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tiompan
Joined:
09-01-2005
Messages: 2707
 OFF-Line
|  New Message Posted!2011-12-14 08:40
Sorry K your'e right , I should have had a closer look
George
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On 2011-12-14 06:41, Klingon wrote:
Quote:
| On 2011-12-12 17:30, tiompan wrote:
The map is of the amount deviation when OSGB 36 data is transformed into WGS 84 . |
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No, this map shows the deviation between the calculated BNG-coordinates and the "real" (originally surveyed) BNG-coordinates. OSGB36 fits Britain better than WGS84 but the British National Grid, based on OSBG36, has some inaccuracies (see my image).
Jan
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Klingon
Joined:
08-08-2004
Messages: 758
from Germany
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|  New Message Posted!2011-12-14 06:41
Quote:
| On 2011-12-12 17:30, tiompan wrote:
The map is of the amount deviation when OSGB 36 data is transformed into WGS 84 . |
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No, this map shows the deviation between the calculated BNG-coordinates and the "real" (originally surveyed) BNG-coordinates. OSGB36 fits Britain better than WGS84 but the British National Grid, based on OSBG36, has some inaccuracies (see my image).
Jan
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tiompan
Joined:
09-01-2005
Messages: 2707
OFF-Line
| New Message Posted!2011-12-12 17:30
The map is of the amount deviation when OSGB 36 data is transformed into WGS 84 . The original OSGB 36 is a more accurate mapping of the UK .
There are a number of online programs that will do the transform easily enough .
George
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On 2011-12-12 16:50, Klingon wrote:
Conversion between WGS84 and OSBG36 is difficult (some really long formulas). Most GPS-receivers don't do the full conversion, they calculate only an approximation.
In 1936 Britain was surveyed without GPS. The British National Grid isn't perfect, there are some distortions (see this image - total deviation of the location).
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Klingon
Joined:
08-08-2004
Messages: 758
from Germany
OFF-Line
| New Message Posted!2011-12-12 16:50
Conversion between WGS84 and OSBG36 is difficult (some really long formulas). Most GPS-receivers don't do the full conversion, they calculate only an approximation.
In 1936 Britain was surveyed without GPS. The British National Grid isn't perfect, there are some distortions (see this image - total deviation of the location).
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h_fenton
Joined:
22-10-2005
Messages: 105
from OXFORDSHIRE, UK
OFF-Line
| New Message Posted!2011-12-10 14:20
Quote:
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On 2011-12-09 22:11, tiompan wrote:
Quote:
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On 2011-12-09 20:42, h_fenton wrote:
On UK Ordnance Survey map a six figure grid reference eg SP 296 310 only makes reference to a square piece of ground 100metres x 100metres, an eight figure grid reference gives 10metres x 10metres.
see: http://mapzone.ordnancesurvey.co.uk/mapzone/PagesHomeworkHelp/docs/easypeasy.pdf
In my experience google earth cannot be relied upon for highly accurate positions, some areas it seems to be spot on, yet others consistently hundreds of metres off. If you look at historical imagery on google earth you may also find as you change the date of the imagery everything moves too.
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Google Earth uses WGS 84 . See above .
George
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sorry George, on the accuracy bit "consistently hundreds of metres off" I should have said in that instance I wasn't just referring to the UK
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sem
Joined:
12-11-2003
Messages: 1722
from Bridgend,S.Wales
OFF-Line
| New Message Posted!2011-12-09 23:46
Cap'n said
"Off the top of my head, from places where I checked, my Garmin gives reasonable position in WGS84 when I use latitude and longitude for display. but then change it to OS grid, and then we shift by magic to somewhere nearby. I guess it depends on where you are."
I'd guess it does Martyn.
Join us "Down Below" and experience some real magic!
Or real BS, depending on where you are.
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tiompan
Joined:
09-01-2005
Messages: 2707
OFF-Line
| New Message Posted!2011-12-09 22:11
Quote:
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On 2011-12-09 20:42, h_fenton wrote:
On UK Ordnance Survey map a six figure grid reference eg SP 296 310 only makes reference to a square piece of ground 100metres x 100metres, an eight figure grid reference gives 10metres x 10metres.
see: http://mapzone.ordnancesurvey.co.uk/mapzone/PagesHomeworkHelp/docs/easypeasy.pdf
In my experience google earth cannot be relied upon for highly accurate positions, some areas it seems to be spot on, yet others consistently hundreds of metres off. If you look at historical imagery on google earth you may also find as you change the date of the imagery everything moves too.
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Google Earth uses WGS 84 . See above .
George
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h_fenton
Joined:
22-10-2005
Messages: 105
from OXFORDSHIRE, UK
OFF-Line
| New Message Posted!2011-12-09 20:42
On UK Ordnance Survey map a six figure grid reference eg SP 296 310 only makes reference to a square piece of ground 100metres x 100metres, an eight figure grid reference gives 10metres x 10metres.
see: http://mapzone.ordnancesurvey.co.uk/mapzone/PagesHomeworkHelp/docs/easypeasy.pdf
In my experience google earth cannot be relied upon for highly accurate positions, some areas it seems to be spot on, yet others consistently hundreds of metres off. If you look at historical imagery on google earth you may also find as you change the date of the imagery everything moves too.
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tiompan
Joined:
09-01-2005
Messages: 2707
OFF-Line
| New Message Posted!2011-12-09 20:33
WGS 84 , (which is fine for a world wide accuracy )will differ by different amounts , depending on where you are in the UK (somewhere in the Atlantic they agree ) , with OS grid refs which are based on the OSGB 36 (which is fine for UK accuracy ) datum .
George
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TheCaptain
Joined:
30-10-2003
Messages: 1490
from near Bristol
OFF-Line
| New Message Posted!2011-12-09 19:24
Off the top of my head, from places where I checked, my Garmin gives reasonable position in WGS84 when I use latitude and longitude for display. but then change it to OS grid, and then we shift by magic to somewhere nearby. I guess it depends on where you are.
Nothing new here, its all down to peeling the orange off the shell of the non spherical globe. Something Magellan and Mercator struggled with hundreds of years ago. We get a bit more accurate, but still not perfect.
I take your point Sem, but its not something I'm going to lose any sleep over !
[ This message was edited by: TheCaptain on 2011-12-09 19:26 ]
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