AutoCAD Civil 3D

[#1]

I am using Civil 3D and Map 3D 2010.

I have some points in a drawing set up in a coordinate system and using a specified point file format, I successfully exported them to another coordinate system for use by our surveyors. I also wanted the pipe route in my destination drawing so I created a shp file from a polyline representing the pipe route in the original drawing using Map 3D Export. When I imported the shp file into my destination drawing there appears to be a slight horizontal difference from the point locations. The shp file doesn't land directly on the points.

To try another comparison, I ran a query on the pipe route to perform a manual coordinate system transformation. The result of this is the exact same as the shp file.

My questions are, which method (exporting points vs. shp files) results in the correct coordinate transformation and why are they different? When exporting points I am not using any specific transformation settings, I am simply selecting the destination coordinate system. When exporting a shp file, Map 3D creates a prj file from the specified coordinate system in the drawing. Are there any other settings I need to adjust to get the same result with both processes? The only thing I can think of is I only have the coordinates out to 4 decimal places in my point file format, do I need to go out farther?

Thanks for your help.

[#2]

A quick way to check the coordinate conversion is with Corpscon. It's the basis of the Map coordinate systems anyway.

The only thing I can think of right off hand is that the Shp file conversion isn't handling a coordinate system based in Survey Feet. But since I don't know what systems your working with. It's only a guess.

Allen

[#3]

Thanks for the reply Allen.

My source drawing is an older drawing in UTM NAD27 US Foot. My destination drawing is in UTM NAD83 State Plane US Foot. Being only 3+ feet off, an International Foot to US Foot error would make sense but the shift is more East-West and both drawings are set to US Foot. I ran through the process of exporting a shp file again with the same result. I also adjusted my grid northing and grid easting coordinates out to 17 decimal places (maximum allowed) with the same result. A check of a couple coordinates with Corpscon should reveal which process is performing the transformation correctly. I'll reply shortly.

Thanks

[#4]

Well. If I had to be suspicious of one. I'd be the shape but I'm surprised about the query. I'm sure you've checked the zones in each drawing. I'll check back in tomorrow to see what the results of the Corpscon run is.

Allen

[#5]

Allen:

Would the Lat-Longs be the same in each version?

Bill

[#6]

Sorry Bill. My mind is off in a different direction this morning and I'm not quit understanding your question.

I had though the OP would be exporting the points in NEZ not Lat-Long. Also which version of what are you referring to?

Now maybe you're talking abut the 2 systems. NAD27 and NAD83. If so. That's a good question. I don't deal with lat-long much so I can't say for sure. I would think the differences in the systems would show up more in the projections from that system rather than in the basic lat & long. But I'm not sure.

Allen

[#7]

Sorry Allen:

I was thinking that the Lat-Long should be the same in each case.   Therefore it would be an easy check with the Map version of the Inquiry Tool.

Bill

[#8]

I think they "should" be Bill. But I don't know enough about how the UTM zones handle Lat-Long for me to want anyone to hang their hat on those results.

Allen

[#9]

I finally got around to comparing some coordinates of the points I exported from UTM NAD27 to UTM NAD83 State Plane using Corpscon. The results show the points that were exported using a point file format from Civil 3D are correct. Both procedures of exporting a shape file and performing a query of the polyline representing my pipe route with Map 3D generate the same result, both of which are off by 3+ feet from the points.

Why is Map 3D different? Are there any settings that can be adjusted in Map 3D or are the Map 3D transformation settings only going out so many decimal places when Civil 3D goes out farther? I would like to get this resolved as we do use shp files often when communicating with the various agencies involved. We have supplied these agencies many shp files in the past and are now realizing they were/are not as accurate as we thought. Any input is greatly appreciated.

Thank you.

[#10]

Can you back out 1 step on this?  I'm curious if you mapexport to Autodesk's "Native" .SDF format, do you get the same results as the .SHP or C3D's points?  

I believe C3D's coordinate library is built around the Map3D coorinate system library, so I don't think the problem lies there.  I used to have problems when I used ArcCAD in the mid 90's - E$RI being single floating point precision versus AD's double floating point precision.  When working in state plane coordinates, my property corners would "float" 1-2 feet.  

If the SDF is "correct," then try "bulk copying" to convert the SDF over to a .SHP.