coordinate transformation results

[demus72]

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.

[AllenJessup]

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

[demus72]

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

[AllenJessup]

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

[wfberry]

Allen:

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

Bill

[AllenJessup]

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

[wfberry]

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

[AllenJessup]

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

[demus72]

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.

[thepworth]

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.

[demus72]

PWorth,

Thanks for the suggestion. I exported to an sdf file and it resulted in the same line location as the shp file. I've attached a crude jpg of a point at a 90 degree fitting, the line is from the sdf file. As you can see, it is 3.07 feet off of the point in this location.

Any other thoughts?

Thanks

[thepworth]

Most Troubling!!!  So, we're going from UTM27 to UTM83.  If you look at how the coordinate systems are set up, we have a cylinder wrapped so the perimeter of the cylinder coincides with longitude lines versus latitude.  What this does is maintain the scale in north south versus projecting the geoid/spheroid to a non-uniform scale east-west.  What I'm getting at is something fundamental in how the projection is transforming.  a transverse mercator projection can be set up with one line of intersection with the spheroid or two.  If you're on a tangent line, then the scale north-east west should be uniform.  with 2 lines, when you're between, the spheroid is outside the cylinder and gets compressed in the east-west direction and is uniform in the north south.  conversely, the east west-scale is expanded outside the tangent lines.  (the joys of a MS in Geodesy....)

So, would you get the same error if your project is closer to one of the UTM defined meridian lines?  I'd go there next.

But that would be at AD's level to confirm.

What you need to do now is open a support case and see if Nathan Moore and the programmers behind the tech guys can answer it.  I'm not aware of any other settings that would cause M3D to be different from C3D on coordinate transformations.  Go through your subscription site first or through your reseller and explicitly state it is priority level 1.  

Please keep us posted!

Todd Hepworth, P.E.

[Sinc]

I think Todd's post is key...

NAD27 Lat/Long should not be assumed to be the same as NAD83 Lat/Long, as they involve different ellipsoids, as well as diiferrent orientation of those ellipsoids upon the structure of the Earth.  They're close in many places in the CONUS, but even there, they vary.  And they get worse if you get out of the CONUS.

[annw2]

I also think the Lat Longs are slightly different.

I was trying to find the vertical conversion on the USGS website & got horizontal instead.  It gave me a conversion in deg min sec.

Found the vertical on the next try.

[thepworth]

The two coordinate systems are based on different geoid models - one CONUS centric & one close to the earth's center of mass.  the different values for ellipsoidal flattening would likely reflect more in the northing/latitude (y) versus longitude/easting (x) direction.  since the transverse mercator is a cylinder wrapped in a polar versus equatorial direction, it would appear that the projection calculations are strange given that the OP is getting different results from C3D versus Map3D.  Hence the call to AD, since they can better answer if there is any difference between the 2 software packages' mathematics/programming routines.

[AllenJessup]

I'm glad to here that the C3D export was correct. I've been doing a lot of back and forth between systems lately and have decided that all the coordinate transformations should be done in Corpscon. I then just adjust whatever comes out on a Map query or a block insert to the points I've imported.

Allen