Elevation problems

R

R.Savich

New Member
I'm using a 1M on the UHF antenna and a fixed height 2m rod for my base. My rover is an LS. I've had this setup for about 2 months, but this is the first time I've needed the elevations for anything. I'm using a low-distortion county coordinate system.

I set four control points around a very small site, traversed between them, leveled between them, and then located all of them with rtk. All points have pretty good sky, but the point where the base was set up is completely open. The base collected data for 534 minutes. I took three shots on each of the other three control points, with just over 180 seconds of data on each shot. I sent the base file to DPOS for post-processing.

After lifting my traverse points to the base, I was surprised to find vertical differences between my rtk elevations and my leveled/traversed elevations of 0.14 feet, 0.18 feet, and 0.24 feet. I'm sure this is operator error. I checked obvious things like my rod height, but I'm not sure where to start looking for the problem. Any ideas? I'm happy to upload some raw data, but I need some guidance about what would be helpful.

Thank you.
 
Matthew D. Sibole

Matthew D. Sibole

Well-Known Member
5PLS
Did you have your LS and 1M oriented to north?

Also I suggest if you need tight elevations to shoot the points multiple times and average those coordinates with cluster average
 
Shawn Billings

Shawn Billings

Shawn Billings
5PLS
Are you storing screen captures with collected points? If so, can you share them here? The graphs may help make some sense of it all.
 
Nate The Surveyor

Nate The Surveyor

Well-Known Member
Was it high on one side of the project, and low on the other side? Or, was it random?
Also, how far apart are the ctrl points?
Thank you,
Nate
 
R

R.Savich

New Member
Matthew D. Sibole said:
Did you have your LS and 1M oriented to north?

Also I suggest if you need tight elevations to shoot the points multiple times and average those coordinates with cluster average
Click to expand...
I probably did not have the LS and 1M oriented to north. I did shoot the points multiple times and cluster average. Each shot was approximately 180 seconds and I shot each point three times.

I don't really need tight elevations because I am holding the elevations from my level loop. This is just the first time that I've located control points with the LS, traversed through them, and then run the level loop through them. I was so surprised by the vertical difference that I started to worry I might have a setting wrong somewhere.

In the future, I will orient everything north. Do you think that the elevation I got with the base is good even if I wasn't oriented north? I actually had the base set up two different days and the vertical difference was only 0.02'. I have no idea what direction the base was oriented either day.
 
Nate The Surveyor

Nate The Surveyor

Well-Known Member
Also, do you have a closed level loop? Or, is this a series of side-shots? Was this done with trig leveling techniques? If so, was each line shot from BOTH ends, or just one?
(I'm fishing for possible problems).
I once had a theodolite, with a broken vertical compensator. It was random, sticking in various ways.
N
 
Nate The Surveyor

Nate The Surveyor

Well-Known Member
Ok... All high, and I take it, this is relative to the base? So, in fact, a bad HI at the base could explain it?, and, the high points, all basicly agreed with each other, and the outlier is the base?
I'm trying to help... Don't take me as trying to criticize. Error trapping is a mechanical procedure, where all the assumptions are reviewed. One thing you can do between the base, and LS, is put the LS at a very short rod height, and shoot the base, under the base tripod legs.... To help trap the error, or confirm things.
N
 
R

R.Savich

New Member
I very much appreciate your fishing for problems! I'm sure I've messed something up. My level loop was closed with a regular auto level (not a fancy one). I also ran a closed traverse through the points with a conventional total station (I did set up on each point).
 
R

R.Savich

New Member
Yes, I think a bad HI at the base would explain it. That's primarily why I'm so worried about it. I adjusted everything to the base, so I would like it to be correct. Don't worry- I don't feel that you are criticizing. I need some help with tracking down this error- and that means questioning everything!

I can't shoot the base under the tripod legs because I'm using a fixed height rod at the base. I've had the base set up on the same point on two different days. I used the same fixed height rod both days, and I got basically (within 0.02') the same elevation both days. Now that I know I have a problem, I wish I had set the base up on a different point on day two. I also wish that I had shot the control points again on day two. It looks like there will be a day three- hopefully I can track the problem down.

I think the most likely issue is that I have a setting wrong in my base that is messing up the antenna height and causing me to get a bad result for the base elevation.
 
Nate The Surveyor

Nate The Surveyor

Well-Known Member
Ok... There are offsets (Like prism offsets in a total station) on the rod heights. Since you are running a 2m rod at the base, then I take it you have the t1m on top of that. My suspect is that the base offset dist is your culprit. It how javad compensates for having the antenna under the t1m. (Base radio antenna) check carefully this item.
A quick dirty check would be to use a 2m pole under the LS, and lean it, with the tip on the base nail/monument. I know it's a slant, but you can compute this. This would tell you what is the problem... The gps, or other.
Quick and dirty can be your friend.
N
 
R

R.Savich

New Member
That seems like an excellent theory. I just looked at the quick start guide that came with the 1M again, and I think it is very likely that I misinterpreted the reference dimension diagram. I will try what you suggested. I'm also going to double-check a bunch of other things. I'll report back. Thanks for all of the great ideas!
 
R

R.Savich

New Member
I put it in as vertical, but the height I used is essentially the same as my slant height. Now I think that if I use a vertical height, I'm supposed to measure to the ARP (antenna reference point), which is 54.4mm lower than the Slant Height Measurement Point. Please correct me if I'm still misinterpreting the diagram. If I raise everything (except rtk shots) by 54.4mm, it appears to solve my problem. You can be sure that I'll be double-checking everything in the field, but it sure seems like Nate figured it out. Assuming he's right- I'm happy to have such an easy solution and sorry for not figuring it out on my own. Thanks again to everyone for the help!
 
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