the z coordinates of a 3D autocad dwg are not displayed in GIS

AnnaVelkov · ‎05-13-2013

Hi,
I am rather a beginner with Arc GIS and I am not very experienced with AutoCAD as well. I am trying to create a TIN (and subsequently a raster) of a road section from a 3D CAD drawing. When I click on the lines in the drawing they give me the correct z coordinate and in general all the information is there.
However when I open the drawing in GISMap (using ver.10) only the points from the point feature (representing the end of each curve segment) have the correct z values. These point though are not enough to create a proper TIN. The lines from the polyline feature have uniform Z value for each line; I mean one line has the same value all along. I tried to add the drawing into GIS in different ways, including CAD to Geodatabase giving it a spatial reference but the result is the same. Apparently I am doing something wrong but don�??t know what.
Can somebody help please?
Anna

KarenHodge · ‎05-14-2013

It might help if you could zip up a small sample drawing and post it here. AutoCAD has multiple types of entities that represent lines. Some, such as lightweight polylines, don't support having different z values at each vertex. Instead, they include a single property called elevation. If, however, these are 3d polylines, they could include a different z value at each vertex.

How are you examining the vertex z values in ArcGIS? You'd have to do some clever stuff to actually see the z values stored on each individual vertex. The z values may actually be correct, but you may be seeing an artificial property of the line feature calculated from the first vertex z value.

If you can post a zipped up DWG file, I'll take a look at it and see what I see.

Hi,
I am rather a beginner with Arc GIS and I am not very experienced with AutoCAD as well. I am trying to create a TIN (and subsequently a raster) of a road section from a 3D CAD drawing. When I click on the lines in the drawing they give me the correct z coordinate and in general all the information is there.
However when I open the drawing in GISMap (using ver.10) only the points from the point feature (representing the end of each curve segment) have the correct z values. These point though are not enough to create a proper TIN. The lines from the polyline feature have uniform Z value for each line; I mean one line has the same value all along. I tried to add the drawing into GIS in different ways, including CAD to Geodatabase giving it a spatial reference but the result is the same. Apparently I am doing something wrong but don�??t know what.
Can somebody help please?
Anna

AnnaVelkov · ‎05-15-2013

Hi Karen,
Thank you for your response.
I have attached here a sample of the drawing. Please note that the lines are going low to negative values and then up again. May be this is creating the problem?
To answer to your question about how I am examining the z values, in ArcGIS I am clicking on the i (identify) button and then on the line, which shows amongs other things the elevation. Or also I open the atribute table for the polyline features. However even if I am examining the z values in a wrong way, when I create a TIN it results in a flat surface.
Thanks a lot for trying to help!
Anna

KarenHodge · ‎05-15-2013

I'll take a look. One more quick question...what version of ArcGIS are you using?

Hi Karen,
Thank you for your response.
I have attached here a sample of the drawing. Please note that the lines are going low to negative values and then up again. May be this is creating the problem?
To answer to your question about how I am examining the z values, in ArcGIS I am clicking on the i (identify) button and then on the line, which shows amongs other things the elevation. Or also I open the atribute table for the polyline features. However even if I am examining the z values in a wrong way, when I create a TIN it results in a flat surface.
Thanks a lot for trying to help!
Anna

AnnaVelkov · ‎05-15-2013

Version 10

KarenHodge · ‎05-15-2013

Here's some information...

The good news is...your Z values are being read correctly and are in the geometry.

The elevation field can only report one value per feature. For many CAD features, that is the correct thing to do since they are planar. "3d polyline" entities (where the Z's vary for each vertex) simply report the first Z value as an elevation. Arguably, that's a bit confusing.

So...first thing you need to know is: ignore the Elevation field.

Here's how I examined the Z values:
- I ran the Geoprocessing tool "Feature Vertices to Points" to generate a point feature class from your CAD Polyline Feature Class
- To minimize confusion, I used the tool "Delete Field" to remove all the extra fields (including Elevation).
- Finally, I ran the tool "Add XY Coordinates" on this point feature class to see all the x, y, and z values for each vertex.

The result shows all the Z values varying from about -998 to about 6.

If you're using the GP tool "Create TIN", the input includes a field that specifies the height. Since your features have varying heights, you'll never get a good TIN using the Elevation field on your features. You'll want to break your polylines up and calculate the height.

Does that help?

Hi Karen,
Thank you for your response.
I have attached here a sample of the drawing. Please note that the lines are going low to negative values and then up again. May be this is creating the problem?
To answer to your question about how I am examining the z values, in ArcGIS I am clicking on the i (identify) button and then on the line, which shows amongs other things the elevation. Or also I open the atribute table for the polyline features. However even if I am examining the z values in a wrong way, when I create a TIN it results in a flat surface.
Thanks a lot for trying to help!
Anna

AnnaVelkov · ‎05-16-2013

Karen, thank you for all the information you have sent me so far �?? I really appreciate it.
There are still some points though which I would like to clarify - I am sorry to be a pain, but since we have started I better get to the bottom of it.
For the moment I cannot examine the z value the way you suggested since I have to wait for the licence for this particular GP tool to be activated on my PC. However, let�??s assume I did it and I see what you see. In the drawing the z values are from about -3.2 to 4.4 or somewhere around. Why the results of z values in GIS go down to -998?
Are you saying that it is not a good idea to create a TIN from the point feature class created as you have described? Or you have created it only to examine the z values? Anyway these -998 values will be quite disturbing.
Breaking the polylines and calculating the height �?? does it have to be done in CAD? Would you be able to give me just few guiding steps so I could ask the CAD team to do it?
One way or another you have been most helpful and I thank you again
A

KarenHodge · ‎05-16-2013

I'm glad to help when I can. I'm no expert at creating TINs, but I'll do what I can.

I think it would be fine to use the points I generated to create your TIN. In fact, that's exactly how I'd do it.

Why are some of the values -998? I confess I didn't look too hard at your drawing, so I don't know. Once you get your license working, you can examine the individual features and see where they are. You could select them out to exclude them from your TIN if they are extraneous stuff.

The key seems to be that you need planar features to create your TIN--points or lines are fine, but each feature should have one z value. You can create these features from ArcGIS or from AutoCAD.

I described one way to do this in ArcGIS. From AutoCAD, you can do a couple of things. Explode will burst your polylines into line segments. However, the start and end points could have different z values, so that probably won't work for you. You could use the commands measure or distance to generate points along the lines, but the points won't be at your exact vertices. That may not be a problem...you know your data better than I do. A quick google search turned up a couple of LSP routines that would iterate over the entity and create points from vertices...which would be closer to what I did in ArcGIS. There are probably several other ways, but one of these should do the trick for you.

Hope that answers your questions! Feel free to ask if you come up with more.

Karen, thank you for all the information you have sent me so far �?? I really appreciate it.
There are still some points though which I would like to clarify - I am sorry to be a pain, but since we have started I better get to the bottom of it.
For the moment I cannot examine the z value the way you suggested since I have to wait for the licence for this particular GP tool to be activated on my PC. However, let�??s assume I did it and I see what you see. In the drawing the z values are from about -3.2 to 4.4 or somewhere around. Why the results of z values in GIS go down to -998?
Are you saying that it is not a good idea to create a TIN from the point feature class created as you have described? Or you have created it only to examine the z values? Anyway these -998 values will be quite disturbing.
Breaking the polylines and calculating the height �?? does it have to be done in CAD? Would you be able to give me just few guiding steps so I could ask the CAD team to do it?
One way or another you have been most helpful and I thank you again
A