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NAD83 projection in Texas  

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INA
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 INA
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My 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!!

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Jack Chiles
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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

 

 

 

 

 

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JaRo
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 JaRo
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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

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INA
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 INA
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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!!!

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JaRo
 JaRo
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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?

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

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MightyMoe
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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 set-up. 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.

 

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MightyMoe
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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. 

 

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