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Create Terrain From MX Elements

OpenRoads works with Civil Geometry Features in the MicroStation drawing file and not with conventional MX models and strings. In the future, when we use the Survey functionality of OpenRoads, features will be created instead of MX strings. Until that time we need to create features from the MX strings. There are processes necessary to convert OpenRoads features to MX strings so that established final design procedures can generate final drawings such as Profiles. Other procedures, such as this document, will convert MX strings to OpenRoads features.

OpenRoads drawings are seeded with 3 DGN models:

  1. Default
  2. Default-3D
  3. MX

It is imperative that the proper DGN model be active.

Creating the Terrain Model from Elements (MX Strings)

In OpenRoads, a Terrain Model is created to represent the existing surface. If the source MX Triangulation is not a continuous surface and voids exist between groups of associated triangles, then importing the triangles will yield an inaccurate Terrain model. Therefore, the Terrain must be generated from the original surveyed strings that reside in the MX TOPO model. Refer to Create Terrain from Triangles if the MX triangulation is continuous.

A drawing needs to be created to house the project's Terrain. Check to see if a 12345-Terrain.dgn has been created during the Create OpenRoads Base Drawings process. Open the 12345-Terrain.dgn in MX. If one does not exist, then from the MX menu select File > New Plan Display... From the Plan Display File Name panel, supply the File Name of 12345-Terrain ("12345" should be the project's Project Number).

Once in, open the DGN's MX model.

Our "Plan Preparation Section" processes survey data and generates MX triangles from it. This is done using an input file called Triangles.inp. This input file contains MX code which masks out (excludes) strings that might not have valid surface elevations and therefore should not be used to generate either the MX Triangles or the OpenRoads Terrain. Copy these masks (019's) from YOUR project's Triangles.inp file and Paste them into the topoterrain.txt text file file. Do not assume that the same mask table is used on other projects. The Topoterrain.txt file will then be used to create an MX model (called TOPO TERRAIN) and that will be used to generate the OpenRoads Terrain. Follow the instructions in the Topoterrain.txt file for the proper location to paste this mask table. Save the new file to your project's MX directory.

Input the new file in MX to create the new MX model. Do this from the MX menu, select File > Input and then specify the new topoterrain.txt file.

Check/Set the Model Default style/feature set to TOPO.pss/TOPO.fns. Do this from the MX menu, select Tools > Model Defaults... Select the TOPO TERRAIN model under Model Names. The Style and Feature Sets are reported at the bottom of the panel, Edit as necessary.

Display the TOPO TERRAIN model. Do this from the MX menu, select Display > Plan with Style Set... Specify TOPO TERRAIN for the Model Name. Set the desired Scale, leave the Display all Strings checked ON and click OK to display.

Remove NULL Elevations

The TOPO TERRAIN MX model may contain strings with null elevations assigned to them and these null elevations need to be addressed. Null points have valid plan coordinates and an unknown elevation. In MX, null points are stored with an elevation of -999. Use Rotate View with the method set to Dynamic in order to inspect the model at a perspective orientation. This will allow a visual confirmation of any of the model's null elevations. Strings with null points will stretch down below the project's surface to sea level.

Null points can be removed by "draping" the TOPO TERRAIN model over the TRIANGLES model. The tool for this job is called NHdraper and is found on the MX menu, select Add-Ins > Personal Add-Ins > NHdraper. Supply the String Model Name; TOPO TERRAIN. Supply the Triangulation Model Name; TRIANGLES. Check the Add elevations only to the existing points check box. Click OK to process the draping. An alternative option to address null points is to Long Section every string that contains a null elevation over the MX triangulation model. This option is best done with a Longsection Input File text file. Check your effort, re-Display the TOPO TERRAIN model to confirm that there are no more null elevations. The final option if necessary, is to individually edit the null points with a derived elevation.

Open the DGN's Default-3D model. Click on Fit View and Zoom to the first area of detail. If necessary, use Level Display to adjust the display of levels of the referenced MX model.

create terrain elements dialog box A different Terrain model will need to be generated for each cluster of associated strings. Use your Element Selection skills to select all the strings for one of these associated string clusters.

Then from the Task menu, select the Terrain Model > Create From Elements button. Fill out the Create Terrain Model panel as shown on the sample panel to the right. Change the Name for each subsequent area or let the command increment it for you. Follow the cursor prompts in order to process this command.

Use the Element Selection tool again to select the new terrain model. The Element Information tool allows one to control the magnitude of the Terrain's triangle trimming. From Edge Method section, use the Length option to adjust triangles that span excessive distances. A larger value will increase the amount of triangles that span across the non surveyed areas. Decrease the value for less triangles. The MX triangles can also be displayed for reference (in the DGN's MX model) to compare against the new terrain model's triangles.

Repeat this Terrain generation process for each separate cluster of associated strings.

When finished creating all the terrains, use Element Information again to change each terrain's Feature Definition to Existing_Boundary.

Open the DGN's MX model. Use MX > Display > Erase Display to clear the MX strings from the DGN's MX model.

Open the DGN's Default model, then open the References panel. The DGN's Default-3D model should already be referenced. Verify that the Snap option is toggled ON so that MicroStation Snaps may be used. The Terrain model's levels may need to be turned on using Level Display (Default-3D) to see the terrain model. Save Settings and Save.

From the Task menu, select the Set Active Terrain Model tool from either the General Geometry or the Terrain Model subcategories. Identify one of the new terrain models to be the Active Terrain model. This will need to be done in each DGN that the Terrain drawing is attached to. Remember to make sure that the appropriate terrain model is the Active Terrain when you design in each area.

calculated features display

Calculated Features Display

Display of the Terrain model is controlled using the Element Information tool. First, use the Element Selection tool to select a Terrain model. Next, click the Element Information button to get the Element Information panel. Expand the Calculated Features Display section, here you may toggle the display of various terrain features as desired.

Move on to the next step of Importing MX Strings as Features.

Text File Format symbol Text File Format (.txt). Visit for a list of free .txt readers for a variety of operating systems.

New Hampshire Department of Transportation
PO Box 483  |  7 Hazen Drive  |  
Concord, NH  |  03302-0483
Tel: 603.271.3734  |  Fax: 603.271.3914