NAD 83???

Possibly a US Foot / Int'l foot thing?

I'm with Mark. Most zones have a large false easting. Switching units shows up as an east west shift...

5' is about par for the course in "sub meter" gis grade units in the woods from what I have seen....

It said US feet in the settings which is what our zone uses. These checks I'm talking about were made in pretty wide open conditions prior to venturing in to do the trail layout. I guess maybe it's just the accuracy of the unit, but I find it strange that the error was in a consistent direction at each point around the job. The checks spanned about 1 hour or more, so it shouldn't have been the exact same satellite constellation the whole time.

Even if the new trails are off by 10' it's not a big deal, but it cost me some time when it cast some uncertainty on my earlier observations and I had to recheck some stuff. I guess I'll just work under the assumption this thing is off by that much.

Sounds like the receiver may be reporting IGS08 current epcoh based coordinates (as a result of using WAAS corrections).

There may be a setting to have the receiver do a 7 or 14-parameter translation in real time which will allow you to closely match your static observations.

However, the trail may have been designed inadvertently (since most GIS folks have NO IDEA that NAD83 involves a projection, a realization and an EPOCH) based from IGS08 observations. The best way to find out is to ask the data provider which realization and EPOCH they used for the design. If they tell you that there is only one NAD83 and imply you are stupid, you can can safely assume they are ignorant and don't have a clue what they are doing.

With the coordinates for the project center I can work you up the 2011_2010.0 vs. IGS08 current epoch difference, and the difference between State Plane coordinates using US Survey Feet and International Feet.

m

Switch your settings in the Trimble unit and shoot a few points, then look for a problem.

Everything may not be as it says.

Paul in PA

I am just a hack when it comes to surveying but I have used those Trimble XH units and noticed similar results.

I am pretty sure that it has to do with the fact that the XH is using WASS corrections and WASS is about 5' off from what you will see from an OPUS solution based position.

As Mark said. ITRF vs NAD83. 2m shift 3D. In my area it's about 2' N 2.5' E and 4' Up.

Dan Patterson, post: 339157, member: 1179 wrote: We are building a park that has some improvements as well as several miles of trails both new and old ones to be rehabilitated. The plans and CAD files are in state plane NAD 83 (not more specific than that).

Anyway, for whatever reason I was supposed to use a Trimble Pro XH GIS grade GNSS receiver to layout the trails in the woods. Being the diligent surveyor that I am, I ran my own independent static GPS observations and tied into the control from the CAD files with very good results (worst checks were +/- 0.1' over a very large area).

Then when I picked up the mapping grade GPS, I of course went to these points with it to verify everything was on the same datum. I missed the first one by about 5' to the east.....ok so I go to the next one 4.5' east. Third point was 5' east. Clearly there was a pattern. I know the thing isn't that accurate but why is it always off the same direction? I checked the coordinate system in the GIS unit and it says NAD 83 state plane. I also recovered an NGS mark about 1500' south of the project limits and it was off east 5'. When I tied it in with my static observations and robot it was dead on. So why the 5' shift? Is it the difference between old and new iterations of NAD 83? Never thought it was that much difference. Is it just the crappy GPS results from the XH unit? I didn't think it could be that because of the consistent nature of the error.

Sounds like you have good site control 🙂

And sounds like some setting in your handheld isn't matching it.
It's simple to check values for IGS08 with a data sheet. See if they are shifted like your handheld.

But whatever you find.............use the site control as it's given, don't shift to your new numbers, any small differences are probably EPOCH differences.

Mighty Moe makes a great point.

One thing I've found with sub-meter receivers in the past is that the errors typically stay consistent for hours. If your receiver can do this, I would localize each day on one of your control points and then check into others throughout the day. I think you'll be surprised at the consistency over time. Localizing would take into account transformation differences and observational errors caused by time and atmosphere. But I would localize every day, perhaps even twice a day if necessary.

When we use our PRO XRS or Geo, we consistently miss more in the Easting than in the Northing. Pretty standard fare from our experience. That's using it to our stuff that was controlled from OPUS.

Thanks for all the great tips guys!

The stuff I'm using that thing for isn't too critical so it's not a big deal. It's mostly just me wanting to understand why it was different and what things may cause that type of error that I hadn't yet thought of.

Dan Patterson, post: 339176, member: 1179 wrote: It said US feet in the settings which is what our zone uses. These checks I'm talking about were made in pretty wide open conditions prior to venturing in to do the trail layout. I guess maybe it's just the accuracy of the unit, but I find it strange that the error was in a consistent direction at each point around the job. The checks spanned about 1 hour or more, so it shouldn't have been the exact same satellite constellation the whole time.

Even if the new trails are off by 10' it's not a big deal, but it cost me some time when it cast some uncertainty on my earlier observations and I had to recheck some stuff. I guess I'll just work under the assumption this thing is off by that much.

Did you post process the data, or is it just straight out of the box?

I don't know about the mapping grade receivers, but the recreational ones all use a null transform from WGS84 to supposedly NAD83, based on the old old military document that said they were the same thing back before anybody had accurate enough data to tell the difference.

I did not post-process anything from the XH

Check your datum. This is likely a GRS80/WGS84 translation issue.

I'm assuming you're running TerraSync? Are you running a real-time correction by chance?

Mark Silver, post: 339180, member: 1087 wrote: The best way to find out is to ask the data provider which realization and EPOCH they used for the design. If they tell you that there is only one NAD83 and imply you are stupid, you can can safely assume they are ignorant and don't have a clue what they are doing.

I am saving this quote!

Shift from US Survey Feet to International Feet is small in NJ because the coordinates at the zone origin are small (N=0m and E=150,000m). For contrast, see origin values for Virginia, Utah, Texas, Michigan, Nevada, and Colorado on page 64 of http://www.ngs.noaa.gov/PUBS_LIB/ManualNOSNGS5.pdf&apos ;">NOAA Manual NOS NGS 5.

For example, see http://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=KV6027&apos ;">point KV6027 in Trenton, NJ.

SPCS(2011): N 501,448.64 E 420,700.85 sFT

(12 inches/foot) / (39.37 inches/meter) = legal definition of US Survey Foot per NGS
0.3048 meter/foot = legal definition of International Foot

501,448.64*(12/39.37-0.3048)=0.31 m
420,700.85*(12/39.37-0.3048)=0.26 m

Computed shift between NAD83 realizations at KV6027:
1986 - 1996: 0.19 ft
1996 - 2007: 0.02 ft
2007 - 2011: 0.07 ft
Computed using http://www.ngs.noaa.gov/cgi-bin/spc_getpc.prl&apos ;">NGS Tools - Lat/Lon to SPC

Todd

Shift from US Survey Feet to International Feet is small in NJ because the coordinates at the zone origin are small (N=0m and E=150,000m). For contrast, see origin values for Virginia, Utah, Texas, Michigan, Nevada, and Colorado on page 64 of http://www.ngs.noaa.gov/PUBS_LIB/ManualNOSNGS5.pdf&apos ;">NOAA Manual NOS NGS 5.

For example, see http://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=KV6027&apos ;">point KV6027 in Trenton, NJ.

SPCS(2011): N 501,448.64 E 420,700.85 sFT

501,448.64*(12/39.37-0.3048)=0.31 ft (12 inches / 39.37 inches/meter)=legal definition of US Survey Foot
420,700.85*(12/39.37-0.3048)=0.26 ft

(12 inches / 39.37 inches/meter) = legal definition of US Survey Foot per NGS
0.3048 meter/foot = legal definition of International Foot

Computed shift between NAD83 realizations at KV6027:
1986 - 1996: 0.19 ft
1996 - 2007: 0.02 ft
2007 - 2011: 0.07 ft
Computed using http://www.ngs.noaa.gov/cgi-bin/spc_getpc.prl&apos ;">NGS Tools - Lat/Lon to SPC

Todd