How I roll: Georeferencing CAD datasets in ArcGIS

It’s the bane of a GIS Professional’s existence – or mine anyway. Here’s how a typical conversation would go:

CAD person: Hi, can you convert this dwg/dxf to “GIS” please?

Me: OK, should be fine. Is it coordinated?

CAD person: What?

Me: Does the drawing have a coordinate system assigned?

CAD person: I don’t know…I don’t think matters.

Me: What LO is it in?

CAD person: Oh! Why didn’t you say so? It’s LO19.

Me: OK. Send it to me, I’ll convert it to a shapefile and send you a map.

CAD person: Cool. So I can expect that in 15 minutes then, because that’s easy GIS right?

Me: …

So most of the people who send me CAD drawings simply do not know what a coordinate system is, or if they do know, why it matters. Here are the steps I follow:

  1. In Catalog, assign the alleged CS to the drawing.
  2. Drag it into ArcMap. If I get the “Inconsistent extent” message, I’ll email the CAD person to ask one of the few senior CAD people to coordinate it for them (somehow, the knowledge of how to do this is never passed on).
  3. Zoom to the extent of the drawing to see where it falls. Invariably, the x and y axes will have been flipped, so the drawing will be in the opposite hemisphere.
  4. Georeference the drawing using control points if I have erven for the study area, or approximate the position using shift, scale and rotate. Confirm with the CAD person that the drawing is now in the correct place.
  5. Look at the layers within the drawing. About 75% of the time, the CAD person would have placed all types of features (roads, pipelines, erven) on the same layer. I then have to select individual colours to create new layers. And yes, sometimes they give different features the same colour.
  6. Check that the data is not nonsensical e.g. labels have not been converted to polylines, legends and neatlines are not included in the drawing etc.
  7. 25% of the time, there will be different layers for different features. I can then export these to feature classes, populate the correct attributes, make copies as shapefiles and generate maps.

There was one time, when someone gave me a drawing that was coordinated, had the right coordinate system assigned, fell exactly in the right place, and had separate layers for each feature type. It was beautiful. The drawing had been set up correctly from the beginning, and the person had worked neatly, knowing that double work could be avoided.

Unfortunately, that happened one time. As it turns out, they tend to have the perception that GIS is quick and easy – anyone can do it! Imagine if I were to tell them that creating a drawing is easy, and could be even easier if they used ArcGIS for AutoCAD, thereby creating everything in a spatial format, readily available to all who need to work with the data in a native format without the need for conversions therefore increasing productivity? That would be the day.

What do I know after all, I’m just a GIS person.

Dilbert CAD Monkey

Yet we’re all fighting the same fight

2 thoughts on “How I roll: Georeferencing CAD datasets in ArcGIS

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