Home PageFacebookRSS News Feed
PocketGPS
Web
SatNav,GPS,Navigation
Pocket GPS World - SatNavs | GPS | Speed Cameras: Forums

Pocket GPS World :: View topic - Why does GPS show the Prime Meridian in the wrong place?
 Forum FAQForum FAQ   SearchSearch   UsergroupsUsergroups   ProfileProfile   Log in for private messagesLog in for private messages   Log inLog in 

Why does GPS show the Prime Meridian in the wrong place?

 
Post new topic   Reply to topic    Pocket GPS World Forum Index -> News And Latest Information
View previous topic :: View next topic  
Author Message
Darren
Pocket GPS Staff
Pocket GPS Staff


Joined: 11/07/2002 14:36:40
Posts: 23848
Location: Hampshire, UK

PostPosted: Thu Aug 20, 2015 10:49 am    Post subject: Why does GPS show the Prime Meridian in the wrong place? Reply with quote

pocketgpsworld.com
Sticking with the subject of maps and datums, a frequently asked question is why GPS does not show the Prime Meridian at Greenwich as zero? This issue also occurs in Google Maps, with both claiming the Meridian is about 100m to the east. But why?

The differences occur because when the longitude was first calculated, the Earth was assumed to be perfectly round whereas it's actually an oblate spheroid, which means it's fatter at the equator.

The error that resulted from this went unnoticed until the late 1960's when the first satellites were launched. A receiver was mounted on the roof of the Greenwich Observatory and calculations showed that an offset of 5.54" existed thanks to the more accurate calculations that were possible. This offset has since been incorporated into the WGS84 datum.

It's largely elementary now unless you're a tourist in Greenwhich park, or a guide at the observatory!

Source: Royal Observatory

email icon

_________________
Darren Griffin - Editor
Back to top
View user's profile Send private message Send e-mail Visit poster's website
yealmgps
Occasional Visitor


Joined: Sep 06, 2004
Posts: 9

PostPosted: Fri Aug 21, 2015 8:31 am    Post subject: Prime Meridian Reply with quote

I was at Greenwich a few months ago and asked the Guide about the offset in the Prime Meridian and he could not explain it!
Back to top
View user's profile Send private message
pdfbt40
Occasional Visitor


Joined: Jan 23, 2008
Posts: 56

PostPosted: Fri Aug 21, 2015 8:43 am    Post subject: Reply with quote

Its actually a bit more complex and political if I read the story correctly.
As more work was done in 1920s to calculate longitudinal references in America, the International Reference Datum was taken from Maryland Observatory, not Greenwich, naturally.
Now with greater GPS and topographical data, a back calculation from Maryland to Greenwich moves the 0 degree latitude from the defuncted meridian by ~100m
Back to top
View user's profile Send private message
pillboxman
Lifetime Member


Joined: Oct 15, 2005
Posts: 123
Location: Somerset, England

PostPosted: Fri Aug 21, 2015 11:31 am    Post subject: Reply with quote

OS False Origin is based near the Isles of Scilly, at 49 45" 58' N: 7 33" 23' W.

If the meridian is wrong how does this effect OS maps (if it does)?

John
Back to top
View user's profile Send private message Send e-mail
challow
Occasional Visitor


Joined: Apr 12, 2007
Posts: 1

PostPosted: Fri Aug 21, 2015 11:41 am    Post subject: Reply with quote

But the best part must be, that the French are still very unhappy that London was chosen over Paris, in a world wide vote in 1864. I fact they were so off that they stubbornly refused to vote and continued to use their meridian for a further 25 years! Eventually capitulating in 1909.

Then in 1994 they had 135 bronze medallions struck and placed in the pavements of Paris, marking their meridian and even as late as 2000 they were still at when they began planting a 600 mile long line of trees making the French meridian through the whole of France.

I suppose you have to admire their misplaced resolve/stupidity in continuing to push a meridian that no one uses anymore, including them Very Happy
Back to top
View user's profile Send private message
pdfbt40
Occasional Visitor


Joined: Jan 23, 2008
Posts: 56

PostPosted: Fri Aug 21, 2015 11:55 am    Post subject: Reply with quote

If its true, I'll see if I can find the article, isn't it also typical of the Americans to steal the International Datum just because their doing the further work and paying for it.

I suppose we should have sent in the gunboats and forced them to move the Datum by 100m to match OUR meridian.
Back to top
View user's profile Send private message
pdfbt40
Occasional Visitor


Joined: Jan 23, 2008
Posts: 56

PostPosted: Fri Aug 21, 2015 12:08 pm    Post subject: Reply with quote

http://www.thegreenwichmeridian.org/tgm/articles.php?article=7
Back to top
View user's profile Send private message
pillboxman
Lifetime Member


Joined: Oct 15, 2005
Posts: 123
Location: Somerset, England

PostPosted: Fri Aug 21, 2015 12:28 pm    Post subject: Reply with quote

Thanks pdfbt40

If I am reading that web page correctly it seems to suggest that it makes no actual difference as all the references are to a false zero anyway.

John
Back to top
View user's profile Send private message Send e-mail
pdfbt40
Occasional Visitor


Joined: Jan 23, 2008
Posts: 56

PostPosted: Fri Aug 21, 2015 1:20 pm    Post subject: Reply with quote

Well yes. As man has known from before Mercator, even to represent a perfect sphere on a flat piece of parchment as a map was bound to be inaccurate. An imperfect, knobbly ellipsoidal sphere is even worse, even if you swap to a 2D grid system. Hence individual datum points and grid systems.
But now we have 3D global data sets, it should be possible to have a single origin. Nationalism, as always, means we can't.
Back to top
View user's profile Send private message
M8TJT
The Other Tired Old Man
The Other Tired Old Man


Joined: Apr 04, 2006
Posts: 10118
Location: Bexhill, South Sussex, UK

PostPosted: Fri Aug 21, 2015 1:44 pm    Post subject: Reply with quote

pdfbt40 wrote:
I suppose we should have sent in the gunboats and forced them to ......
We did in 1775. And lost.
Back to top
View user's profile Send private message
pdfbt40
Occasional Visitor


Joined: Jan 23, 2008
Posts: 56

PostPosted: Fri Aug 21, 2015 5:57 pm    Post subject: Reply with quote

M8TJT wrote:
pdfbt40 wrote:
I suppose we should have sent in the gunboats and forced them to ......
We did in 1775. And lost.


But worked in mid-1800 when we protected our illegal drug running......
Back to top
View user's profile Send private message
Lincslad
Lifetime Member


Joined: Oct 09, 2004
Posts: 20

PostPosted: Fri Aug 21, 2015 6:18 pm    Post subject: Reply with quote

I am also fatter at the equator nowadays. Confused
_________________
TomTom XL V4 Europe
Back to top
View user's profile Send private message
JimmyTheHand
Frequent Visitor


Joined: Apr 16, 2005
Posts: 386

PostPosted: Sat Aug 22, 2015 9:50 am    Post subject: Reply with quote

pillboxman wrote:
OS False Origin is based near the Isles of Scilly, at 49 45" 58' N: 7 33" 23' W.

If the meridian is wrong how does this effect OS maps (if it does)?

John


It doesn't impact OS maps - they are based on the OSGB36 datum (Ordnance Survey Great Britain 1936, based on the Airy 1830 ellipsoid) which I believe is a better fit for UK than WGS84 used by the GPS system.

My first hand held GPS (Garmin GPS 38 - and if you thought the phone is slow to lock on) allows you to set world models and co-ordinates systems separately - in which case you could be 100+ metres out if you had wrong World model set. Modern Units generally don't allow to make this mistake for grid references
_________________
J.
Back to top
View user's profile Send private message
MaFt
Pocket GPS Staff
Pocket GPS Staff


Joined: Aug 31, 2005
Posts: 14843
Location: Bradford, West Yorkshire

PostPosted: Mon Aug 24, 2015 11:49 am    Post subject: Reply with quote

I quite like how the affable Tom Scott puts it in his "Thing You Might Not Know" series: https://www.youtube.com/watch?v=DmvHZ4omB2A

MaFt
Back to top
View user's profile Send private message Visit poster's website
pdfbt40
Occasional Visitor


Joined: Jan 23, 2008
Posts: 56

PostPosted: Mon Aug 24, 2015 2:39 pm    Post subject: Reply with quote

No holding back then - Despite lots of different systems, and International conferences and agreements he didn't mention (fortunately), The Americans stole the reference global meridian !

And even then, I've never found a decent USA road map OR someone intelligent enough to explain house street numbering!
Back to top
View user's profile Send private message







Posted: Today    Post subject: Pocket GPS Advertising

Back to top
Display posts from previous:   
Post new topic   Reply to topic    Pocket GPS World Forum Index -> News And Latest Information All times are GMT + 1 Hour
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum

Powered by phpBB 2.0.11 © 2001 phpBB Group
phpBB port v2.1 based on Tom Nitzschner's phpbb2.0.6 upgraded to phpBB 2.0.4 standalone was developed and tested by:
ArtificialIntel, ChatServ, mikem,
sixonetonoffun and Paul Laudanski (aka Zhen-Xjell).

Version 2.1 by Nuke Cops 2003 http://www.nukecops.com

Make a Donation



CamerAlert Database

Click here for the PocketGPSWorld.com Speed Camera Database

Download Speed Camera Database
20.115 (30 Nov 22)



WORLDWIDE SPEED CAMERA SPOTTERS WANTED!

Click here to submit camera positions to the PocketGPSWorld.com Speed Camera Database


12mth Subscriber memberships awarded every week for verified new camera reports!

Submit Speed Camera Locations Now


CamerAlert Apps



iOS QR Code






Android QR Code







Terms & Privacy


GPS Shopping