PDA

View Full Version : Greenwich prime meridian 95 m off?



rosenblut
08-06-2005, 10:48 PM
The International Meridian Conference
Washington DC, USA - October 1884:

At the Conference the following important principles were established:
It was desirable to adopt a single world meridian to replace the numerous one's already in existence.

The Meridian passing through the principal Transit Instrument at the Observatory at Greenwich was to be the 'initial meridian'.

But look for yourself
http://www.googleearthhacks.com/dlfile399/Greenwich-Royal-Observatory,-London.htm

The observatory is 95 m to the left of the prime meridian!?

I didnīt think that the continental shifting was that worse...

Greetings
Alex

Steeefan
08-06-2005, 11:19 PM
You are right, I also think that this is wrong. I was standing on the prime meridian ~6wks or some so I can confirm that it truely passes through the Royal Observatory

JBSurveyor
08-11-2005, 11:49 AM
Back to the "map datum" problem (see my other posts).
All coordinate systems are relative to their origin and the mathematical shape used to model the earth. This is commonly referred to as a map datum.
In the good old days before GPS a map datum would usually fit the shape of the country it was endemic to.
In the modern geodesy of geocentric (relative to the gravitational centre of the earth) datums eg WGS84 there is usually a significant difference between the two.
Hence data capured on one system and projected without transformation onto another will look a bit askew.
I assume that is the case with the Greenwich meridian in GE and the satellite image.
Somone at GE needs to do some "rubber sheeting".
Regards

ditkanbeter
12-17-2007, 03:04 PM
Watch this video on youtube: http://www.youtube.com/watch?v=oxL4RGqNEJs

The Prime Meridian, also known as the International Meridian or Greenwich Meridian, is the meridian (line of longitude) passing through the Royal Greenwich Observatory in London
It is the meridian at which longitude is defined to be 0 degrees, but is it?

This footage shows that according to a GPS device the real 0 degree longitude is 200 meters off.
Does anyone have anything to explain?

Marc

Captain Hornblower
12-17-2007, 04:02 PM
The meridian IS where the meridian is supposed to be. The video just shows, that this GPS device has an error of ~200 m for whatever reason.

ditkanbeter
12-25-2007, 10:28 PM
@Captain Hornblower:

What do you say about this image from goole earth then:
http://www.ditkanbeter.nl/images/greenwich.jpg
And GPS only has an average accuracy of 25 meters.

Even better, on the video it was almost exact and maybe only 2 meters off.

sladys
12-25-2007, 11:05 PM
The Captain is right what he says.

The grids in GE is only a guide and can vary from place to place. So the right line of the Meridian in GE should go through the building at the observatory. You see it on the photo.

The same you see if you look at the borders. The borderlines in GE aren't following the real places for most of them.

http://www.damer.com/pictures/travels/britain1981/uktravels/gmt-greenwich.jpg

Forkboy2
12-26-2007, 04:53 AM
@Captain Hornblower:

What do you say about this image from goole earth then:
http://www.ditkanbeter.nl/images/greenwich.jpg
And GPS only has an average accuracy of 25 meters.

Even better, on the video it was almost exact and maybe only 2 meters off.

There are 100s of different ways of defining Lat/Lon. Not all of them match up. As for the Prime Meridian, a quick search at Wikipedia revealed the following.

The zero meridian used by satellite navigation systems (on the WGS84 datum) is 102.5 metres (336.3 feet) to the east of the line marked at Greenwich.[1]. WGS84 uses the zero meridian as defined by the Bureau International de l'Heure,[2] which was defined by compilation of star observations in different countries. The mean of this data caused a shift of about 100 metres east away from the Prime Meridian at Greenwich, UK.[3] The plane of this geodetic meridian passes through the centre of the Earth, unlike the plane of the astronomical meridian which contains the direction of gravity (indicated by a plumb line) which points opposite to the direction of the zenith, to which astronomical instruments are aligned.

Captain Hornblower
12-26-2007, 07:33 AM
*thumbup*
nothing to add...