Understanding your reference data – Using a DXF/DWG file

Most (if not all) modern survey instruments will accept a DXF file as a source of reference data. Introduced in 1982 as part of AutoCAD 1.0 it was intended to provide an exact representation of the data in the AutoCAD native format (DWG) but in a format that could be opened by other third-party applications. However, since AutoCAD has developed and now supports more complex object types the DXF file is becoming less practical. The format itself hasn’t been updated since 2007, a full 13 years ago, and is therefore also unlikely to be tinkered with soon. It remains however one of the most used ways surveyors and site engineers receive their data. But it does have its problems. First let’s talk about size. Because a DXF is a report generated from a native DWG file it can easily bloat into becoming a very large file indeed. What can start life as a 2mb DWG can expand into a 20mb DXF file once it has been saved in that format. This immediately becomes a problem for a surveying instrument with limited memory and processing power. Unrequired data held in the drawing can compound the problem as well, with engineers being given unedited drawings which include layout details or even parts of the site they do not require. Empty layers and complex blocks of data can also add to the complexity of the file and therefore the size of the output. We also often come across the problems of data being placed far outside the working environment. It’s a quick shortcut for a CAD technician to pop the key for their layout far away in the drawings modelspace, but it’s unlikely to be removed when the drawing is given to the site engineer. The result is that when the engineer zooms to the extent of his drawing his site disappears. The only clue to its fate is the scale bar at the bottom of the screen which has shot to 20,000m. The only onboard solution being to hunt around for the dot which represents your site. So, while the industry has adapted to use the map screen on their instrument for their setting out, the data they are provided with is often not fit for purpose. Leaving the site engineer to edit the drawing (if they have access to a CAD package) or asking the provider for a trimmed down version. Ideally the drawing should have any unrequired data removed, any blocks exploded, and the drawing purged of empty layers before being saved as a DXF. Copying the relevant data then pasting it to its original coordinates within a new drawing also solves several problems. You could also consider using the original dwg file itself. Many modern instruments will now work with the native AutoCAD drawing file which is smaller and more versatile than the DXF. However, this will not solve problems caused by extra data within the drawing itself. While the presence of the DXF format has made engineers lives easier in how they interact with the data they are presented with, it has also brought its own set of problems which have to be adapted to. Next time we take a look at BIM and the IFC file.