CAD Data Made Useful

Who draws in millimetres? And why is that facility drawn in some arbitrary location with no spatial reference what so ever?

In the world of engineering drawings spatial reference is almost irrelevant, after all that’s the surveyors concern, they’ll tell them where needs to go. The engineering plan is there to explain the size and materials etc of a particular feature. So what can we do with that data? How can we integrate it with the data that is already in our spatially aware GIS? Well with a few simple steps we can easily bring this data into our maps and make use of it.

The Georeferencing Toolbar

If you’re often working with CAD data you’ll need to familiar yourself with this useful set of tools. This tool bar is made up of tools that simply allow us to position our data in the right place as well as precision tools for applying geographic coordinates to control points.

georeferencing tool bar

Often CAD data has origin coordinates of (0,0,0). This is because as we have said the drafting hasn’t been done with spatial references in mind. This is why when you bring the data in it is often not even in the same hemisphere to where you would expect and is outside of your data extent. By selecting ‘Fit To Display’ from the drop down menu the CAD dataset will be scaled and shifted to the current display.

two point

Now that we have the CAD dataset in the same general area we are working in we’ll want to fit the dataset in with our existing GIS data. We do this by means of a ‘Two-point Transformation’, what this will do is move the dataset into the correct position but it will maintain the aspect ratio with no

stretching or bending. This will not affect the original CAD drawing but will simply display it correctly with in our map.

two point 2

The output from this transformation process is a world file. A world file (*.wld) is a simple text file that stores the coordinate pairs that defines where the CAD data is coming from and where it is being moved to, it defines the transformation.

wld file

A world file is needed whenever the CAD drawing is created using a local coordinate system. The bonus of the world file is if you need to perform a shift in different CAD datasets that are going to be transformed in the same way (the from/to movement is the same, relative to the data) you can use the same world file. This is referred to as universal world file. Just be sure to save the world file as ESRI_CAD.wld in the same folder as the CAD drawings.

Now that the data is displaying in the correct position we can now use it like any other GIS data we have. This means we can symbolise it as we wish, we can snap to it if we are editing a feature class or we can run queries on the data. None of these processes will affect the original drawing as all the changes are saved in the *.MXD file. Also the one limitation we do have is editing, if we want to do this we’ll have to export the CAD data into either a geodatabase feature class or a shape file.

Of course there is much more you can do with CAD data this is only the start. If you would like to learn more we have a one day course to cover this and many more CAD functions. If you would like to learn more about Working With CAD Data or any other training courses you can check the out here

Kym J

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