CRTN Confusion

[Jim Frame]

I don't know if this belongs in the T-LS forum or the J-Field forum, but here goes:

When using the California Real Time Network in my T-LS, I have the Base Reference Frame set to NAD83(2011). According to the CSRC mountpoint document, CRTN stations are referenced to NAD83(2007) Epoch 2011.00. I've been assuming that the differences between these reference frames won't introduce enough error to worry about. However, I'm seeing a significant difference between the CRTN published position of the mountpoint I most often use (UCD1, PID AI4467) and the position reported by the T-LS.

The CRTN source document shows UCD1 as:

38 32 10.449957 121 45 4.380424 0.0915

But when I press the Stats button on the T-LS when connected to UCD1, it shows the base at

38 32 10.45142 121 45 04.38223 0.0851




This amounts to roughly -0.15' N, -0.18' E, and 0.02' EH. I'm not concerned about the height difference, because it's so small and because UCD moves more than that vertically anyway, but the horizontal is disturbing.

In an effort to figure this out, I connected to UCD via BNC and converted the XYZ position in the RTCM message to lat/long via the NGS online tool:

=================================================================
X (Meters) Y (Meters) Z (Meters) ELLIPSOID
INPUT = -2628825.7516 -4247933.4662 3952176.6501 GRS80
=================================================================
LATITUDE LONGITUDE ELLIP_HT ELLIPSOID
------------- -------------- ---------- ---------
N383210.44996 W1214504.38043 0.0914 GRS80

Thus the RTCM message position substantially agrees with the CRTN published position.

Why is the T-LS reporting such a different position? Is there a conversion going on that I'm not taking into account, or is my assumption about reference frame parameters off base?

 

[Shawn Billings]

What transformation are you using Jim? I suspect you are using ITRF>NAD83(2011). This is based on the 14 parameter transformation between ITRF and NAD83. J-Field now supports HTDP. I would recommend using the HTDP transformation and set the epoch to 2010. I may be wrong, but the "small" discrepancy horizontally would have me believe this is your issue. Generally it doesn't matter which transformation you use IF you are using your own base station, but it becomes more significant when using another base station or network.

 

[Vladimir Prasolov]

Hi Jim,
It looks like base station reference frame was set to WGS84(ITRF2008) instead NAD83(2011)@2010. Please verify the default NAD83(2011)@2010:

If Epoch is not 2010.0000 please add NAD83 system one more time with that epoch.

Also push "Edit" in "Setup" and push few times "Next", to verify Base Reference Frame:

 

[Jim Frame]

It looks like base station reference frame was set to WGS84(ITRF2008) instead NAD83(2011)@2010. Please verify the default NAD83(2011)@2010:

That was it! Somehow the epoch had gotten set to a particular date in 2014.

I set the default to NAD83(2011) as indicated:

But I'm still a little confused about the current project setting:

Why does the Current Project setting allow an SPC zone to and geoid model to be specified, but the Default doesn't (or doesn't seem to)?

 

[Matt Johnson]

Hi Jim, the project coordinate system is set in the Collect or Stake Prepare screens by tapping the Project button:

If you choose Edit Current Project you can change and view the project coordinate system:

The project coordinate system is the default coordinate system that is used for each page. The coordinates systems for each page can then be changed to something other than the project coordinate system if desired by choosing the Page button in Stake and Collect Prepare screens.

The Coordinate System button found on the home screen is just a list of coordinate systems. Highlighting a coordinate system here does set the coordinate system for the project. Current Project list all the coordinate systems used in the opened projected, all Projects list all the coordinate systems you have used in all projects and Default are some predefined default coordinate systems. I hope this clarifies things for you. I was a little confused by this as well when I first started using J-Field.

 

[Shawn Billings]

Still, I believe the root cause of your issue is that your NAD83/California Zone 2... coordinate system is using transformation "WGS84(ITRF08)>NAD83(2011)/SPCS California Zone 2..." and it should be using "HTDP WGS(84)>NAD83(2011)/SPCS California Zone 2..." for best results.

I believe you will need to create a new coordinate system for California Zone 2, this time selecting HTDP for the transformation, and setting the epoch to 2010. Use the rename button to somehow differentiate the name of your HTDP system and your non-HTDP system (which is actually using the 14 parameter transformation).

 

[Jim Frame]

Still, I believe the root cause of your issue is that your NAD83/California Zone 2... coordinate system is using transformation "WGS84(ITRF08)>NAD83(2011)/SPCS California Zone 2..." and it should be using "HTDP WGS(84)>NAD83(2011)/SPCS California Zone 2..." for best results.

I missed the HTDP part. How's this: