CSRC question - GPS basis of bearings

A.Westerlund
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CSRC question - GPS basis of bearings

Postby A.Westerlund » Mon Feb 10, 2020 3:56 pm

You perform a survey like its 2020. GPS, scratch that, GNSS everywhere. You know what the CORS are. You can download RINEX files. Hell, you even know what a properly wieghted least squares adjustment is. Now, you've decided that having rotations and translations 6 ways from sunday just to match a record map is recipe for disaster.

You're going to use "state plane" for your basis of bearings, maybe you'll scale to ground in your first adjustment, maybe you won't. Either way, all worlds will be the same, data collector, processing sofware, CAD - its so awesome, its like the TV is in color or something.

Unfortunately, you know that 8801-8819 of the public resourses code exist. All because you're the only one that got your book from CLSA instead of ACEC. Damn it.

Question 1:
My understanding is that a bearing is a CCS83 value, triggering 8813.3 which requires coordinates and publsihed/stated accuracies for the refence stations which have controlled the values on your map. Now that you have shown a CCS83 coordinate on your map(pucblished reference stations), you are subject to 8815.5, which requires mapping angle, combination factor, and elevation for at least one point.

Am I wrong about any part of that? Seems like the most common thought is that if you don't show coordinates of your survey, you are not required to state any of that. The Basis of Bearings paper on the CSRC website seems to prove my point, but there is so much opposition. Am I wrong?

Question 2:
Someone told me that the CSRC did an update while I was living under a rock, so I tried it out. Yeah, stuff fits much better, using those coordinates.

The CSRC listing only shows accuracies for the ITRF data. How do get CCS83 data and show the accuracies per the requirement of 8813.3 using the CSRC data? I can't show the NAD83 values as controlling my survey and the accuracies per the ITRF adjustment, can I?

Hold on, I just found this coordinate system in TBC called ITRF to NAD83. Is that what I'm supposed to use?

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Ian Wilson
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Re: CSRC question - GPS basis of bearings

Postby Ian Wilson » Mon Feb 10, 2020 4:33 pm

Great questions, Antonio!

I ran into the same issues when I got my first set of GPS receivers. In the end, because of the mess with trying to comply with the PRC, I gave up. I fixed two found monuments on a reference map to use for a Basis of Bearings and turned in maps based on one of those points being 10,000 x 20,000 and kept the good coordinates for myself.

I remember the line from Jack Nicholson in "A Few Good Surveyors", "Coordinates? You can't handle State Plane Coordinates!" Great movie.

The PRC is being "adjusted" to the times. Hopefully, that'll be in the statutes soon.

Best to Claudia!
Ian Wilson, P.L.S. (CA / NV / CO)
Alameda County Surveyor

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SPMPLS
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Re: CSRC question - GPS basis of bearings

Postby SPMPLS » Tue Feb 11, 2020 6:03 am

I recommend that you contact Dr. Bock at the CSRC (ybock@ucsd.edu) to confirm , but I am almost certain that the NAD83 accuracies for the 2017.50 epoch positions are the same as the listed ITRF accuracies. This section of the project report confirms that, I believe.

Lat2sig(mm), Lon2sig(mm), Hgt2sig(mm)
ITRF2014 and NAD83 geodetic coordinates uncertainties. The geodetic uncertainties provide a more
intuitive representation of the errors. Note that they are provided in millimeters, while the coordinates
are expressed in meters. The X,Y,Z variance-covariance matrix is transformed to the N.E,U variancecovariance matrix by error propagation by means of equation (2). The uncertainties are the square root
of the variances on the matrix diagonal expressed at the 95% confidence level (multiplied by 1.96). As
expected the north and east uncertainties are about three times smaller than the vertical uncertainties.
It should be noted that the N,E,U covariances are negligible and not shown. Recall that the presence of
low correlations is the rationale for performing separate time series analysis for each of the geodetic
coordinates (Zhang, 1996). Using the same logic, the uncertainties are one dimensional, for each
component. Note that we assumed that the coordinate uncertainties for ITRF2014 and NAD83 values are
equivalent, since the transformation between the two is assumed to be without error


Project report:
http://csrc-old.ucsd.edu/docs/csrsEpoch2017_50_ProjectReport.pdf

A.Westerlund
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Re: CSRC question - GPS basis of bearings

Postby A.Westerlund » Tue Feb 11, 2020 6:26 am

Thank you, I have not read that document yet. Will read and digest soon.

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Steve Martin
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Re: CSRC question - GPS basis of bearings

Postby Steve Martin » Wed Feb 19, 2020 8:17 am

It is good that you are reading the PRC, even if it is only because you bought the CLSA binder, Antonio.

We use the California Coordinate System (CCS) here at EBMUD for all of our surveys because that enables us to relate one project to all of the others, and import the data into a GIS for mapping purposes. We do not scale "up to ground" because that defeats the purpose, is not the CCS, and I would argue, is not truly ground either. But I do not want to argue.

We use a CCS Basis of Bearings. I can send you our standard BOB statement if you like.

I'll include a "Diagram of ties to the CSRN to Establish a CCS83 Basis of Bearings" on my Record of Survey maps to graphically show how the basis of bearings was extended onto the project site. Usually this includes only the GNSS vectors used in the project control adjustment. I'll also include the CSRC published coordinates for the CRTN stations.

I have included a statement such as "Note: CSRC published 1 sigma accuracies for stations WWWW, XXXX, YYYY, and ZZZZ are all sub-millimeter in each of the earth-center, earth-fixed (ECEF) cartesian coordinate system components (X, Y, Z) per CSRC's CSRSEPOCH2011.00.xls file."

Sub-millimeter is good enough for me. You can put the actual numbers in there, however I think a general statement works.

I have also forgotten to include such a statement several times and no one has called me on it.

I do not include coordinates on new control points with accuracies in the survey as I want to be clear that I am not establishing new CCS83 control which would then trigger additional written data to justify those results. It is only a CCS83 basis of bearings, not a control survey.

That is one area where the Public Resources Code is not clear enough, the difference between using CCS83 and establishing CCS83 control. Perhaps we can clarify that in a future CLSA sponsored bill.

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David Kendall
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Re: CSRC question - GPS basis of bearings

Postby David Kendall » Wed Feb 19, 2020 9:42 am

I appreciate what Steve Martin says about grid vs scaled to ground measurements. Also the distinction between a control survey and a CCS BOB is important

I have never tried the grid BOB on a map but my understanding (based on discussion with other surveyors) is that unless you are stating coordinates, the CSRN ties are unnecessary.

Please correct me if I'm wrong. If I state a grid basis of bearings on a map with a proper datum accompanied by two monuments on the ground then the survey ought to be retraceable in theory.

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Steve Martin
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Re: CSRC question - GPS basis of bearings

Postby Steve Martin » Wed Feb 19, 2020 2:51 pm

David,

Per the PRC:

8813. After December 31, 1999, and prior to January 1, 2006, any survey or map that uses state plane coordinates shall be based on, and show, field-observed direct connections to at least two horizontal reference stations that are one of the following:

Included in the CSRN. (my shorthand notation: CSRN = NAD83+GPS+(NGS or CSRC)< 2cm)

Located outside the State of California and meet all the requirements for inclusion in the CSRN, except for the requirement that they be inside California.

Shown on a subdivision map, record of survey, or a map filed with the county surveyor by a public officer and whose horizontal positions have been determined by Global Positioning System survey methods in accordance with first order or better FGCS standards and specifications and whose state plane coordinates are based on field-observed direct, nontrivial connections to at least two stations that are included in subdivision (a) or (b).

8813.1 modifies that somewhat:

8813.1. After December 31, 2005, any survey that uses or establishes a CCS83 value or values shall meet all of the following requirements:
(a) The survey shall be referenced to and shall have field-observed statistically independent connections to one or more horizontal reference stations that is or are one of the following:
(1) CSRN station.
(2) Geodetic control station located outside of the State of California that meets all the requirements for inclusion in the CSRN except that the station is outside California.
(3) Existing CCS83 station that:
(A) Is shown on a map filed with the applicable county surveyor by a public officer, subdivision map, corner record, or record of survey.
(B) Meets all the requirements for inclusion in the CSRN, except that the station and its data are not published by NGS or CSRC.
(C) Has an accuracy, conforming to the applicable CSRN requirements, stated for the station's value.
(4) Existing CCS83 station that:
(A) Is shown on a public map or document that is compiled and maintained by the applicable county surveyor.
(B) Meets all the requirements for inclusion in the CSRN, except that the station and its data are not published by NGS or CSRC.
(C) Has an accuracy, conforming to the applicable CSRN requirements, stated for the station's value.
(b) If an accuracy is to be claimed for the CCS83 value or values established, the claimed accuracy shall be an accuracy standard published by FGDC or FGCS.

My use of a control diagram to show the field-observed statistically independent connections is a way of communicating to the following surveyors how I met the letter of the law.

PLS9196
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Re: CSRC question - GPS basis of bearings

Postby PLS9196 » Wed Feb 19, 2020 4:56 pm

Great points, back to the original question, isn't all our underlying GPS data collected in ITRF and translated by our data collectors to our chosen flavor of data output?


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