Why doesn't my GPS say E/W 000 00.000 on the Prime Meridian Line at Greenwich?

Question

It reads E 000 00.079. But I really don't get why.

Anybody advise? Please reference your answer.

I know there is an actual reason for the difference, but I don't know what it is. It will read all zero's for everybody 102m from the line.

But why is it 102m out?

(oh, and not worried about the equator or the N/S coords)

GPS error & drift could account for 10-15m of variation.

But EVERYBODY gets 102m out.

Co-ord format is WGS84.

Answer 1

There is a reason, and it's not an error in the GPS.

The international reference meridian (used by GPS and other systems) is 102.5 metres to the east of the line marked at Greenwich. The reference longitude line is defined by the Bureau International de l'Heure, which compiled star & quasar observations in different countries. The average of this data caused a shift east away from the Prime Meridian at Greenwich, UK. The BIH calculated this average sometime before 1984; I don't know the exact history.

In other words, observatories around the world had determined their longitude from star observations and Greenwich time (not Greenwich longitude). When these longitudes were analyzed, it was found that there was some error with each one. The BIH added a common correction to each one, to make the average of the errors come out to zero; and the correction factor works out to 102.5 meters at Greenwich.

It was actually a bit more complicated than that, but that's the basic idea.

Answer 2

0° N by 0° E/W would be on the Prime Meridian Line south of Greenwich on the Equator. That position is in the Atlantic Ocean off the coast of Nigeria.

If all you are concerned about are the E-W coordinates (and not the N-S position) it looks like you are only a few meters away from 0. The coordinates you provide (E 000 00.079) have an error somewhere, as I can't tell if they are in decimals or d-m-s. You could try moving a few meters to the west until you get an exact zero, but you also need to consider your projection and the GPS error of a few tens of meters.

Hope that helps!

Answer 3

It's because the GPS system doesn't use the traditional Greenwich meridian as its reference. Due to variations in gravity, surface topography, and tectonic drift, traditional map making and navigation references don't provide an accurate-enough frame for current navigational needs. Particularly those of U.S. Dept. of Defense - who developed the GPS system.

As a result, the navigational reference frame chosen for GPS was the most current version of the World Geodetic System - WGS84. WGS was originally developed during the 1950s, with the first version (WGS60) finalized in 1960, to meet the needs of various space programs, civilian air and sea navigation, etc.

Under WGS84, Longitude 000 00.000 lies about 102 meters east of the traditional meridian line that runs through the old Royal Observatory at Greenwich. The spot is marked on the eastern pathway with a trash can - intentional or not?

Answer 4

Because of inaccuracies I suppose. The GPS signal from the satellite has an continuous changing error due to the atmosphere that's only corrected for once in a while. I think it's very hard to get exactly on the GPS meridian, because of this. As soon as you are, the GPS will show slightly different again. If you'd plot your GPS position while you are staying put in one place, you still can see yourself slightly moving over the chart, because of the continuous changing errors in the GPS system.
As far as I know the inaccuracy of the raw GPS signal is about 15 meters for civilian purposes. For military (USA only) use the inaccuracy is 5 meters. They don't want the enemy to have the same accuracy as them, and in times of war the GPS for civilian purposes can even be switched of completely. That's why the European Union still wants it's own satellite navigation system (Galileo), which has a very high accuracy, also for civilian purposes, but it seems they can't get the finances together to finish the project.

Answer 5

GPS coordinates have inherent error - they are not completely EXACT - unless you do some really specialized surveying. You may have not had a lot of satellites, your signal might have been weak, there might be a difference in your receiver and the other receivers.

Answer 6

Interesting topic!

Answer 7

Lots of great answers already for this