BuildDTM.tml Create a DTM from Contours Quickly and Easily A DTM (Digital Terrain Model) is a computer-generated, 3-dimensional model of a surface created from points that have x, y and z coordinates. In Terramodel a DTM is used as the basis for many advanced features such as calculating volumes and interpolating cross sections. (Terramodel User's Guide, Chapter 7. "What is a DTM?") If you import contour data into Terramodel from another program and want to create a DTM, this TML is for you! BuildDTM.tml will do the following: Find all 3D Plines and convert them to sets. Find all Sets with single elevations and treat them as contours. Filter contours to create an accurate surface with the smallest amount of data. Find all spot elevations if they are points, point blocks or just text. Create a DTM. The BuildDTM command: Explanation of Each Item: 1. Contours: http://tmlstore.com/docs/builddtm.asp?print=1 (1 of 7) [1/3/2003 2:54:38 PM]
Contour Layers - BuildDTM.tml will search the project and list ALL layers containing 3D plines or sets with points of the same elevation. Often, contours are drawn on different layers and finding all layers with contour data can be difficult and time consuming. Maximum segment length - When a set line is created for the DTM from a contour line (pline) then points are created along the set. This setting controls the maximum distance between points in straight segments. Maximum deviation from original line - When the contours are converted to sets, BuildDTM filters the contours and removes excessive vertices to prevent "monster" sized projects. This option helps preserve the integrity of the original contours. The new set will not deviate from the original line more than the distance specified here. A smaller distance here will create more points. http://tmlstore.com/docs/builddtm.asp?print=1 (2 of 7) [1/3/2003 2:54:38 PM]
Segments per spline - If contours are splined, the number of points considered along each spline segment is specified here. This should always be 3 or greater. The higher this number is, the longer the process will take. Delete old lines - Since the contour lines will be converted to sets, this option allows you to delete the original contours. This will help keep the file size minimal. However, if the original contours are deleted, comparing the new DTM and the original contours will not be possible. 2. Spot elevations: Spot Elevation Text: Data imported into Terramodel may have spot elevations that are text objects. Terramodel requires points with elevations for the DTM. BuildDTM will search for Text objects with numeric values or elevations and create points with elevations on the DTM. Spot Elevation Blocks: Points can be imported as blocks. BuildDTM searches for blocks with elevations and creates points with elevations on the DTM. Spot Elevation Points: Points with elevations will be found and used as DTM points. Relayer Points: Points with elevations that are found can be relayered to the DTM layer to prevent duplicate points. Copy Points: Points with elevations that are found will be copied to the DTM layer and the original points will remain on their original layers. http://tmlstore.com/docs/builddtm.asp?print=1 (3 of 7) [1/3/2003 2:54:38 PM]
3. Linking: Max distance along edge links: This option specifies the maximum edge distance of links along the perimeter of the DTM. This option is very important for non-rectangular shaped DTM's. For example, in the case of a U shaped DTM or a Horseshoe shaped DTM, if the Max link distance is set too high, then links could enclose the DTM. http://tmlstore.com/docs/builddtm.asp?print=1 (4 of 7) [1/3/2003 2:54:38 PM]
Display Links: This option will turn on the links for visual inspection which is a very important part of evaluating the Digital Terrain Model. 4. New DTM Layer: This is the layer for the new DTM created by BuildDTM. It is recommended to create a New layer for this setting. 5. Treat elevation zero as 2D: http://tmlstore.com/docs/builddtm.asp?print=1 (5 of 7) [1/3/2003 2:54:38 PM]
If you are not at sea level, then you want to check this option. Often, 2D linework and points are assigned an elevation of zero. Terramodel will see this as a valid elevation and could result in an inaccurate model of the surface. DTM Tips: Once the DTM is created by BuildDTM, generate a 3D View of the surface. Select 3D Visualizer from the DTM menu. Select the DTM layer created by BuildDTM then click OK. You may need to created two DTM's. A DTM from Existing contours and a DTM from Proposed contours. This task is a little more difficult since you must separate the Existing data imported from the Proposed data. With BuildDTM, you simply run the command twice. Each time selecting the appropriate layers for each DTM. Advantage of using BuildDTM: Finding all layers for a DTM can be very difficult. The BuildDTM command quickly identifies all the possible DTM data. Use layer lists created by BuildDTM to verify valid contours and spot elevations. Isolate each layer list used by BuildDTM. The default name of the layer lists are: Contours, Spot Text, Spot Blocks and Spot Points. ILL.tml is available from www.tmlstore.com which will isolate layers in a layer list in one step. Without ILL.tml, follow these steps to isolate a layer list: 1. Click on LSET which is found in the top left corner of the Terramodel screen. 2. Click Select All 3. Uncheck the Visible checkbox to turn off the visibility of all layers. 4. Click on Select by Llist... 5. Highlight the Contour layer list and click OK. 6. Click on the Visible checkbox to turn on the visibility for the selected layers. Click OK. 7. Inspect the contour layers used to build the DTM. 8. Repeat these steps 1-7 to inspect the spot elevations. http://tmlstore.com/docs/builddtm.asp?print=1 (6 of 7) [1/3/2003 2:54:38 PM]
Programmer's Notes: Building DTM's from contours can be very frustrating and tedious. Not all data imported from other programs are on one layer. BuildDTM offers a solution to Terramodel users that will quickly find DTM data convert it to 3D points and sets and place it on one layer. And it is easy! When comparing BuildDtm to the Filter command that comes with Terramodel, I found that BuildDtm can reduce the number of points on a DTM created from contours as much as 75% with similar accuracy. Results will of course vary in different terrain. http://tmlstore.com/docs/builddtm.asp?print=1 (7 of 7) [1/3/2003 2:54:38 PM]