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3 Ways to Save in CAD to GIS Conversion

by Mike on May 12, 2014

Every day on some project somewhere, someone is burying a piece of infrastructure that may or may not go in exactly as planned (and in our experience, almost always goes in at least slightly different).  Meanwhile, almost just as often, someone else is planning or building something new in an area where uncertainty or even mystery exists regarding infrastructure that was built and/or buried in the vicinity.
In both cases, the imprecision and uncertainty costs money – if not today, then almost certainly eventually.  Present-day methods and practices of as-built surveying and construction record-keeping should be doing better and many expect they do, but real-world experience tells us it just isn’t happening as intended and where it could, there is even room for cost-effective streamlining to do the job better for less.  All that’s required is a thorough understanding of the technology on both sides of the great divide – the divide between facilities delivery and facilities maintenance
With that knowledge, facility planners and facility operators can save dozens if not hundreds of expensive data conversion hours on every project by transforming ready-for-construction CAD files into ready-for-lifecycle maintenance GIS data.
Here are three simple ways to save:
  • Coordinates
Be sure to create the data in a known coordinate “real-world” (not assumed) system (or move the data to it).  State Plane, U.S. Feet – typically, with fractions thereof expressed as decimals, not inches.  Common mistakes include– “scale factor” scaling, using international feet, and forgetting to convert from architectural units (which are actually inches, not feet). 
Scale factor scaling is the practice of applying a scale factor based on a geoid calculation.  Unless you have a project actually geographically big enough for it to matter (like, miles across), don’t use one.  Just select a known control point and rotation, and keep all your column grids and pipe lengths at their actual, not scaled, lengths.
Most civil projects will be created in CAD units of feet and decimal parts thereof – this is good.  If the facility drawings are in architectural units (feet and inches), well, in AutoCAD, the units are inches.  Scale these projects by 1/12.
  • Attributes
Attributes in GIS has a broader definition than in AutoCAD.  CAD users are familiar with “attributes” being fields of data applicable only to “blocks” or symbols.  In GIS, attributes are any of the tabular data fields encoded with any points, lines, or polygons.  Luckily for us, AutoCAD Map uses Object Data definitions to include GIS-ready attributes on any CAD features, including lines, arcs, polylines, or polygons.
The big question, then, is which of these Object Data attributes to include (in addition to any “block” attributes).
We can make this easy, or we can make it hard.  Some standard data models have a lot of attributes that seem to have been contributed by a committee, where someone was keeping points for whoever could add the most.  What is essential are the basic descriptive quantities and classifications – pipe size, material, type or class.  Some metadata – including the date and source of the information, precision, etc -. is also essential.  AutoCAD’s MAPEXPORT command  facilitates including Object Data fields as ESRI Shapefile attributes.
  • Leave a link or path to the “other side”
If there is the slightest chance that this data will ever “round trip” back to CAD from GIS, encoding the cookie crumbs that lead back to the CAD-specific attributes come in very handy – saving time and money when needed.  Since they are almost effortlessly captured in the export process, do it – CAD layer name, linetype, original drawing name for everything, and block name and rotation for symbols are essential.  AutoCAD will also allow these fields to be used in the MAPIMPORT command to format incoming GIS data back into its original CAD specifications.
Facility folks wishing to mitigate cost and improve efficiency can use CAD-to-GIS conversion of construction documents to drive future maintenance operations in a much more useful format.  But as this series describes, there are many ways to go about that – some more cost-effective than others.