Another thing that has intrigued me is that when we use conventional surveying equipment, points on the earth stay pretty much the same distance apart relative to each other. We measure from point to point with a total station and EDM and everything works and we are happy. Now we have GPS and our fixed reference points are not attached to the ground, per se. We have discovered (or better yet, realized and can account for) that the earth under our feet actually moves according to something called "tectonic plate movement." We have discovered that a point "here" right now was "over there" a few years ago. We have been able to measure the "direction," which is not linear, and "velocity" our tectonic plates move. Thank goodness, the distance between those "two" points is very small. However, did it really move at all? When points are relative to each other, we can use conventional and GPS to determine that yes, they are the same distance apart as they were twenty years ago. However, the GPS coordinates are slightly different.
So, we have yet another correction factor to add to our other correction factors to correct or account for those millimeters that .......the earth moved under our feet.
That sounded like a song...... o.O
I remember my professors talking about this circa 1997 ... from what I recall, the NGS also tracks "velocity" of CORS and HARN points. Honestly, as a person who uses GPS as a tool in boundary surveying, I've never cared much about it. As you said, for most type of surveying, it's more important to know the relative bearing/distance between two boundary markers, than it is to know the lat/long. But that said, I keep the metadata for each location so that if at some point in the future the lat/long does become critical, I suppose I could recreate that, too.
Check this out. It's the NGS tool to tracking the changes in position over time.
http://www.ngs.noaa.gov/TOOLS/Htdp/Htdp.shtml
> the NGS also tracks "velocity" of CORS and HARN points.
While it is true that the NGS "tracks" the CORS, the HARN/HPGN/FBN/CBN stations are NOT "tracked." In order to do that, the stations would have to be periodically re-occupied and adjusted.
Bear in mind...the vectors used to generate the NAD83(2007), NAD83(2011) etc., positions of these stations, are [were] in most cases observed years (sometimes decades) in the past. the velocities are computed using the HTDP "model" current at the time of the particular re-adjustment.
The HTDP link above contains a LOT of very good information about the nature and status of the HTDP Model, and how it can be used to predict (and/or retrodict) positions based on the limitations of the underlying velocity Model.
Like ALL models, it is not perfect, but it has come a LONG way in recent years.
Loyal
This is another reason that coordinates should not be allowed to control boundary - especially long term. For the most part annual movements are small - in my area I think the magnitude is on the order of 3mm per year. But over 100 years that translates to 300 mm or about a foot. Meta data is critical, but how often does correct meta data get passed along. And then NGS will no doubt redo things at least 4 times in the next 100 years. Glad I'm not going to be around.
The establishment and maintenance of title has a measurement component. In a sense, the PLSS is a coordinate system itself. The system is controlled in a manner that gives great weight to local control. Even metes and bounds are a series of polar coordinates controlled by nearby monuments. The common element being deference to controlling monuments.
Shifting to a purely coordinate based system would ravage private property rights. There is no practical mechanism for making the switch. I suspect coordinates will find their way into the order of calls. I see no path to the top...
Coordinates for boundary......Nooooooo!!!!!!!!
Southern California (W'ly of San Andreas Fault)...
about 2 inches a year NW.:-O
The reason we are still using NAD83 (whatever flavor) instead of WGS84 is that NAD83 is tied to the North American tectonic plate and WGS is world-wide. Most of the crustal motion that affects raw GPS coordinates in WGS84 is taken out when converting to NAD83, so most of the continent sees little change in NAD83 coordinates. Sorry, folks on the west coast.
An interesting philosophical question is: how do you define motion on a spheroid that has ll of its pieces moving in various directions? WGS84 has chosen to take some kind of average of stations around the world as their reference. While that's sort of arbitrary, there doesn't seem to be any better solution.
While I have some problems with your statement wrt WGS84, on the issue of accounting for differential plate motions see: http://www.earth.northwestern.edu/people/seth/demos/BRICK/brick.html
Do a search for Euler Poles for more info.
As for North America, there was (is?) an effort by the acronym SNARF which sought to implement a continental subset of ITRF (which is much closer related to NAD 83 than WGS). There are similar systems like EUREF and in South America (SIRGAS). SNARF is "stable North American reference frame.
On the general question of this thread, I am curious how boundary surveys account for issues of crustal changes? I understand that issues like accretion and meandering are addressed in the literature, how are earthquake events addressed?