Vertical discrepancies - Construction Staking

avoidthelloyd

Active Member
Calling all guys who like to figure things out.

Pardon my ignorance here, but I want to get some feedback from you all regarding a situation I had a couple days ago. I have done a LOT of construction staking with my LS+ in past 8 years. Just so everyone knows, it has been banged up some and sent to the shop a few times, etc. As you know, when you visit your construction sites, you check in to your control points to make sure you are good to go before staking. I do not recall having any issues like this in the past on previous jobs using my LS.

On this trip, I tied into the control with about 0.08' elevation bust. I found that the tip of my rod was dull and after re-measuring my rod, it fixed most of that. This was about 10am in Oklahoma on Wednesday. I set the building corners to FFE and some other items requested. After I left and got to my next stop, I was called and told that my elevations were off according to the dirt guys.

I returned and used the same setup. It turns out that the dirt guys had a bad FFE elevation in their model so our fills were not gonna match, BUT when I shot the same corners I had just set about 2 hours previously, my elevations were about 0.16' different now? About 12:30pm.

I had a row of trees south of my base at about 45 vertical degrees. I went back and added a meter pole under my base to increase the sky view. I went back, tied the control again with about 0.08' off vertical and saw the same results.

I moved my T3 base to another control point and rechecked at about 3pm and I was almost 0.20' vertical different from my initial staking. I did change rods here, but remeasured the HR. This was pretty consistent across all the nails I set that morning.

Like I said, I have not experienced this scenario before and am very careful and routine in my setups and HI/HR vertical measurements. My question is, am I expecting too much from my LS? Why haven't I had this much discrepancy in the past? I use simple manually calculated points to stake to.

It's wide open sky view and I used both precise topo and boundary profiles and got the same elevations. I'd appreciate any insight, thoughts or feedback.
 

Matt Johnson

Well-Known Member
5PLS
Are you aligning both the base and rover to north when staking the points? If not it will reduce the accuracy and precision significantly.

How do you have the RTK engines configured? I would expect the best accuracy if each engine is using all 4 available constellations with 2 signals per constellation.
 

avoidthelloyd

Active Member
Are you aligning both the base and rover to north when staking the points? If not it will reduce the accuracy and precision significantly.

How do you have the RTK engines configured? I would expect the best accuracy if each engine is using all 4 available constellations with 2 signals per constellation.
I have not heard of the alignment causing that much difference in results? All 4 are being used.

Can you share a link to the details on the alignment issue? I appreciate you.
 

Mark Wheeler

Active Member
Matt, I understand there is a slight offset in the instrument. What are the reasons that this would affect rover position and elevation significantly more than the really minute offset? Every brand stresses this, but I am curious to know why and what sort of precision issues you have experienced?

Mark
 

Matt Johnson

Well-Known Member
5PLS
The antenna calibration models used are absolute antenna calibrations meaning that they have calibration data for both the azimuth and elevation angle so for this to be applied correctly the antenna need to be aligned to north. Here is the antenna calibration data for the LS: https://www.ngs.noaa.gov/ANTCAL/LoadFile?file=JAVTRIUMPH_LSPA_NONE.atx

I did some testing of this a long time ago and found it introduce more error and degrade precision but I have not done any recent testing to check magnitude of error introduced by not aligning the receivers to north. This is a fairly simple test to do with LS and its ability to automatically collect points over and over.

For the LS, the screen should face north. For the T-3, the side with the buttons should face north.

All 4 are being used.
Are you using Auto Setup RTK for configuring the RTK engines manually. The Auto Configure does not configure each engine to use all the constellations. I always recommend the engines be manually configured.
 
Top