AutoCAD Civil 3D General Discussion

AutoCAD Civil 3D General Discussion

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Registered: ‎10-20-2011
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Need help w/ datum/coordinate sys problem

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10-20-2011 11:54 AM

Hello all,


I am a GIS Analyst at an engineering firm trying to help one of our engineers determine what the heck is going on in some files he's working with. We have a concrete pad design file that is purportedly in Ohio State Plane North, NAD 27. We needed contours for the design, so we downloaded the raw LAS Lidar point cloud from the Ohio OGRIP server. I had him bring it into a blank CAD file which was assigned the coordinate system of the Lidar, which is Ohio State Plane North, NAD83 HARN. He imported the point cloud, classified it, and generated the contours.


I brought the file into ArcMap, set my coordinate system to NAD83 HARN, imported the file, and exported the polylines to a shapefile in order to perform the two-step transformation from NAD83 HARN to NAD83 to NAD27 in ArcMap. I then exported the NAD27 version back to CAD, and checked at each transformation step to ensure that my files were lining up per the assigned coordinate system.


When the engineer adds the converted contours in NAD27 to the concrete pad file using NAD27, the pad is shifted away from where it ought to be in the contours. In one file the units are in inches, in the other they are in feet. My Lidar tile index, aerial image, DEM, and the contours that he created from the LAS file all line up for me in ArcMap no matter if they are in NAD83 HARN or the final NAD27. We need to determine if the original concrete pad design was done wrong, but I don't know where to tell him to start. We have been going back and forth all day and are both ready to kick the computer. :smileywink:


Could a kind soul take pity on a GIS'er, and toss some applicable commands or lingo my way that will better help me understand what is going on? I've taught myself enough CAD to understand basic file manipulation, but this sort of troubleshooting on the CAD side is beyond me.


In ArcMap, we can look at the coordinate system settings to determine if different files with the same coordinate system have slight variations in the Prime Meridian, and stuff like that. I'm wondering what could be going on with the concrete pad file - it's close, but just not close enough.



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Post 2 of 3

Re: Need help w/ datum/coordinate sys problem

10-20-2011 12:04 PM in reply to: ctulley

This might be of some help...


We recently had a problem with the units not being consistent. Check all drawing settings and make sure they are all set to "US Survey Feet" or "International Feet" respectively. Make sure to stay consistent because although the difference is quite small in a local coordinate system, when working in state plane coordinates the differences can be quite significant. I believe the difference between the two can be 4' in a milliion feet. So you can see where the problem lies.

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Post 3 of 3

Re: Need help w/ datum/coordinate sys problem

10-20-2011 01:12 PM in reply to: davevoith

Be aware that Civil 3D/Map does not dynamically transform data in differing coordinate systems or units unless you use an FDO connection. Aside from FDO you will need to re-project your data, either in ArcMap or using Map's transformation tools and methods. In other words, all of your data must be in a common CS and units to align properly. If you are positive that all of your data is in the right CS then you may want to file a support request.

Neil Wilson (a.k.a. neilw)
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