AutoCAD Civil 3D

[#1]

I'm trying to create a surface from a DEM (as a *.adf file)

I've managed to figure out how to convert between coordinate systems. (a UTM system to a State Plane)

How do I control the elevation values? My DEM has x,y coordinates in meters, but elevation values are in feet...

So far what's happening is that I end up with all the z values multiplied by 3.08... (I assume it is trying to convert the meters to feet..., but since I already have feet in the DEM ... no good)

I bet there is a simple solution to this.. but I'm struggling to figure it out...

-d

[#2]

There have been several posts about this problem. As far as I know there is no way to adjust the vertical units in the transformation settings. You may find a workaround in the posts.

Try a search on DEM Elevation Units.

[#3]

Thanks Neil,

I had hoped that perhaps there had been some progress on this issue since the other posts. I can do coordinate transformations to the DEM before I pull it into C3D to compensate (*sigh), but I'm not entirely sure what AutoCAD is doing with the z elevations. It appears to assume that if the coordinate system is specified in meters, that the elevation must be also... thus the fact that my elevations appear to be scaling by about 3.2...

Do you think it'd be 'safe' to say that I could just pre-scale the elevation values by 1/3.2 ...?

thanks,

-d

[#4]

Have you tried creating a surface, then adding a DEM file?  This tends to work much better than creating a surface from DEM.  If you create a surface, then add a DEM file (under Definition), you are prompted to add the file, then you have to specify the CS Code to match the DEM file.  Make sure you specify the correct UTM zone and in METERS.  If you have a coordinate system set to your drawing, it will transform it correctly.

[#5]

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eladkcem wrote:

Do you think it'd be 'safe' to say that I could just pre-scale the elevation values by 1/3.2 ...?

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That depends on how accurate the DEM is and how accurate you need the results to be. Theoretically transforming from one datum to another should be done with an algorithm that compensates for each X,Y. But if you don't mind some error creating in, it might be OK. You'd also want to know if the DEM is in International Feet or US survey feet as the scaling would be different.

Allen

[#6]

@trogers:

I am creating the surface first. I set the style to no_display (it's a big DEM). I've also added a data clip boundary to keep the size of the file manageable. I am using the correct CS settings. (Nad 83 DEM to State Plane dwg) The surface ends up located in the correct place in x and y space. The z values, however, are multiplied by 3.2. There doesn't appear to be anyway to adjust how elevation values are imported...

Thanks though.

-d

[#7]

First off, while it is true that you will have more control over the transformation settings if you first create an empty surface and then add the DEM data, it won't provide a means to compensate for the different vertical units.

The only solution I can think of is to apply the transformation via the settings when you create the DEM surface and then generate the triangles and rescale them in the Z direction and use the resutling triangles to create a new surface.

Of course with DEM's you will have lots of triangles so you can plan on long processing times for each step.

[#8]

The only way I can think of scaling the triangle in the Z only is to make them a block and change the z scale on the block. I'd have to try that to see how it would work. Is there another way?

Allen

[#9]

Neil/Allen,

It's a bit of a work around, but it does appear to work.

I exploded the surface twice, and then made a block out of the 3d faces (surface triangles).

Inserting the block with a scale of 1,1,0.3048006096 changed the z-coordinates. I then recreated the surface, and voila, the points have the correct elevations.

A solution.

but...Shouldn't there be a better one...?

-d

[#10]

There absolutely should be a better one. I'm sure the development team is aware of the problem but it would be good to submit a problem report.