Are lines of Latitude and Longitude on exactly the same points as 100 years ago?

[spaceboy0]

I notice on GoogleEarth that the Canada - U.S. boundary is not exactly on 49degrees. In fact, south of Vancouver the Americans are cheating by about 800 feet! At Blaine, Washinton the boundary is actually a bit north of exactly 49 degrees.

I confirmed these measuremenets one day with my GPS receiver.

Further east of Vancouver the boundary approaches 49 degrees and then wavers slightly north and south of 49 as you move east all the way out to Ontario where of course is diverges south, but that divergence out there is by design and political convention.

First off, when they built the Peace Arch Monument south of Vancouver 100 years ago or so, how were they able to determine or guesstimate where exactly the 49th parallel was?

There was no GPS back then and Sextants couldn't have been that accurate.

I guess they mismeasured. They wanted to be spot on 49 but instead were off by 800 feet!

Is the fact the Earth wobbles and the North and South Poles wobble around the reason why particular points on the Earth change their Latitudes and Longitudes over many decades?

I would think the North Pole and South Pole are in exact spots that don't shift around irregardless of any wobble effect, and that the lines of Latitude and Longitude on the Earth never change position!

I am aware of the shift of the Earth's north and south magnetic poles. However I'm trying to understand teh shift of the geographic poles.

I had heard that at the South Pole the exact point of the Pole isn't where it was when the first Station was set up there 40 years ago...

[grant hutchison]

There's a thing called True Polar Wander: the globe of the Earth is shifting a little relative to the rotation poles: it amounts to a few metres in a century. And continental drift moves the continents around relative to the poles and Greenwich meridian: again, at a few metres per century.

If memory serves, the movement at the south pole involves the polar marker moving with the ice of the polar plateau, which carries it away from the rotation axis.

Grant Hutchison

[spaceboy0]

So the fact that the Peace Arch border marker is 800 feet north of true 49 is more a result of the lack of precise measuring instruments back 150 years ago when they built it rather than any effect of continental drift?

[Delvo]

That stuff alone isn't enough to explain what you describe. Two straight lines intersect only once at most, with the distance between them consistently increasing farther away from the intersection point. So any waving back and forth with fluctuating distances and reversing orientations (which one's on which side of the other) means one of these lines is not merely misplaced, but also not straight...

[neilzero]

By specifing exactly, the answer is no, but the shift likely is typically one inch per century. We improve accuracy with a transiet, by repeating measurements many times and averaging the readings which cluster, and add appropriate finagle factors, such as the movement of tetonic plates. Civilian GPS is not much more accurate.
A very large telescope, might make a very accurate sextant. Neil

[mugaliens]

Plate tectonics! The Earth's crust moves! Over time, even rock can bend without breaking.

GPS coordinates are based on what's known as the World Geodetic System 84 Datum (http://en.wikipedia.org/wiki/WGS_84).

[geonuc]

Plate tectonics! The Earth's crust moves!

That was mentioned. But it doesn't nearly account for 800 feet movement in that short time period.

[Drunk Vegan]

Why isn't important. What's important is - HONOR!

Reclaim that 800 feet.... for Queen and country! Or some such.

[spaceboy0]

The U.S. Customs building through which cars have to be cleared to enter the U.S. is right on 49 degrees 00 minutes.

It's the big white Peace Arch Monument and two side markers that are about 800 feet north of that 49 degrees 00 minutes point.

You can see the white Peace Arch Monument on GoogleEarth; check it out for yourself, just follow Interstate 5 up to the Canadian Border (Blaine, Washington). Note the U.S. Customs building (looks kinda greenish) is the one right on the 49 degree 00 minutes mark and that building was I believe built back in the 1950s.

[BioSci]

From wikipedia: http://en.wikipedia.org/wiki/49th_parallel_north

Parts of the 49th parallel were originally surveyed using astronomical techniques that did not take into account slight departures of the Earth's shape from a simple ellipsoid, and the surveys were subject to the limitations of early to mid 19th-century technology. As a result, in some places the surveyed 49th parallel is as much as several hundred feet from the actual geographical 49th parallel for the currently adopted datum, WGS84. The Digital Chart of the World (DCW), which uses the Clarke 1866 ellipsoid, reports the border on average at latitude 48° 59′ 51″ north, roughly 270 m (290 yd) south of the modern 49th parallel. It ranges between 48° 59′ 25″ and 49° 0′ 10″ north, respectively 810 m (885 yd) and 590 m (645 yd) on either side of the average.