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I'm a bit unclear on what exactly assigning a coordinate system does in the model space of autocad (we are using 2015 civil 3d). Our local county coordinate systems have a scale factor associated to with the projection something along the lines of 1.00000534678....

Does that mean by assigning the coordinate system my drawing units change in the .dwg? As in 1 unit no longer equals 1 and now equals 1.00000534678....

I've done some tests and that does not appear to be the case. So, if not, what does assigning a coordinate system do? Why would it be beneficial for a land surveyor to assign a coordinate system in the .dwg? We pretty much have no reason to transform from one coordinate system to another. Other than using the automatic background map display (aerial imagery, road maps etc), I see no real reason to assign coordinates systems. Am I missing something?

We collect data in the field with GPS and start all projects with the county coordinate system assigned to the data collector so all of our coordinates are exported from collector into our .dwgs and are already in the proper coordinate system.

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A scale factor should be used to convert "ground" (i.e., measured) distances to "grid" (i.e., projected) distances. To convert ground coordinates to grid coordinates, you also need to translate (i.e., shift) and rotate them. It sounds like your GPS data collection system is already doing this for you (to relate your collected data to your background map). I don't know much about Autocad but I assume that if it is working with grid coordinates and is given the scale factor then it can divide any grid distances in the model by the SF to yield the "true" ground equivalents.

  • ground distance * scale factor = grid distance – Martin F Dec 16 '16 at 5:18

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