NAD83/11

[Bruce Dawson]

I am wondering about the correlation between the different adjustments to NAD83 that are out there and what J-Field is using as my default, which is NAD83/11. Specifically, in my neck of the woods, there are lots of published control monuments that have Washington State North Zone State Plane coordinates in NAD83/91. I have been working with my system, using NAD83/11 in J-Field as the coordinate system, and am finding that checks to these published coordinates in NAD83/91 are about 0.1' off in Northings. I am putting my base unit on a published NAD83/91 coordinate, and checking other NAD83/91 monuments.

Is this typical? Do I need to use OPUS or something to establish new NAD83/11 coordinates for any monument I want to use, then work in that adjustment? Or can I use the existing 91 adjustment values and not worry about it? I know this might be splitting hairs again, but I want to start off right. I seem to recall that other RTK systems I have used have had the option of selecting different adjustments for each job.

Thanks for any input!

Bruce

 

[Shawn Billings]

If you are entering NAD83(91) coordinates into the base, and the base and rover are both set to the same reference frame, NAD83(2011), then the coordinates at the rover will be NAD83(91).

The 0.10' miss in Northings could be any number of things:

• Error in original control
• Local Displacement of monuments since being determined
• Error in Today's RTK work

Error sources in today's RTK work could be in the base setup (tribrach) or in the rover (level bubble, pole). If you are using Tilt Correction at the rover, it is absolutely critical that you do the level calibration and Level Offset calibration. If you are not using Tilt Correction, or you are not intending to use Tilt Correction, make sure Correct for Tilts is turned off.

You should be able to do much better than a tenth of a foot. You are right not to compromise on this. The system is capable of much, much better.

 

[Bruce Dawson]

Thanks Shawn, that helps. Yes, I have done a calibration, but do not have tilt on for those measurements. It's just one monument that I have checked, I will do some more to see what it looks like. Checked my hardware recently, all the bubbles look good.

 

[Kelly Bellis]

SX0084 FIRST HORIZONTAL, SECOND ORDER, CLASS 0 VERTICAL


Hi Bruce,

Shawn's got you all squared away it sounds like, but you've got me a little curious; what sources of control data are you consulting that are stuck using 24-year old reference systems? Have you, as an independent check, occupied any currently published control such as SX0084, and if so, are you still seeing a tenth of a foot bias in the northings?

The other element of your discovery that I'm curious about, where is your base located and by what means are corrections being received?

Regardless of those picky things, a tenth of a foot doesn't sound unexpected after looking at the effects of those 24 years and our incredibly dynamic planet (units are in meters):

PREDICTING DISPLACEMENTS BETWEEN TWO DATES
This is a pretty cool tool - have fun!

I am wondering about the correlation between the different adjustments to NAD83 that are out there and what J-Field is using as my default, which is NAD83/11. Specifically, in my neck of the woods, there are lots of published control monuments that have Washington State North Zone State Plane coordinates in NAD83/91. I have been working with my system, using NAD83/11 in J-Field as the coordinate system, and am finding that checks to these published coordinates in NAD83/91 are about 0.1' off in Northings. I am putting my base unit on a published NAD83/91 coordinate, and checking other NAD83/91 monuments.

Is this typical? Do I need to use OPUS or something to establish new NAD83/11 coordinates for any monument I want to use, then work in that adjustment? Or can I use the existing 91 adjustment values and not worry about it? I know this might be splitting hairs again, but I want to start off right. I seem to recall that other RTK systems I have used have had the option of selecting different adjustments for each job.

Thanks for any input!

Bruce

 

[Bruce Dawson]

Hi Kelly,

Well, there is a county wide control network done years ago that many surveyors use (me included) that is based in NAD83/91. Those are the published coordinates I am referring to. I occupied one of those monuments with my base unit and broadcast corrections to my Triumph LS via UHF. That monument is actually SX0085 which I think does have NAD83/11 values. I just used the county position to try things out. There is a second county control monument within the area about 1 mile away that I checked against and my RTK position was 0.1 out. I may try the same experiment but use the NAD83/11 position for SX0085 and see how SX0084 matches.

 

[Shawn Billings]

How long did you occupy the point, Bruce? Good open skies at both ends?

 

[Bruce Dawson]

Yeah Shawn, base was active for about 3 hours under wide open skies. I was tying in a bunch of other mons and the check tie was open as well. I did a 10 epoch occupation with at least 5 engines resolving.

 

[Shawn Billings]

Ten epochs might be a little short for a control shot. I'd expect better than a tenth but probably not a lot better. If you want to get all of the goodie out of RTK, I'd stay longer on a control point - like about 120-180 epochs. Two or three minutes may sound like a long time, but when you think of the time required to get to a control point and find the control point, two or three minutes is probably won't impact your productivity much. And, rather than scratching your head trying to figure out why a point is checking as well as you thought it should, your investment of a couple of minutes may save you more time later. From my experience, magical things start to happen at about 180 epochs. For soft topo point (dirt shots, edge of pavement, etc.) those ten second shots are fine.

 

[Bruce Dawson]

Yeah, I kind of wondered about that. Like I say, I was just playing with the gear to get familiar with stuff. Actually, getting as close as a tenth in that short occupation is pretty impressive!

 

[Kelly Bellis]

From my experience, magical things start to happen at about 180 epochs.

Is it one of the keys to the Stumpwater Portal?