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# Google Earth

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This might have been discussed already, but if so, then I missed it.

How do the co-ordinates given at the bottom of the Google Earth map translate into hddd mm.mmm?

e.g. if I go to a random point in Edinburgh, Google Earth gives the co-ordinates as 55 55 24.74 for North and 3 22 06.85 for West, which is more digits than we use for our co-ordinates on gc.com. So how do you compare them?

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GE uses degs mins and seconds for lat/log co-ords as opposed to degs mins that we tend to use for co-ords.

To convert between them take the GE seconds and divide by 60 then add to the mins ie using your example

55 55 24.74 . translates to N55 55.4125 i.e. (24.74)/60 = 0.4125

Hope this makes sense.

Edited by Big Wolf
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AB,

Google Earth is very tolerant of co-ord formats.

Just type in some GC-style co-ords and it will figure out what you mean.

eg:

let's say you want to plot N 56° 12.233 W 003° 08.544

just type in those co-ords (or cut&paste, whatever) into the wee search box at the top left of the screen in our format and it will understand what you want. It cleverly parses almost any format into the style that it uses itself. Even if you've got something like 56.123456789N 3.987654321W, it will still understand where you want to go.

Google Earth is a clever bit of kit. Much too good to remain free forever, methinks.

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If only Google Earth was a little clearer over the UK it would be a great piece of software. Just cannot zoom down onto area clear enough.

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Depends on the area Gralorn. My area was 'upgraded' around christmas and the quality is fantastic.

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If only Google Earth was a little clearer over the UK it would be a great piece of software. Just cannot zoom down onto area clear enough.

Fact

Google earth is not accurate due to the orthorectification, which is used to correct the satellite images, if the images were taken beneath the stratosphere then atmospheric distortion would have been less and accuracy increased. This is why the military still use spy planes for accurate location trips on targets

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If only Google Earth was a little clearer over the UK it would be a great piece of software. Just cannot zoom down onto area clear enough.

I can identify my car outside my house, can't really complain.

Google earth is not accurate due to the orthorectification, which is used to correct the satellite images

Depends on your definition of accuracy I suppose, its more than good enough for everything I have used it for. A GPS isn't highly accurate, yet we all manage to find caches with one.

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Google earth is not accurate due to the orthorectification, which is used to correct the satellite images

Depends on your definition of accuracy I suppose, its more than good enough for everything I have used it for. A GPS isn't highly accurate, yet we all manage to find caches with one.

Discussion

GE can be over 100m out in some locations due to the orthorectification, but in other areas it can be almost spot on. It is the way that the images have been rendered from a flat surface (i.e. photographic plate or CCD) onto a computer generated model which distorts the image one a curved surface.

Edited by Moote
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Depends on your definition of accuracy I suppose, its more than good enough for everything I have used it for. A GPS isn't highly accurate, yet we all manage to find caches with one.

The article from which Moote got his information uses the word "accuracy" in the context of various examples given therein.

(I know this, because I wrote it.)

By any reasonable standard of judgement, the registration of GE's hi-res imagery is extraordinarily good, and we're very fortunate to have such a tool available, free gratis.

My tests (rather scattered and random) showed a range of position variation from around 15m, down to better than 3. One later test - on the reference marker at OSGB headquarters in Southampton - gave a variance of 156cm (+/- 15cm)

The article concluded that using Google Earth to locate a cache, is about as reliable as using a consumer GPSr to do it.

Which is just as it should be.

-Wlw

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Depends on the area Gralorn. My area was 'upgraded' around christmas and the quality is fantastic.

So was mine, to an OLDER photo .

You used to be able to see my lovely new patio and conservatory (circa 2004) now both are missing

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You used to be able to see my lovely new patio and conservatory (circa 2004) now both are missing

Joke

Missing! Eckington, phone the police or get that suitably dressed Lactodorum on the case!

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Depends on your definition of accuracy I suppose, its more than good enough for everything I have used it for. A GPS isn't highly accurate, yet we all manage to find caches with one.

The article from which Moote got his information uses the word "accuracy" in the context of various examples given therein.

(I know this, because I wrote it.)

By any reasonable standard of judgement, the registration of GE's hi-res imagery is extraordinarily good, and we're very fortunate to have such a tool available, free gratis.

My tests (rather scattered and random) showed a range of position variation from around 15m, down to better than 3. One later test - on the reference marker at OSGB headquarters in Southampton - gave a variance of 156cm (+/- 15cm)

The article concluded that using Google Earth to locate a cache, is about as reliable as using a consumer GPSr to do it.

Which is just as it should be.

-Wlw

Thats quite interesting, and I agree with an earlier statement. Although GE is not perfect, for a free program I can't complain!

You mention the varience from the co-ords and the pictures varying from spot to spot. Something I have noticed a few times is that checking/unchecking the 'Terrain' box to make the map 3D can somewhat distort the view looking straight down, with some areas looking either pinced or punched, and surrounding areas being stretched or squshed due to the deformation. I presume to get the most 'accurate' (i use this word carefully in this forum) view, this should be unchecked. Please correct me if I'm wrong.

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You mention the varience from the co-ords and the pictures varying from spot to spot. Something I have noticed a few times is that checking/unchecking the 'Terrain' box to make the map 3D can somewhat distort the view looking straight down, with some areas looking either pinced or punched, and surrounding areas being stretched or squshed due to the deformation. I presume to get the most 'accurate' (i use this word carefully in this forum) view, this should be unchecked. Please correct me if I'm wrong.

There's a short answer to this, and also an almost unbelieveably long and boring answer which no-one in their right mind would want to read.

The short answer is that that it makes no difference.

-Wlw

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You mention the varience from the co-ords and the pictures varying from spot to spot. Something I have noticed a few times is that checking/unchecking the 'Terrain' box to make the map 3D can somewhat distort the view looking straight down, with some areas looking either pinced or punched, and surrounding areas being stretched or squshed due to the deformation. I presume to get the most 'accurate' (i use this word carefully in this forum) view, this should be unchecked. Please correct me if I'm wrong.

There's a short answer to this, and also an almost unbelieveably long and boring answer which no-one in their right mind would want to read.

The short answer is that that it makes no difference.

-Wlw

Since we're on the GE subject, I do have a relevent and interesting point to make, or a question W|w may know the answer to. When using GE to track a TB's path, why does it sometimes give 2 'stops' when there should only be one. For instance, looking at a TB that has been to one of our caches, there were 2 locations, each with a corrosponding 'stop number' one of which seemed a fair distance (maybe 200') from the cache location. Is there a reasonable (and hopefuly not too long and boring) expaination for this?

Edited by Geo-Kate
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Google earth is not accurate due to the orthorectification, which is used to correct the satellite images, if the images were taken beneath the stratosphere then atmospheric distortion would have been less and accuracy increased.

Three misunderstandings in a single sentence.

Firstly, orthorectification increases geospatial accuracy. It does not decrease it.

Secondly, the high resolution imagery of the UK which GE purchased from the Geoinformation Group is taken from conventional aircraft flying at a typical height (not altitude) of approximately a mile.

Thirdly, the effect of atmospheric refraction is a term which which is part of the orthorectification algorithm, but it is miniscule and not detectable at the quarter-metre level of the dataset which you see on GE.

The hi-res imagery providers use both orthorectification and georectification to process the imagery before they sell it. Your GE prog then uses a completely different dataset from radar alrimetry flown on a NASA Space Shuttle mission to create the "terrain" perspective. It is a secondary form of georectification.

The low-res background imagery is processed completely differently because the LandSat data is a continuous stream, rather like an endless carpet roll, instead of a series of snapshots as flown by the aircraft.

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When using GE to track a TB's path, why does it sometimes give 2 'stops' when there should only be one.

This would be at your Shorne Wood cache, right?

-Wlw

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Since we're on the GE subject, I do have a relevent and interesting point to make, or a question W|w may know the answer to. When using GE to track a TB's path, why does it sometimes give 2 'stops' when there should only be one. For instance, looking at a TB that has been to one of our caches, there were 2 locations, each with a corrosponding 'stop number' one of which seemed a fair distance (maybe 200') from the cache location. Is there a reasonable (and hopefuly not too long and boring) expaination for this?

This is because geocaching.com deliberately makes the coordinates in the KML files inaccurate. It seems that an error is added each stop individually, so they appear to be in slightly different places.

You can see this if you download the geocache browser KML and zoom in on a cache - when it refreshes the cache icon appears to jump about a bit.

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Something I have noticed a few times is that checking/unchecking the 'Terrain' box to make the map 3D can somewhat distort the view looking straight down, with some areas looking either pinced or punched, and surrounding areas being stretched or squshed due to the deformation. I presume to get the most 'accurate' (i use this word carefully in this forum) view, this should be unchecked. Please correct me if I'm wrong.

The simple answer is no.

The accuracy is unchanged, whether you select or deselect the "terrain" tickbox.

It is merely a choice of two different way of portraying the same data to the same accuracy. Without the tickbox unchecked, you are seeing the imagery in a cartographic projection which is analagous to a flat map projection such as Mercator or transverse Mercator. Ticking the box gives you a perspective which takes into account your selected height of eye and the relative altitude of the terrain.

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When using GE to track a TB's path, why does it sometimes give 2 'stops' when there should only be one.

This would be at your Shorne Wood cache, right?

-Wlw

Well, no, it was Dartford heath I was refering to, but I just checked a TB that has been to Shorne Wood, and it too has two completely different locations, both suoposedly the cache location.

Edited by Geo-Kate
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geocaching.com deliberately makes the coordinates in the KML files inaccurate.

Why does Geocaching.com deliberately falsify the co-ords?

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Well, no, it was Dartford heath I was refering to, but I just checked a TB that has been to Shorne Wood, and it too has two completely different locations, both suoposedly the cache location.

OK - I thought you were referring to a certain TB which was in the cache when you moved it a short distance.

That apart, Craig's explanation above is the correct one.

-Wlw

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geocaching.com deliberately makes the coordinates in the KML files inaccurate.

Why does Geocaching.com deliberately falsify the co-ords?

Thought

I think that GE just moves one as there are 2, but I have noticed also that points can move slightly everytime you open GE

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I think that GE just moves one as there are 2, but I have noticed also that points can move slightly everytime you open GE

It's done by geocaching.com, to make the location of TBs "fuzzy".

This - and other measures - have been taken to stop smart-alecs like me finding caches (containing TBs) before they're even listed.

-Wlw

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I think that GE just moves one as there are 2, but I have noticed also that points can move slightly everytime you open GE

It's done by geocaching.com, to make the location of TBs "fuzzy".

This - and other measures - have been taken to stop smart-alecs like me finding caches (containing TBs) before they're even listed.

-Wlw

Opinion

No I have noticed it with the Coords of my house, Sometimes it moves up to 100m from where it is

Edited by Moote
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Why does Geocaching.com deliberately falsify the co-ords?

So that people cannot us GE to get accurate coordinates for caches. They want people to go through the web site.

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Opinion

No I have noticed it with the Coords of my house, Sometimes it moves up to 100m from where it is

I can't comment on the location of your house, but the statement I made (related to geocaches) is still correct.

Jeremy's name for it is "obfuscation"...

-Wlw

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Opinion

No I have noticed it with the Coords of my house, Sometimes it moves up to 100m from where it is

Yes, but living on a boathouse doesn't help!!

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