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State Plane to Local Coordinate transform

Some of us need to export the LAS point cloud to Civil 3D back to local coordinates, since all our CAD files are local not state plane. 


For example the local coordinate system from my planset is:

Northing Conversion: 2624946.571 (local to state plane)

Easting Conversion: 1312508.894 (local to state plane)

Scale: 0.999899046


When I use those factors with my GCP’s they are where I expect them to on the Pix4d map and I’m able to use the basic GCP editor.  Since the drone captures everything in state plane this is all fine and well for the processing.  My problem is the output of the LAS (which I want to import into Civil 3D), I manually enter the site calibration in Pix4d but the results don’t make sense.  


I tried use the following in the site calibration:

Northing Conversion: -2624946.571 (difference from state plane to local)

Easting Conversion: -1312508.894 (difference from state plane to local)

Scale: 1.000100964 (inverse from original scale)


But the math doesn’t work, going back from state plane to local requires the inverse scale to be multiplied then the Transform be added/subtracted. 

I believePix4d assumes the following;

(State Plane Northing or Easting + Northing or Easting Transform)*Scale=Local Coordinates, This is what I use to transform the GCP’s to state plane for processing.

but to transform back to local from state plane I need;

(Local Coordinates*inverse scale)-Northing/Easting Transform, so I can import the LAS into Civil 3D.

I suppose the *fix is to edit the EXIF data in the drone photos prior to processing, not modify the GCP’s, and not display a background map thus processing everything in the local coordinates.


Thank you,

Ryan Baumgartner

Hello Ryan,

First of all, the site calibration tool is intended to be used only for projects with image geolocation and GCPs of high accuracy.
The initial purpose of the tool is to get the necessary transformation parameters for a site that is flown several times: the first time, GCPs are necessary to get the 7 parameters, and then these 7 parameters can be used in subsequent projects where only the image geolocation is available.
Additionally, only parameters that have been computed performing the site calibration process in Pix4D Desktop are supported as it is not in our knowledge how other providers calculate the parameters.

In case that the GCPs refer to the local coordinate system, you do not need to provide any transformation parameters from the state plane to the local coordinate system. You can simply add the GCPs in your project, selecting Arbitrary coordinate system both for the GCPs and for the outputs. You will need to mark your GCPs using the Basic Editor.
The software will respect the GCPs values and will try to fit the model as well as possible to the GCPs. Then, all your outputs will still be in an arbitrary reference system but this system will be based on the system defined by the GCPs.

Could you please give it a try following this workaround and let me know if this works for you?