Using OS Terrain 5 data to create a digital terrain model
Increasingly, with the availability of 3D design functionality, and tools to determine clashes between proposed road infrastructure assets, there is a need for you to start and carry out your design on an accurate representation of the location. This includes using a digital terrain model (DTM), also referred to as a surface, onto which the mapping and your designs can be placed at real land elevations.
Ordnance Survey provide two layers which include land elevations. ‘Terrain 5’ provides spot heights at 5m squares, which is accurate for most road design schemes. Whilst not sufficiently accurate to determine drainage requirements, this can allow you to ensure that vertical sight lines can be taken into account, and provides an increase in the accuracy of material estimation.
KeyOSC provides a ready way, within AutoCAD Civil 3D, to use OS Terrain 5 spot height elevation data, to create a digital terrain model.
1. First, zoom to the required view. In the next steps, you will see that the function will create spot heights covering exactly the rectangular area of the current AutoCAD view from which you will create your surface. Once the spot heights have been plotted, you can delete any that you do not wish to be included in the DTM, for example if the rectangular view framed by the AutoCAD window is not what you want, for example if a road corridor is all that is needed.
2. Start the KOS_TERRAIN5 command. This prompts for you to specify the folder in which your Terrain 5 data is held. This is supplied by OS in various file types, the most important of which is the ASC file covering 1 km squares. All of the files covering your area should be placed in a single folder. Browse to this folder to tell KeyOSC where the data can be found.
3. KeyOSC will determine the data for the current view and will create an AutoCAD point at each position, at 5m intervals. Each point has an elevation, read from the Terrain 5 data.
The next step is to use built-in AutoCAD Civil 3D functionality that provides for a surface (DTM to be created from the points plotted. This functionality is not available in ‘vanilla’ AutoCAD.
4. Use the Civil 3D Toolspace to create a new surface. If you do not see the Toolspace, go to the ‘View’ ribbon tab, and make sure the Toolspace icon has a blue border and slight blue colour, indicating the palette is visible.
i) Right-click on the ‘Surfaces’ item and click to ‘Create Surface…’
ii) In the ‘Create Surface’ dialog, add details as required. A good name is recommended so your surface can easily be identified as the base surface later on, once road corridors and other surfaces are added to your design.
iii) Expand the items below the new surface branch to ‘Definition’, and right-click on the ‘Drawing Objects…’ item, then ‘Add…’ to define the surface from the points created by the KeyOSC command.
iv) Ensure that the object type selected is ‘Points’ and click OK (a description is optional).
v) Now select the points required to form the DTM and press enter to let Civil 3D create your DTM.
5. Lastly, you may wish to delete the points. One easy way to select the points for deletion is to select a single point and then select the remaining ones by choosing ‘Select Similar’ from the right-click menu, then finally pressing Delete on your keyboard.
When you now use KeyOSC to load mapping into a region (polygon or buffer shape) this will be draped onto the new surface. Make sure that the appropriate options in the MAPOPTIONS command have been selected including, if you require this, for buildings to be created as 3D objects from OS Building Height data if you have this.
Note: you will need to save, close and re-open the drawing before you can load KeyOSC mapping.