Multiple rod heights during a single observation

Jim White

Member
Is it possible change rod heights during an observation? In difficult environments, the height of the GPS can make a big difference in acquiring locks. Is there a way stop the observation, adjust the rod height, and then continue the observation into the same bucket groups? This would allow many more fixes to be added into the buckets for a better final solution.

I understand this may add complications to post processing, but for RTK solutions it would be a great advantage.
 

Adam

Well-Known Member
5PLS
It's not possible. You can however take multiple shots with different heights.
 

Jim White

Member
I think this is an important concept for Javad to consider.

When working in canopy, every fix is valuable in filling the buckets. Changing the rod height certainly can help gain more fixes in less time. Time is money. It is a shame to have reject a height with 4 or 5 fixes in order to try and find a better one.

From my observation, I assume that buckets are filled with measurements at either the ARP (or phase center of the GPS). However the user is measuring coordinates on the ground, and the the GPS being located a specific height above the point. If the buckets were filled with the ground coordinates instead of GPS receiver coordinates, then the rod height could be subtracted and all fixes could be utilized.

The user could start an observation at 6.00 feet and get a few fixes, stop it, adjust to 5.50 feet, resume it for a few more fixes, re-stop it, and continue the process until the enough fixes are gathered for a successful location.
 

Joe Paulin

Well-Known Member
Changing the rod height would surely ruin the static session and the ppk results, which many of us use to verify the rtk results. I for one, don't want to give up having the ppk results by changing the rod height part way through the session. Just my $0.02.
 

Jim White

Member
Joe, just because you have the option doesn't mean you need to use it. I typically redundantly RTK critical points as a check so more fixes in a shorter period is my goal..
 

Aaron S

Active Member
I'm not saying you shouldn't or can't, I'm just curious. I work in canopy almost exclusively, and never considered that a foot or two vertically on rod height would have much of an impact on getting fixes - maybe it does? I usually just set it as high a possible and hope for the best.

It seems like changing the rod height during observations would be like shooting on different datums simultaneously. I follow your logic, but I wonder how that would work mathematically.
 

Aaron S

Active Member
With the old Topcon gear, we used to slowly raise the antenna, then lower, til it got a fix.

I suppose if you were strictly doing RTK and not post-processing, you could move it up and down freely until you got the first fix, then lock it in place. Come to think of it, when I'm on a point and having trouble getting fixed, I'll spin the rover around until I get a fix, then don't move it. I think there's a guideline that says if the LS is facing a certain direction it gets fixes faster. But I can't remember off hand which direction, or even why that's the case.
 

Jim White

Member
I typically play with the rod height until I get a fix. For boundary work, I normally don't really care about the rod height, as it all gets flatten to a plane in the end, and the small changes wont affect the scale factor at all. I do track heights if I suspect that slope distances were originally used.

In terms of the math, its just like shooting a point with multiple rod heights. You subtract the rod height from each shot to get the elevation of that spot on the ground.

Just for clarification, this idea is for phase 1 only when you are collecting a series of discrete fixes. It probable cannot be done in phase 2.
 

Adam

Well-Known Member
5PLS
If you don't care about the vertical why not change the vertical compenent of the confidence gaurd and jack it up and down all you want? I don't agree with doing this but give it a try if you want. It may work. I haven't tried it. I never change the rod height as I don't see it making that much different in the difficult places I work. If I have good visibility down low thats where I put it. If I have good visibility a little higher than thats where it goes.
 

Nate The Surveyor

Well-Known Member
A note of caution: If I'm not mistaken, one of the metrics that is used in quality control is the vertical component.
Opening up the vert as Adam says above may foil, or prevent this component from functioning as designed, and may allow a bad shot through.
Just a thought.
N
 

Darren Clemons

Well-Known Member
I don’t adjust the rod height at all during a session, but as Nate mentioned when I had the ”old yeller” brand I did.... it that was, of course, an entirely different system and all we were waiting for was “one” fix. The entire design and setup on the LS is for it to basically remain stationary the entire session time to let it “figure things out”.
I do, however, turn the LS 45 to 90 degrees on the rover pole when it is struggling and very, very often that will help get a “pop”. I can’t explain how just a slight turn will do this, but I’ve seen it work way too many times to shrug it off as coincidence. It must be that the signals get a slightly different angle coming into the LS around the obstruction or the canopy that we work in.
 
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