Following a GPS survey

Remarkable repeatability as long as you have good sky. A lot shakier when there are obstructions. But still, often, very good.

Must be nice. One of the issues I'm running into with GPS derived survey work I did a decade ago is the areas I work in is somewhat in a state of geological flux. In one area the amount of movement over the last 8-10 years is negligible and a mile way I'm detecting movement up to half a foot or more. Do I hold the decade old refence frame or if I'm expanding it out do I reference it to the current and just let it float? It's like hitting a target moving in slow motion in random directions. Where I hit the bullseye a decade ago, now I'm hardly on the paper.

Sounds like you and the other surveyor have great procedures and he obviously gave you the information needed to follow him aka scale etc. Which is how it should be done. Meta data good procedures. Etc. Way to go. Awesome to hear things like this for sure.

We see movement also. I did a thread a while back about some that we see since we have GPS. I don't have a good answer for what to hold. I set a brass cap at the SW corner of a township in 1979. We have located the Brass Cap 4 times with GPS since 2000 and we are seeing a 1' per 10 year movement. So where was the monument in 1979, and where was it in 1881?

Difficult to answer. There isn't anywhere close to witness it. It would have to be over 600' in any direction since the entire local area is moving.

Since GPS everything has tightened up. I keep hearing on this board about horror stories. I don't see them and I work in a 400 mile radius of the office. It's trivial to use GPS compared to the equipment I started with. The amount of effort it takes to apply GPS to a survey compared to a T2, chain, distance meter, NGS monuments, solar observations, ect is maybe 10:1 easier. Add to the field equipment the computer advantages and it's night and day easier.

So true. Much easier. All one needs to do is take a little time and gain the understanding of GPS and how whatever particular manufacturer handles it’s approach and you can do some very quality work in a short time for sure. I am still showing the numbers to my boss. He made a crew run a level loop guess what. All the control that they ran levels through we hit with base and rover around.03 ft or better. All for a topo on swampy ground. For something that a half a foot would have been truthfully good enough. Good procedures. Understand its limits Redundancy and the time to run a mile worth of levels was so unnecessary for that job. Know when rtk vrs or rtk base and rover or traverse or static work is needed and not needed. Knowing when a tack hammer or sledge is needed is half the battle.

"So where was the monument in 1979, and where was it in 1881?"

It hasn't moved at all. It is still at THE CORNER. Simple as.

Do I hold the decade old refence frame or if I’m expanding it out do I reference it to the current and just let it float? It’s like hitting a target moving in slow motion in random directions. Where I hit the bullseye a decade ago, now I’m hardly on the paper.

We're lucky here in the PNW that movement is (mostly) all in the same direction.

I'm really curious to see how well the Intra-Frame Velocity Model works when the new datums drop. I do remember how much stuff would move up in AK. Saw at least two deep-rod mons drive to refusal get frostjacked by ~0.4' in a single year, and some pretty good horizontal movement over the relatively short time I lived there.

I'm also looking forward to being able to use the actual survey epoch for my values and it not being viewed as utter heresy. I did the survey now, not 13 years ago - how about I reference where the mons are now?

I just completed a two-section breakdown in some nasty woods - a logging area with old growth throughout ~90% of our project area. We nailed everything down with lots of RTK under the trees.

I had a HARN (1991 epoch) value for one of our section mons, from a ~2001 DNR survey....for kicks I used HTDP and NCAT to transform the adjusted position of a section corner back to HARN, and got about 0.04' of difference. When procedures are good, around here the tools work pretty darn well.

@rover83 Under canopy with today’s gnss is amazing even doing rtk. When i was in the mountains and down in a valley I had set up to get on datum but as they were logging I mapped a road which was in open for that area set some control to come off of in different areas for the traverse set some points for like you say fun in thick old mature hardwoods. All base and rover rtk R12 R12i. When we traversed around and through all the control traverse kits good procedures 4 rounds direct and reverse. Cross tied the control. The next afternoon i had moved the base and wanted to get another opus just to have a check. I went ahead and re located all my control as I was done except for some sketches and pictures I had my helper doing. Those points in the trees on side of mountains were not a few hundredths difference at most in direct distance and most came out to be so minute after adjustment i could have never broke out the robot. I think redundancy and time gap difference base set up and let it do what it does. Flat out blows my mind what can be achieved. I still have not convinced our other offices as they still set two pairs locating a few times and then traverse. Of course they scale to ground every time so scale factor 1.000 for robot state plane gnss two different job files every day. I am like just use one job file for each day and scale it later. Compass rule only.

In my experience, the horror stories are a consequence of having no %&$*@*$ clue what one is doing and no practical understanding of the fundamentals of the technologies being employed.

You need better surveyors in your area.

hmmmmm, not a disturbed monument then?

This thread began by extolling the virtues of GPS measurements. No problem there. It then evolved into using measurements to prove monument instability. This is the devils playground. One of the first things I was taught is the corner is where you find it. Monuments are not constructed to resist surface motion and those who depend on them for boundary location should not be subject to an abstract measurement to overturn its current location. If that were true there would be no need for monuments.

It then evolved into using measurements to prove monument instability.

I didn't see anyone advocating for not accepting monuments due to local surface movement, it seems to be more of a discussion about how much easier it is to detect the movement than it used to be.

I agree. The R12i can provide highly accurate values under normally impossible canopy. Of course, you still need to conduct proper redundancy and time gaps in GPS ties to be certain. But that's a heck of alot quicker than traversing through those arees. We use one controller and job per project, whether it's convention or GPS or the more common GPS/conventional job. Once the zone and SF are set we create a project template in TBC and use if for ALL jobs on that project.

From a practical stand point for me it comes down to awareness that change is a constant and no measurement is absolute, just a snapshot in time. I'm certainly not going to put myself in the position of trying to make the argument with a layman that because his monument has drifted any distance over time, that it is any less valid in determining where his boundaries are or should be. Early in my career on one my first real high profile surveys, a family was squabbling over their deceased father's land holdings. Everyone wanted that coveted bluff top lot with magnificent views and they'd been fighting over it for years. I was tasked with retracing the original 1950 BLM survey that created it. Long story short, the bluff had eroded back more than a 100' over the subsequent years and only 15 square feet of that lot remained. Needless to say, didn't go over well. The entire family ripped me a new one and I had to tell them simply, 'Look, I just measure land, I don't make it and I don't make it go away. You're going to need to take this one up with a higher power than me.'

Not that moving monuments were really my intention starting this thread, but many should remember the BLM seminar case that made the rounds dealing with a moving 1/4 corner and their solution (which I totally disagreed with). The monument was moving quite a distance and they had "experts" testify about soil conditions and such.