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NAD83 projection in Texas
Posted by INA on June 7, 2019 at 9:16 pmMy friends and colleagues!… I’m working in San Antonio, Texas, and I have a problem…. I configure my Trimble GPS with NAD83 (2011) projection, working with TxDOT VRS network (Trimble). When I check any Control Point of TxDOT, I have diference about 2 thousand feet… What will happen? The construction project of TxDOT says that reference control point have been measured with VRS TxDOT network in NAD83 (2011) projection… Any Idea?? thank you very much!!
mathteacher replied 5 years, 4 months ago 7 Members · 14 Replies 
14 Replies

Betcha money the problem is in the difference between Surface and Grid coordinates. About 2200 feet is the difference in the Houston Area.
Good luck!
JR Chiles

Do you know what the surface adjustment factor is (from TxDot) for the county you are working in?
edit: or tell me what county you are in and I will look it up.
James

I’m working close to San Antonio, and the project says that coordinates H&V established from the TxDOT VRS system. distances and coordinates are in US Survey Feet displayed in surface values using a surface adjustment factor of 1.00014. Datum NAD83 2011. I’m working with trimble TSC3. I configure coordinate system with NAD83, but diference of 2000 feet.
thank you very much for the interest!!!

Quick calc:
Zone 4204:
Two points near San Antonio:
Inverse the two points using 680′ for the elevations creates a ground/grid scale factor of 1.000163, that means if the SPC is multiplied by the Scale Factor it will increase the Northing by 2220′ and the easting by 350′. A lower elevation will decrease that shift.
You need to figure out how the DOT handles ground/grid shifts, if they are multiplying the coordinates by a scale factor that is probably your answer, that is typical DOT procedure and they normally state the SF.
If you are 2000′ South South West from the given coordinate that is probably the answer.

Posted by: INA
I’m working close to San Antonio, and the project says that coordinates H&V established from the TxDOT VRS system. distances and coordinates are in US Survey Feet displayed in surface values using a surface adjustment factor of 1.00014. Datum NAD83 2011. I’m working with trimble TSC3. I configure coordinate system with NAD83, but diference of 2000 feet.
thank you very much for the interest!!!
Divide the North coordinate and the East coordinate from the plans by 1.00014 and see how close they match what you shot.
James
edit: are you using Trimble Access or SCS900?

Looking at the TxDot list for the South Central zone which includes Bexar county, there are not any counties listed that have a SAF of 1.00014 .
That means they either updated the county list for the San Antonio district much like the Dallas district did a few years ago, gives them more accurate ground distances. OR, the had a consultant surveyor doing the surveying for design that considered themselves smarter than TxDot!

thanks… one question: wich coordinate systema in Trimble GPS I have select? “United States/ITRF to NAD83?? thanks a lot!

They are telling you to select the Texas Coordinate System. Do that, then apply the Scale factor that they give you to apply. This is all in Trimble’s setup. You should seek out someone who knows how to do this. Whatever you do use the existing DOT control monuments for XYZ if you are staking a DOT project.
I would not care what the VRS gives you except as an interesting factoid. If it was all done correctly the two numbers should be close, but DOT control would override.

TxDot allows their contractors to use their State GPS TxDot system for use on TxDot projects.
IMVHO, the people that are using the system to do boundary surveying are practicing unfair trade practices because it is not an open nor a pay to use system.
0.02

By the way, you do know that TXDOT coordinates are published in “surface coordinates”?

He mentioned that they are surface coordinates, but I don’t think he has any idea what that means.

Always a fascinating topic. Look at 3 points (I know, not an adequate sample) near San Antonio: AY1896, AY1924, and DG5157. For each of these points, the reciprocal of their scale factors is 1.00014, rounded to 5 decimal places. Multiplying the coordinates of these points by 1.00014 does produce about a 2,000 foot difference when compared to the SPCS coordinates.
But, using the reciprocal of the scale factor as the SAF raises them to the ellipsoid, not to the ground. To get a ground factor, you need the reciprocal of the combined factor or else account for elevation some other way. AY1924 has no elevation information, but the reciprocals of the other two combined factors are 1.00016664 and 1.00016397.
How do survey computers account for elevation in cases like this?

You should not be using the ITRF to NAD83 coordinate system if the VRS is already broadcasting NAD83; you should just be using US State Plane 83. Change the coordinate system setting from Grid to Ground and put in the referenced scale factor.

So, pdf page 36 of this publication https://www.sanantonio.gov/Portals/0/Files/CIMS/DesignGuidance/CIMSFebruary2012DGM.pdf says that the SAF for Bexar County is 1.00017, which squares with the AY1896 and DG5157 combined factors. It’s a 2012 publication, though.
It does appear that there is more than one problem here.
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