. Granted that all I have to go on is your graphic, but I'd think the numbers would show the deviation.

 

Give this a try. Get your numbers just like before. Look at 'em and decide what ones are at the outside extreme. Toss those that are and then average the rest. After that do your comparison to the maps.

 

I still think you are over thinking it. Many thousands of caches have been hidden with GPS receivers less accurate than those available now. Somehow we have managed to find most of them.