Coordinate System Transaltions

[mikeosborne]

I am having an issue with two drawings. one drawing is set up in NC State plane NAD83 cooridnate system, and the is set up in UTM Zone 18 NAD83 it is the same site(there is a long story that goes along with this on why I am dealing with this, it is what is now).

If it is the same site and the coordinate system and units are set right in the respective drawing shouldn't I be able to xref one into the other and have them drop on top of each other? or am I missing a step?

[sinc]

Well, strictly speaking, drawings in different coordinate systems cannot be XREF'd on top of each other. They need to be transformed into the other coordinate system.

The problem with transforming drawings is that the process is a disaster in C3D. You can transform normal drawing elements using Map queries, but those don't work on any C3D elements. You can transform some C3D elements using LandXML export/import, but not all. So it can end up being quite the mess.

Practically speaking, you can often attach a drawing from a different coordinate system, then translate/rotate/scale it to fit, and you'll have something that's close enough. However, there is nothing in Civil 3D that will automatically do this. So if you want to try this option, hopefully you have coordinates in each system for your control points. Then use the control points to translate/rotate/scale the XREF into your current drawing. Technically, this isn't the same thing as transforming your drawing to the new coordinate system, but as long as you're looking at a small area, it might introduce little enough error to be acceptable.

However, if you're feeling really gung-ho, there IS a feature in core Autocad that kind of does this. In order to use it, you have to turn on the geomarker (set GEOMARKERVISIBILITY = 1). Then, when you attach your XREF, you should be able to select the option to "Use geographical data" to automatically translate/rotate/scale your XREF as it is attached. Just be sure to do some checks if you try this method - I've seen it return some pretty funky results. The results seem to get much worse if the extents of either drawing is too large.