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Accuracy of Google Map and cache co-ordinates


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Hi

I have recently started Geocaching and found a few caches and placed my first cache . Now , my question is this . My ( only ) cache co-ordinates I have hidden are accurate as far as I know , yet when I look at the Google map and zoom the picture to magnify the site , then the picture shows the cache at a point about 30 metres from the true position . Does this mean that you should ignore the Google map picture , and rely solely on the co-ordinates ?

 

cheers,

Barlicker

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Yeah, the images on google are never 100% spot on.

It's best if you are placing a cache if you use a GPS to record the coords, as this is what the finders will be using.

 

cheers , Jaz , Its just that the reading the comments in the log for my cache tend to make me think that folk have been paying more attention to the "off licence" ( Google map position ) than to the posted co-ordinates.

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Hiya Barlicker, i noticed you were the last visitor before me on the worden park cache, you're the one who hid it so well i ended up nettling my hand :unsure:

 

I found google maps were good for local caches i already knew the location of but i think the GPS would be more accurate, maybe add a bit of a broader clue?

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Hiya Barlicker, i noticed you were the last visitor before me on the worden park cache, you're the one who hid it so well i ended up nettling my hand :unsure:

 

I found google maps were good for local caches i already knew the location of but i think the GPS would be more accurate, maybe add a bit of a broader clue?

 

Hi Jindivik , The worden park cache - I couldnt find it , my mrs found it , she put it back , I will pass on the comments . I didnt think there were nettles at that site tho. It was very well hidden when we were looking for it.

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Hiya Barlicker, i noticed you were the last visitor before me on the worden park cache, you're the one who hid it so well i ended up nettling my hand :unsure:

 

I found google maps were good for local caches i already knew the location of but i think the GPS would be more accurate, maybe add a bit of a broader clue?

 

 

I will double check the co-ordinates . and maybe review the clue :lol:

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I have a cache at which I can get a good GPSr reading - down to 9' or so. I have taken quite a few 'average' readings, and yet googlemaps shows my coords about 20' away. I suppose that's nothing really. At least googlemaps is more accurate than streetmap, at least as far as rivers and streams are concerned! ;)

 

Jon

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Here's what to do:

 

Convert your Lat/Long to OS grid co-ords using a reputable offline utility such as Memory Map or Grid InQuest, or an online utility from a reputable source such as nearby.org.uk or gps.gov.uk. Ignore the crapulous grid co-ords at the top-left corner of the cachepage. They are complete garbage and are a disgrace to DoubleSpeak Inc. They contain many errors massively greater than a mere 20' and should be ignored.

 

Plot your grid cords on one of the several freebie OS mapping services, such as magic.gov.uk. Those maps show the grid co-ords to single metre precision and are accurate to the OS normal standards which are comparable to a reasonably good quality GPS fix.

 

If you want to check whether one of the georeferenced image services such as GoogleEarth or VirtualEarth are producing accurate co-ords, simply scale off the co-ords of a recognisable point on the OS map, such as a fence or wall corner or intersection and compare them to the indicated co-ords on the orthorectified image at GoogleEarth. You can apply that translational shift anywhere reasonably close to your co-ordinated point, so long as there isn't a very steep change in height.

 

The accuracy you can expect from the Ordnance Survey depends on the scale of the survey. For urban and rural areas surveyed at 1:2,500 the errors are <± 2.8 metres. For mountain and moorland areas surveyed at 1:10,000 the OS errors are <± 4.1 metres. These accuracies are comparable in magnitude to the typical error of a single GPS fix.

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Google Earth is pretty much spot on and can be used for many caches. I discovered this when my GPS went in for repair and have found it fun since seeing how many I can find on GE - I take along the GPS as backup. Mind you, don't use the geocache icon which is deliberately offset, rather cut and paste the coordinates into the GE search box, it's remarkably accurate.

 

I've also used GE to check my own readings when cache setting. Again seems pretty accurate most times.

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cheers , Jaz , Its just that the reading the comments in the log for my cache tend to make me think that folk have been paying more attention to the "off licence" ( Google map position ) than to the posted co-ordinates.

 

One of those logs was mine; my excuse: Urban Canyon. The GPSr just overshot by a bit and had me walking straight towards the offie. It was a real wrench to turn round when the arrow had settled down and was pointing the opposite way.

 

Geoff

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I'd always thought of urban canyons as occurring between huge skyscrapers, not something there are many of in Barlick ;-)

 

Ah! Ok, you got me on a technicality. It must have just been the magical attraction of the off-licence :laughing: .

 

To be fair though, we did find the cache before we went in. And we were thirsty, or at least we knew we would be eventually.

 

Geoff

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Hiya Barlicker, i noticed you were the last visitor before me on the worden park cache, you're the one who hid it so well i ended up nettling my hand :anibad:

 

I found google maps were good for local caches i already knew the location of but i think the GPS would be more accurate, maybe add a bit of a broader clue?

 

I have updated the clue for my cache , and checked the co-ordinates and updated them :anibad: . However the new co-ordinates , whilst changing the last 2 decimal places only , has moved the cache ( and Google Earth map picture ) by a stagering 478 feet :P . Now I wasnt expecting this , as I thought i'd altered the figures by just a few feet . I just hope folk can find my cache now . Any feedback as to the accuracy of the gps figures will be greatlly appreciated . The original figures were taken from a different gps unit , so I am not too sure what to make of all this . ( You can tell I am a novice to all this Geocaching milarky stuff ) :laughing: I am pretty sure the original figures were pretty good , its just that 478 feet seems an awful lot . Any help and comments will be appreciated

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I have updated the clue for my cache , and checked the co-ordinates and updated them :anibad: . However the new co-ordinates , whilst changing the last 2 decimal places only , has moved the cache ( and Google Earth map picture ) by a stagering 478 feet :anibad: . Now I wasnt expecting this , as I thought i'd altered the figures by just a few feet . I just hope folk can find my cache now . Any feedback as to the accuracy of the gps figures will be greatlly appreciated . The original figures were taken from a different gps unit , so I am not too sure what to make of all this . ( You can tell I am a novice to all this Geocaching milarky stuff ) :laughing: I am pretty sure the original figures were pretty good , its just that 478 feet seems an awful lot . Any help and comments will be appreciated

 

Yikes! I didn't think they were that far out. It's a built-up area and the offie had a strange effect on me but I don't remember it being more than a few metres out.

 

Geoff

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Crikey there's more to this mapping lark than I ever thought possible
I think I'll have that engraved as my epitaph!

 

How do you know so much about this stuff?

A couple of years of studying Geodesy at postgrad level and a third of a century of offshore experience of working as a Hydrographer were a good start. Devising and assuring position fixing methods to align and connect two pipelines in a couple of thousand feet of seawater so that the boltholes of the flanges line up in three dimensions and can be connected accurately enough to secure oil or gas at a couple of thousand psi in an environmentally sensitive location concentrates the mind wonderfully. After a few decades of doing that sort of work the simple matter of position fixing onshore in daylight to an accuracy of a couple of metres becomes a piece of the proverbial.

 

One of the sidelines I do in addition to my 'day' job is occasionally working as a civilian instructor on the directing staff of four units of the UK land forces teaching Advanced Land Navigation to specialist troops or their instructors . My studes aren't awarded marks out of ten, they either pass or fail the course. One of several practical tasks they have to accomplish before passing is to measure the co-ords of a given location in the field to an accuracy better than 7 metres without GPS. Almost all of them pass the course, usually measuring the point to much better than the required standard. For one of the units, a TA outfit in lowland Scotland, I usually give them a task to do an accurate four legged traverse to a point which just happens to have a geocache. They then have to prove to me that they have been there by counting the boughs on a certain tree and then they have to measure the co-ords of the cache. The cache owner is, I'm pretty sure, unaware of the undeclared visitors to his cache though he may be puzzled by a certain mark in the logbook! The co-ords which the successful studes produce are usually within 3 or 4 metres of the actual position. They're not allowed to use anything other than a sighting compass, a piece of knotted string, a pencil, paper and simplified compact trig and log tables and I check their working to ensure that they haven't cheated (much!), but most of them get the co-ords right to an accuracy that a good cacher would be proud of.

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Yikes! I didn't think they were that far out. It's a built-up area and the offie had a strange effect on me but I don't remember it being more than a few metres out.

 

Geoff

 

Sorry if my log made you think the coords were out - I was refering to Geoffs tendancy to be a few metres away from the cache when the find takes place! It was also raining so we read the clue without trying to find it first. The cache was in the first place I looked. The GPSr we use needs you to be moving for the arrow to work accurately, but I'm not sure which arrow Geoff followed when he headed for the offie*!

 

Jill

 

*The reason the Off Licence gets a mention is that it had over 100 'real' bottled beers, and a nice selection too!

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.........Devising and assuring position fixing methods to align and connect two pipelines in a couple of thousand feet of seawater .........

Boy, The Forester's caches must be hard to find and require specialist equipment! :rolleyes:

 

Funny you should say that! A couple of years ago I seriously investigated the possibility of placing a geocache on the seabed in a very deep part of the Norwegian Trench. I obtained permission in principle, subject to detailed review of course, from one of the reviewers. I obtained a redundant underwater camera housing, which was rated to 2000m depth, as a cache container. As Client Rep I am consulted by the Party Chief when selecting a suitable location for test dives of work class Remote Operated Vehicles (ROVs). We usually choose somewhere a little bit interesting for test dips, such as a mine-like object which has previously only been seen on sonar and hasn't yet been visually identified, or perhaps a poorly doccumented wreck where we can do a little bit of thievery such as nicking the ship's bell for display in the mess or even popping off a shape charge to liberate the phosphor bronze propellor to sell to a dodgy scrappie for some drinking vouchers for the lads.

 

Cache maintenance wouldn't be much of a problem at my chosen site as there's a major gas field fairly nearby which is often attended by ROVs and I can easily contact either the Party Chief or the Client rep if I'm not aboard myself. The only major problem I ran into was resolving the "permission" issue. Strictly speaking placement of a cache on the seabed would be classed as "dumping" under the UNCLOS regs. That would require government permission. I've dealt with the Noggie government on such matters before and the bureaucracy is a major pain. Another problem to solve would be ensuring that the cache can survive being clobbered by a trawler. That can be achieved but is quite tricky.

 

Such a cache would certainly rate a 5/5 as the "terrain" is certainly harsh and the "special equipment" involves a million Pound ROV and a £20M support ship which has a current dayrate of around £36,000!

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