by Geoff Woodhead
In my last blog I spoke about the OpenData product range from Ordnance Survey. If you missed it you can find it here. In this post I am going to focus on one particular product from the range, Terrain 50. For any Civil 3D user whether beginner or expert, Terrain 50 could be the best free download since the UKIE Country Kit.
So what is Terrain 50? Terrain 50 as its name suggests is a low resolution Digital Terrain Model (DTM) product, designed for landscape visualisation and analysis over large areas. The data has a grid spacing of 50m and was last updated in 2013, a giant leap forward from the free DTM product Ordnance Survey historically offered as part of the OpenData range (Land-Form PANORAMA whose source dates back to the 70’s and 80’s).
So now we know what it is, how do we use it and what will it do for us? The data supplied is essentially a point file of the ground height arranged on a 50m grid. Unlike typical point files, DTM’s are handled a bit differently in Civil 3D. This is because DTM’s don’t record the Easting and Northing for every point, the benefit to this is in the file size, only storing 3 out of 15 characters gives us a huge reduction. So if they don’t record every point what do they do? They record the total number of rows and columns in the grid, the grid origin and spacing size in the ‘header’ of the file and then a grid of surface heights, this allows the software to calculate the Easting and Northings for the grid and apply the surface height from the DTM file.
Lets get some Terrain 50 data into Civil 3D to see what it looks like. There are two ways we can bring DTM files into Civils, and the most obvious way isn’t necessarily the best. Before we move on it is worth noting that DTM (Digital Terrain Model), DEM (Digital Elevation Model) and DGM (Digital Ground Model) are just different ways of describing data that represents the bare surface of the earth. DSM (Digital Surface Model) is not the same as the above, it records the earth’s surface and all objects on it.
For the first method of import the option you choose will depend on whether you use the Prospector or the Ribbon to create your surface. If you use the Prospector when you right click on Surfaces you will need to select the Create Surface from DEM… (red box on Fig. 1) option whereas if you are using the Ribbon after clicking Surfaces on theCreate Ground Data Panel you will select the Create Surface From DGM (green box on Fig. 1) option. Rest assured that no matter which option you select the same task is performed.
No matter which of the two options you selected you will now be shown the Grid Surface from DEM dialog box. The DTM files Ordnance Survey have provided are considered by Civil 3D to be Esri ASCII Grid (*.asc) files, selecting this from the Files of type: dropdown box and navigating to the location you saved them to should give you a view akin to that depicted below in fig. 2. You can now select the file and then choose open.
Having navigated to, selected and opened our file a surface should have been created and data imported into it. Zoom Extents will show us our imported data. The surface you see in front of you is a Grid surface and so will not necessarily be best for use in our design as they cannot contain an irregular network of points, meaning we cannot add any data to this surface that is not a DEM file. It can however be pasted into TIN surfaces.
DTM files can also be imported straight into TIN surfaces. Having created yourself a surface in the normal fashion, expanding the surface and its definition in the prospector will reveal the DEM files data type. Right clicking DEM files gives us the ability to Add…, selecting this will provide you with the Add DEM File dialog box depicted in Fig. 3. We now need to click the button to the right of the DEM File Name: field (red box on Fig. 3) which will provide us the Grid Surface from DEM dialog as shown earlier in Fig. 2. Using the same process as earlier we can select our DTM file, choose open and the data will be imported into our surface. The benefit to this method is that we can add additional data and are not limited to this only being from DEM files.
Now for the bit we really wanted to see. How does this data look?
Fig. 4 – Surface displayed as wireframe
Fig. 5 – Surface in shades of grey
Fig. 6 – Surface in 10m & 50m contours
The data file I have used is for British National Grid Reference NN17 and is the area surrounding Ben Nevis. As always I hope you’ve found the content of this post useful and have as much fun with this dataset as I have.