Transforming C3D Objects - Your Approach

[SurvBase]

How are folks transforming C3D objects (today we'll focus on surfaces) from one coordinate system to another?
Map seems to be unable to query most (if not all) C3D objects directly.

My kluge for surfaces is to:
1. be in the source dwg with the appropriate coord system assigned
2. display the surface as Triangles
3. explode to 3DFaces - save as
4. open target dwg with the appropriate coord system assigned
5. use map to query the 3DFaces in
6. build the surface (again) from the 3D faces

While this works, it's not an ideal situation as: 1) it doesn't cope well with the addition of data to the model in the original coordinate system; 2) you end up with two surfaces (which will get out of sync) that are supposed to represent the same area, and; 3) it's labor intensive.

With the large quantity of data in State Plane/UTM Coordinates there ought to be an efficient way to transform C3D objects created in their source coord systems to low distortion projections that are suitable for civil design and right-of-way mapping/acquisition. Tune me up - what's the cool C3D method to handle this fundamental need?

[jpaulsen]

I may want to try to use LandXML? I've used it in the past to convert from imperial to metric but I'm not sure how, or if, you can use it for a coordinate transformation.

[brian.hailey933139]

Land XML will not do a coordinate transformation. It will do a translation however.

What's the difference? Transformation moves each point from the coordinate in one coordinate system to the correct coordinate in the new coordinate system. Translation moves the object from one point to another point and then scales it and rotates it. In other words, if you translate a square, you end up with a square. If you transform a square, you probably won't end up with a square, just something that looks a lot like a square.

I'm pretty sure I have this right, if not, I'm sure someone will correct me.

[bhaugi]

Land XML will not do a coordinate transformation. It will do a translation however.

What's the difference? Transformation moves each point from the coordinate in one coordinate system to the correct coordinate in the new coordinate system. Translation moves the object from one point to another point and then scales it and rotates it. In other words, if you translate a square, you end up with a square. If you transform a square, you probably won't end up with a square, just something that looks a lot like a square.

I'm pretty sure I have this right, if not, I'm sure someone will correct me.

Very well stated Brian. You have to be very careful with transformation and take the time to understand what is going on. Some transformations as well as some coordinate systems have inherent amounts of error. I recently dealt with one that just converting from NAD27 to NAD83 had 12 to 18 cm of error just in the translation. This is why you must always no the source of your data as well as any transformation that may have occurred.

To more answer the original question, look into the transformation tab in the Civil3D settings. Be mindful of project datum or project scale factors. Civil3D (at least 2008 version) doesn't like to convert from a project datum to GRID.