Fairly Interesting Observation

[+The Lavender Hill Mob]

I recently placed 6 caches on the same day within hours of each other.

 

When I got home and checked the coords on GoogleEarth 4 of the caches coordinates were withing 6 to 10 feet of where the cache was placed, what you'd expect.

 

Two (the first 2 placed) were off by at least 100ft for the first cache and about 50ft for the second! The GPS was left turned on between marking the cache coords and none of the caches are in woodland.

 

I changed the coords of those two caches once tweeking the original coords to ones closer the caches in google.

 

Does anyone else check their cache coords on GoogleEarth and noticed this type of oddity?

 

TLHM

Edited March 30, 2008 by The Lavender Hill Mob

[+keehotee]

I had exactly the same problem a few weeks ago when setting a series.

GPS was on for the entire time, ten caches placed, and used BeeLine to average 300+ readings over 5 or ten minutes for each one - but didn't check them on GE until the DNF's started. Then found that a couple of them were way out....... both of them were in clear sky - although they were fairly near to overhead power lines (?) - and five of the series were under trees, but no probs have been posted with those!

 

Curiously - one of the errant positions is fairly near to another cache, and I spent weeks tracking it down because both Memory Map and my GPS placed it about 50 feet from the hiding place - even though other cachers had found it with no probs. This was with both my GPS's (1 Garmin, 1 Fortuna)

[+The Lavender Hill Mob]

I had exactly the same problem a few weeks ago when setting a series.

GPS was on for the entire time, ten caches placed, and used BeeLine to average 300+ readings over 5 or ten minutes for each one - but didn't check them on GE until the DNF's started. Then found that a couple of them were way out....... both of them were in clear sky - although they were fairly near to overhead power lines (?) - and five of the series were under trees, but no probs have been posted with those!

 

Curiously - one of the errant positions is fairly near to another cache, and I spent weeks tracking it down because both Memory Map and my GPS placed it about 50 feet from the hiding place - even though other cachers had found it with no probs. This was with both my GPS's (1 Garmin, 1 Fortuna)

Interesting.

 

The reason I posted the OP was becasue we did a cache this morning where the owners had posted a note asking for who ever found the cache next to check the coords because the FTF's had said that the posted coords were off by about 150ff and they were.

 

Perhaps it's due to sun spots, lay lines or little green men

[+Stuey]

I tweaked some co-ords once when I plotted the cache on Memory-Map (Ordnance Survey 1:25,000) it looked like it was on the wrong side of quite a wide road. Assuming I had made a mistake with my readings, I adjusted it so it looked right on the map. When it went live, people reported co-ords were quite a way out and only found it using the hint. So, I put it back to my readings and it was fine.

 

Don't always trust a map.... even the OS get it wrong.

[+Master Mariner]

I would always exercise caution when using Google Earth (GE) as the positions obtained from it can be many, many metres in error. The following may help to illustrate this.

 

Start up GE and zoom into Canada Tower (51° 30.300'N, 000° 01.175'W) at an altitude of about 400m. I don't think that anyone will disagree that the (many) corners of the tower are at the same geographical position as the tower does not taper for most of its height.

 

If you use the measuring tool in GE and measure a distance up one of the corners of the tower you will get a distance of about 40m. So, the corner of the tower is at the same geographical co-ordinates but a difference of about 40m exists. This is obviously down to perspective and can be seen becaue of the presence of the tower. What has now to be considered is which position is correct, that at the top of the corner or that at the bottom?

 

Many months ago I asked Google about this but have not received a response.

 

I assume, although could be totally wrong, that all the images shown in GE have co-ordinates adjusted to a zero metre height. In the case of Canada Tower that would indicate the co-ordinates near the base are more accurate. So, in a wide sweeping statement GE is more accurate at or very near to sea level.

 

The point that I am trying to illustrate here is that in a hilly or mountainous area aerial shots are probably not directly over where a cache is hidden although it may look that way. If the Canada Tower affect is considered then the co-ordinates chosen could be well off. Canada Tower is only a couple of hundred metres high so by the time you get into the hills/mountains you could be looking at thousands of metres. If Canary Tower was 1000m high then the error between the top and bottom of a corner could be a couple of hundred metres.

 

I hope that the above makes sense and would welcome any comments.

 

Edited to add: The OP mentions setting six caches of which two were in "error". I assume there was little significance in height difference but is it possible, and it is hard to tell, if the caches fell in different aerial shot tiles?

Edited March 30, 2008 by Master Mariner

[+keehotee]

Edited to add: The OP mentions setting six caches of which two were in "error". I assume there was little significance in height difference but is it possible, and it is hard to tell, if the caches fell in different aerial shot tiles?

All my caches were along Exeter Ship Canal, so to all intents and purposes at exactly the same elevation.

The neighbouring cache is alongside the Exe, and again at the same height (to within a meter or so). They're all probably within 2 or 3 meters of MHW.

 

Curiously - one of the errant positions is fairly near to another cache, and I spent weeks tracking it down because both Memory Map and my GPS placed it about 50 feet from the hiding place - even though other cachers had found it with no probs. This was with both my GPS's (1 Garmin, 1 Fortuna)

Just checked this one again - and it is 24m out on both GE and MM (phew, not just my GPS's)

Edited March 30, 2008 by keehotee

[+The Lavender Hill Mob]

Edited to add: The OP mentions setting six caches of which two were in "error". I assume there was little significance in height difference but is it possible, and it is hard to tell, if the caches fell in different aerial shot tiles?

Take a look, Sat was directly overhead. These were the two with errors N 51° 38.097 E 000° 33.380 & N 51° 35.922 E 000° 33.941. This was one of the ones which was spot on N 51° 39.214 E 000° 32.529.

 

Essex is pretty flat so there's not much difference in height.

[+Master Mariner]

Edited to add: The OP mentions setting six caches of which two were in "error". I assume there was little significance in height difference but is it possible, and it is hard to tell, if the caches fell in different aerial shot tiles?

Take a look, Sat was directly overhead. These were the two with errors N 51° 38.097 E 000° 33.380 & N 51° 35.922 E 000° 33.941. This was one of the ones which was spot on N 51° 39.214 E 000° 32.529.

 

Essex is pretty flat so there's not much difference in height.

 

I was just trying to offer an explanation as to why I would trust my GPS over Google Earth.

 

Interestingly, if you plot your second position in GE you can see that the aerial view is not directly overhead. About 150m to the west of the posiion is a pylon and you can see that the picture was taken from the SW of the pylon otherwise it would appear symmetrical from directly overhead (ignore the shadows). Given that the elevation here is only 30m or so the error caused by the perspective would only be minimal.

 

An interesting idea would be for you to find a lighthouse on the Essex coast and obtain its position from the Admiralty Light Lists and check your GPS against that. Have a day at the seaside and try it every hour or so and see if/what error you get. You could also see how the lighthouse looks, positional wise, on GE.

[+The Lavender Hill Mob]

An interesting idea would be for you to find a lighthouse on the Essex coast and obtain its position from the Admiralty Light Lists and check your GPS against that. Have a day at the seaside and try it every hour or so and see if/what error you get. You could also see how the lighthouse looks, positional wise, on GE.

Essex use to have a lighthouse on the coast but it was stolen! It ended up as a plant stand in someone's back garden in Tilbury

[+Delta68]

The 'ruler' feature on Google earth can be way out as well.

We recently set a letterbox hybrid specifying starting co-ords where a micro stated direction and (straight-line) distance to go to find the cache. This was about 520ft using the gps Google Earth measured it at about 300ft!

Quite a difference!

Yes, this did mean an early morning dash out to the cache to recheck

[Alan White]

I've always found GE to be absolutely spot on. It's more accurate than the GPSs. Whenever I've compared - as I've just done for yesterday's finds - the cache location as on the GPSs with GE the coords are always within a few metres.

 

And the ruler correctly measures the width of our garden

[+Team Sieni]

There was a similar discussion here

http://forums.Groundspeak.com/GC/index.php...=182151&hl=

that you may find interesting.

 

(By the way, this isn't a snotty "see the original thread!" "you're off topic" "neh neh" response, just thought it might be interesting )

[+The Lavender Hill Mob]

There was a similar discussion here

http://forums.Groundspeak.com/GC/index.php...=182151&hl=

that you may find interesting.

 

(By the way, this isn't a snotty "see the original thread!" "you're off topic" "neh neh" response, just thought it might be interesting )

Hmmm, it's the same discussion, never saw that one.

 

Anyway the 2 offending caches were published with the coords changed to GE coords and I've had no complaints that they were off! So I'd have to assume that the coords (when input) in GE are as, if not more, accurate than the GPS! And/or the GPS at the time the waypoint(s) was marked was affected by Sun spots, lay lines or aliens

[+Nick_Scots]

I've found GE to be off, especially since my local hiding area is low res.

 

MS Virtual Earth is much better res.

 

I check my Beeline on my Acer PDA using my Garmin Etrex Cammo with WAAS enabled.

 

The BEELINE never seems more accurate tham 17.5ft, the circle. I've tweaked the 'arrive' distance to less than 3m.

 

I have seen other threads about turning off static navigation.

 

Nick

[+ForsdykeFinders]

It was one of our caches in Southend that the Lavender Hill Mob kindly helped correct for us. Strangely although the first cache we placed was out, the second one we placed about 20 mins later has been confirmed as spot on......weird!

We dont have that many caches but is the first one we have had to correct.

[+Nick_Scots]

It was one of our caches in Southend that the Lavender Hill Mob kindly helped correct for us. Strangely although the first cache we placed was out, the second one we placed about 20 mins later has been confirmed as spot on......weird!

We dont have that many caches but is the first one we have had to correct.

 

Two cachers found a cache of mine today, GE shows on the other side of a canal.

 

MS Virtual earth is more accurate.

 

Nick

[+TheWhoUK]

I just had some experiance of this. I published a cache in london I had a 8-9 sat fix at final cache location. left GPS on and marked a 1st waypoint which required the finder to solve a puzzle.

 

The final cache seems to be out about 50m not much you may think but in london it another street so throws people out. managed to update with help of FTF cacher and google earth as I'm in wales on leave at the moment so unable to check myself.

 

the way point co ords spot on though.

 

SPOOKY

 

TheWhoUK