Greenwich Meridian

[+Quasar]

On a recent holiday in England, I visited Greenwich, the home of longitude. It's an interesting place, and along with the historical exhibits, you can stand on the meridian line, and have your photo taken with one foot in either hemisphere.

 

I took my GPS along, to watch it zero out, but to my disappointment, when standing on the line, it said 000 deg 0.089 minutes West, or about 70m from the meridian. Is this to do with the datum? I was using WGS84 as usual. I tried a few other datums and the closest I could get was one called "EUR" (presumably european of some sort), which said I was 8m away - within the usual margin for error.

 

I had presumed that all datums would use the equator and greenwich meridian as an origin, so the line would be the same in all of them. Can anyone shed some light on this?

 

Glenn

[+Quasar]

I use a Magellan GPS315

[nv0u]

Check out this link. The main paragraph you want is near the bottom.

 

Essentially, you were standing at the Airy Transit, and the International Meridian is 102.5 meters east of that.

 

Here is a more in depth discussion of why it is "off" as well.

 

That's pretty cool information, actually... Thanks for asking that as I had no idea either!

[nv0u]

Check out this link. The main paragraph you want is near the bottom.

 

Essentially, you were standing at the Airy Transit, and the International Meridian is 102.5 meters east of that.

 

Here is a more in depth discussion of why it is "off" as well.

 

That's pretty cool information, actually... Thanks for asking that as I had no idea either!

[+Quasar]

Thanks for the info.

 

So they use measurements from quasars to determine the International Terrestrial Reference Frame for positioning! I knew I chose this name for a reason :-)

[Kerry.]

quote:

---

Originally posted by quasar:

 

I had presumed that all datums would use the equator and greenwich meridian as an origin, so the line would be the same in all of them. Can anyone shed some light on this?

---

 

The equator (as such 0 deg's) is a moving target as far as the datum is concerned. The equator "follows" the datum not the other way round.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[Kerry.]

quote:

---

Originally posted by quasar:

 

I had presumed that all datums would use the equator and greenwich meridian as an origin, so the line would be the same in all of them. Can anyone shed some light on this?

---

 

The equator (as such 0 deg's) is a moving target as far as the datum is concerned. The equator "follows" the datum not the other way round.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go