3 ms·

If GCS84 has the zero point 102m east, what are they using as a reference from which to determine that? How do they choose the position of their zero line? I’d

by simonh 5y ago

If GCS84 has the zero point 102m east, what are they using as a reference from which to determine that? How do they choose the position of their zero line? I’d have thought the zero line would be the reference from with other points would be determined. Unless they used the international date line as the reference perhaps.

jandrewrogers 5y ago
As I recall, the WGS84 meridian is registered to the center of the Earth from space, whereas the classic Greenwich meridian is essentially registered to the local direction of gravity at a point on the surface. If the local direction of gravity passed through the center of the Earth then the meridians would be the same. However, the Earth's gravity is non-uniform which causes the local direction to be slightly deflected away from a path that would pass through the center of the Earth as is the case in Greenwich. The WGS84 prime meridian is parallel to the Greenwich meridian but slightly shifted so that it actually passes through the center of the Earth. Because WGS84 meridian is not registered to a point on the surface (for good reason), it was always going to drift away from the Greenwich meridian over time regardless.
walnutclosefarm 5y ago
The actual explanation is very involved, but it boils down to the fact that due to gravitational anomalies in the area around Greenwich, the "straight up" vertical that was used to establish the Greenwich meridian as the origin for universal time does not pass through the center of mass of the earth. The coordinate system used by GPS (WGS84) has the center of mass of the earth as it's origin. The plane that defines the reference meridian (the 0th meridian that is) in that system has to be parallel to the one used to establish universal time, in order to maintain UT consistency. So, when you extend the plane of the WGS84 coordinate system from the center of mass of the earth, parallel to the vertical used at Greenwich, which, again, does not point exactly at the center of the earth, the plane cuts the earth's surface 102 meters East of the original 0th meridian.
- simonh 5y ago
  This is why I love HN, thanks (and jandrewrogers).

by simonh 5y ago

jandrewrogers 5y ago

As I recall, the WGS84 meridian is registered to the center of the Earth from space, whereas the classic Greenwich meridian is essentially registered to the local direction of gravity at a point on the surface. If the local direction of gravity passed through the center of the Earth then the meridians would be the same. However, the Earth's gravity is non-uniform which causes the local direction to be slightly deflected away from a path that would pass through the center of the Earth as is the case in Greenwich. The WGS84 prime meridian is parallel to the Greenwich meridian but slightly shifted so that it actually passes through the center of the Earth. Because WGS84 meridian is not registered to a point on the surface (for good reason), it was always going to drift away from the Greenwich meridian over time regardless.

walnutclosefarm 5y ago

The actual explanation is very involved, but it boils down to the fact that due to gravitational anomalies in the area around Greenwich, the "straight up" vertical that was used to establish the Greenwich meridian as the origin for universal time does not pass through the center of mass of the earth. The coordinate system used by GPS (WGS84) has the center of mass of the earth as it's origin. The plane that defines the reference meridian (the 0th meridian that is) in that system has to be parallel to the one used to establish universal time, in order to maintain UT consistency. So, when you extend the plane of the WGS84 coordinate system from the center of mass of the earth, parallel to the vertical used at Greenwich, which, again, does not point exactly at the center of the earth, the plane cuts the earth's surface 102 meters East of the original 0th meridian.

simonh 5y ago
This is why I love HN, thanks (and jandrewrogers).