I get this question quite a lot. The issue is that I have a digital design model from an engineer. I have the coordinate system information for the project location which I set in TBC. I turn on the background map service / image service, and my design is offset from the map service in Northing and Easting. What is the best way to get them to align? This also affects the Google Earth output. TBC is using the Coordinate System information to convert the project data into WGS84 coordinates for use in Google Earth, so if the position is not right in TBC then it is not going to be right in Google Earth either.
The cause of the issue is the the Google Earth and TBC Map Services are set up in Grid and in this case the digital design information for the Data Prep or Takeoff have been provided at Ground, and at this location (in Texas) there is a scale factor of 1.00013 (actually 1.0001307298 according to TBC and yes the number of decimal places is important when you are working with 8.3 coordinates).
In the video example, the Coordinate of the Road Start point is different to the coordinate of the Road Start point in Grid - in this case the coordinates of the start point of the alignment are
The approximate coordinates )Measured using the measure tool in TBC just eyeballing the start point in the Map (which I am not exactly sure where that is supposed to be) of the Road Start Point in TBC Background Map Service are
The deltas between the two are
DN: -1720.883 (From Alignment POB to Map POB)
If I use the scale of 1.00013 provided in the plans that would give me a Map POB coordinate of N/1.00013 and E/1.00013 as follows
This would give a set of Deltas as follows
So close to the measured distances but not the same exactly - this is likely caused because 1.00013 is not accurate enough - TBC if you compute the scale factor says it is 1.0001307298 so If I use that then the deltas become
If I use these deltas in the Local Site Settings command to adjust the False Northing and Easting values then the alignment is outside the East Edge of Pavement rather than down the centerline of the existing road - which may be the correct location if this is a new design alignment.
So in this case either measure the deltas using the measure tool and use those to adjust the False Northing and Easting in the Local Site Settings command or use the scale factor (computed by TBC rather than the one used in the plans as the number of decimals provided are of insufficient accuracy to compute the shift accurately.
If you check the TBC coordinate system settings against the published EPSG Coordinate System they are identical, so they are correct and not the cause of the issue here (the issue is purely the Grid to Ground Scale Factor issue)
For Takeoff purposes to overlay the project on Google Earth or to use the Background Map Service in TBC, simply applying the offsets between the two either using a measured N and E offset or using the Ground to Grid Scale Factor to compute the offset of a known location eg the Road Alignment POB is sufficient for the purposes of a Bid Overlay