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Civil 3D and Coordinate Datums
All,
I have questioned the need to set the coordinate datum in Civil 3d for point import and export. So decided to perform my own test by:
I am importing and exporting points using different datums, with the intent to reproduce coordinate errors based on the datums used. However, I am not experiencing any coordinate changes between the different trials, comparing nothing and eating values imported and exported. This makes me ask if it is even necessary to set the coordinates in a CAD file for just importing and exporting points. I can imagine that coordinates would be necessary for transformations but not for exporting or importing point data.
If it proves necessary to set coordinates in CAD, is there a best way to demonstrate this, so my hard head can understand it better?
Can someone shed some light on this topic?
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5 Replies
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The reason is to allow the XY coordinate to convert easily in CAD to LAT, LONG. If you change the projection but insert the same XY coordinate, it should show a different L,L #. Depending on the change it could be a very small number change or a huge one if you’re changing zones. So by importing the same XY value, all you’re doing is moving the position in the real world. N1,000,000 E1,000,000 In Wyoming state plane will be in a way different location than the same number in a Colorado zone., but it will still be 1,000,000×1,000,000.
Check the lat long on each point, they should change with the different datum.
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If your drawings are all simple and isolated you can just use “No Datum, No Projection” and get along just fine. But if you start mixing and matching external references or aerial imagery that is in this projection and that projection – as you might if employing GIS data – you will want to pay close attention to those settings.
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All,
Thank you for that feedback. I will test the recommendations.
Concerning lat-long locations, I thought they were mostly used to measure on a 3D geographic datum, but I see this also in the Civil 3D prospector as point location data. So, the lat-long must be interpolated on the projected grid surface. Is my understanding correct?
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@cturlington Yes, you are correct
Latitude and Longitude of a given point tends not to change – other than physical movement – it takes a change of spheroid model
Northing and Easting coordinates depend on the projection – change the projection and you will see those numbers change, sometimes quite significantly
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All,
Thanks a million for all your feedback.
I have experienced the change in lat-long values but have been unable to force a change in the Northing-Easting values by changing the datums/projections and even units. My guess is that because the coordinate systems are grid-based in Civil 3D, the units from the origin are already fixed. So, changing the datum will not change the distance to the origin unless I move the points in the file. However, I would think exporting the points with an odd datum should create a CSV point file with distances relative to the odd datum. Does that sound correct?
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