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Site Calibration

 

 

 

 

TPC's Site Calibration lets you use both geodetic positions and local site (ground) coordinates in the same survey.

 

Site Calibration (10:13) - Learn how to use a site calibration to combine geodetic positions from Google Earth with local site coordinates

For example, you can set up your survey to use a State Plane or UTM CRS, but work entirely in the local site (ground) coordinates of a construction project at starting coordinate point 10000/10000.  Just tie some project monuments or control points with your GPS / GNSS equipment, use them to create a calibration and TPC does the rest. Now all the points you tie with your GPS / GNSS equipment will have local site coordinates in TPC. And when you go to stakeout points, TPC just reverses the process, creating the appropriate geodetic position for each local site point you export.


Geodetic Positions and the Survey's CRS (Coordinate Reference System)

Depending on the CRS you select for your survey, a geodetic point of 43°54'19.429"N / 124°05'45.937"W would generate a grid coordinate of 836264.996Y / 3973780.945X when transformed. The distance between two such coordinates would be a grid distance. TPC would then use the Distance Type selected for a traverse, drawing or COGO routine to report the grid or ground distance of that inverse.

Many large area infrastructure projects, like DOT highways, pipelines, municipal / county / state lands survey use CRS coordinates to coordinate positions across the entire project. TPC is very comfortable with this kind of survey and has all the tools to keep grid and ground coordinates and distances separate.

See Calibrations and Coordinate Reference Systems.


Local Site (ground) Coordinates

Local Site (ground) coordinates are just what they sound like - a local Cartesian grid for a small area project, using an assumed center of the coordinate grid.  Here, the grid is not the same as the projected grid of a CRS. It is simply an X/Y Cartesian grid we are all familiar with.

Local Sites are useful because the math is simple, inverses between coordinates match distances measured on the ground and work well with CAD construction drawings and plans.


Example

Let's say you have a Local Site (ground) survey, which has a local ground coordinates starting at 5000 / 5000.  You've entered all the record data based on the Local Site coordinates, computed additional COGO points and now have imported an XRef to the CAD drawing used for the project construction.  Everything is coordinated to reference the Local Site coordinate system.

Now lets say that you want to use your GPS / GNSS equipment and its related geodetic (LLh) positions with the survey.

Without Site Calibration

Without Site Calibration, this would be nearly impossible. The CRS coordinates computed from the geodetic position will coordinates like 836264.996Y / 3973780.945X, nowhere near your local site coordinates. In fact, if you import those geodetic positions into your Local Site survey and plot them, you will have two little dots on each side of the screen - one dot for the local site points and the other dot for the geodetic points.

Without site calibration, imported GNSS data would compute UTM or State Plane coordinates based on the survey's CRS. In fact, without a CRS, TPC would not be able to generate coordinates from the geodetic positions.  So you would have GNSS points at some CRS coordinates like 1,500,000 / 700,000 and local site coordinates at 5000 / 5000.

With Site Calibration

With site calibration turned on and the appropriate calibration created and selected, all GNSS point transformations compute calibrated local site coordinates.  Now your geodetic points are computed and plotted at their calibrated positions in your Local Site.


Related Topics

Calibrating Coordinates
Calibrations and Coordinate Reference Systems
CRS

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