Problem:

I have a file with a raster image draped onto a existing terrain as well as a design corridor. There are some spikes in the existing terrain and the raster shows through the proposed roadway in places. How can I make sure the proposed road is displayed on top of the existing?

Solution:

Since we are working in a 3D model for the display of the draped raster and displaying of elevations of the terrains, we can't control the display of the models if one is higher than the other in elevation.  Whichever surface is above the other will appear.  As a solution, create a clipped terrain using the corridor as a clipping element to create a void in the terrain.  Once the clipped terrain is created, the raster will need to be draped onto this newly created clipped terrain nistead of the original ground terrain.

1. Select the 'Create Clipped Terrain Model' command. Once the command is opened, make sure you have a Feature Definition selected and a Name for the Clipped Terrain Model that will be created.

2. When prompted to 'Locate Reference Terrain Model', you select the terrain model you wish to use as the base of the new terrain.  In this example it is the original ground terrain.

3. Next you will be prompted to 'Locate Elements'.  Select the corridor so it will use that element as a clip reference in the clipped terrain.  You can select multiple elements if you desire, then right-click when done to accept.

4. You can specify a horizontal offset and vertical offset if needed, but in this case we will leave the values of 0.

5. Specify the Clipping Method as 'Internal'.  Left-click to accept and finish the process. 

6. Now that you have a new clipped terrain with the terrain information removed from the boundary of the corridor, you will need to reassgn the terrain in which your raster is draped.  Once you reassign the terrain to this new Clipped Terrain you will see that the raster doesn't appear in the areas where the clip is void from the corridor element and you should no longer have overlapping graphics.  Note: Also make sure your Original Ground Terrain is also turned off so it does not display and cause conflict.

Background Information

Here is a tutorial providing some background on how to drape a raster onto a Digital Terrain Model.  This will provide the steps needed for displaying the raster image draped onto a terrain.

http://communities.bentley.com/communities/other_communities/askinga/w/askinga/3567.aspx