Importing topographical survey data into Autocad allows you to create contour diagrams as well as 3D terrain models. There are many different types of digital data collectors, but they all take surface data information from the field and convert it into a format which can be imported into Autocad and Autocad Civil 3D, usually an ASCII file with a .txt file extension.
This file can be opened up in Notepad or Excel in order to examine it in detail. It consists of a series of columns, for instance a PNEZD file is a comma delineated file with columns for the Survey Point number, readings for Northing, Easting and Elevation, and one for the Point Description. Another file could have columns for X, Y and Z coordinates. An alternative method is to output the Point file from Excel as a CSV file, which Autocad will also read. On importing into Autocad, this data will appear as Point Group or Point Cloud.
There are now three methods of proceeding in order to create contour lines. The industry-specific program Autocad Civil 3D, which is one of AutoDesk’s many vertical CAD programs, has several built-in tools for creating contours. For example, we have a Create Ground Data tab in the Ribbon. This includes several Point Creation tools. On hitting the Create button we browse to where the file is saved, then click the Import Points button, and choose either a CSV or TXT file, and select Add Points to a Points Group. Then zoom out by typing Z and E for Extents. We may need to change the annotation scale on the bottom right Status bar. Note also the panel on the left – here we can right-click on Points and choose to Edit Points.
These tools are not included in the basic standard Autocad program, however. We first open the file in Notepad and add the word Line to the top of the page, then save it as an SCR file. In Autocad we then type Script into the command line and navigate t the SCR file, whereupon the point group is created.
To create contour lines we need to download a contour creation plugin. There are several Survey CAD plugins available from Softonic and Softwel, to name just two third party developers. These are often freeware and quickly downloaded. The plugin will then open as a toolbar within Autocad.
The third method of creating contours is to estimate the contour lines and draw them in manually. First we look closely at the point cloud, picking out the highest and the lowest points in the group. We then decide on the intervals between these two heights; a good number would be around 10m intervals to start. Then we use the Polyline tool to draw contours between the points, for instance, draw a line in the space between points 20m and 25m points. Then draw a line between points above 25m and those below 30m points, etc. Gradually narrow down the spaces between those lines, depending on how many points you have to work with. Whilst this is not as accurate as the computer generated contours, it is the traditional surveying method, and still has merit.
Whichever method you choose to use, the contours can then be given a z-axis elevation, and used to generate 3D modeled terrain, either within Autocad itself, or exported to a program like SketchUp.