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   Computer Resources GIS Manual  

Working with CAD Data in GIS

Here at the GSD we have a wealth of detailed planimetric (building and pavement) and contour data available in GIS for All of New York City and the greater Boston Area. Students are always asking me. Hey, how can I extract parts of these databases and convert them to CAD? Well, here are the basic steps:

The Basic Steps:

Related Documents

Overview

 Over the past decade, it has become common for Utility companies and municipalities to compile fairly detailed information on building footprints, pavement edge, and contours covering entire cities. Geographic Information Systems are used to manage these large datasets because of its capabilities of dealing with expansive amounts of data referenced in real-world coordinate systems and the ability to attach rich sets of semantic attributes to geometric representations. Here at the Design School, it is common for students to want to incorporate these datasets into their CAD work. This entails learning a little about GIS, and converting data formats. It is relatively easy to convert some GIS data to a flat dxf layer. It is also possible to do some tricks that will make use of the GIS database to set the elevation of contours, or to control the CAD layers that the converted features will end up in.

All of this will require a little patience, and some reading of documentation and maybe a little trial and error, but in all, this will be much easier than digitizing sontours and planimetric context detail from scratch!

Find your Data and Open it in ArcMap

The GIS specialist at the GSD maitains a collection of data on a shared volume that can be mapped to your PC as \\terra\geo Within this folder you will find sub folders of data for New York City (nyc), Boston, Cambridge, etc. You can find a brief description of the contents of these folders in this index. Once you have mapped the geo drive, you can explore its contents with ArcGIS.

You can find ArcGIS and the other tools you will need installed on any of the computers in room 516. Start ArcGIS and use the Add Data button from under the file menu. At first, the drive you mapped to geo won't show up. YOu will need to use the "Connect to Folder" button See Picture. to add the new drive to your file browser.

Zoom in to your area of interest and export the data to a new shapefile

Usually CAD users do not want to download a whole city coverage of contours or building footprints as a DXF file. These files would be too big to be maneagable in a CAD program. Thus, the first step in exporting your data is to zoom in to your area of interest.

The export process happens layer-by-layer. The layers are displayed on the left side of the map window. If you have questions about which layers are which, you can turn them off and turn them back on one at a time to see what they are. Once you find the layers you wish to export, right-click on it and choose 'Data->Export.' Make sure the pull-down at the top of the screen is set to 'All Features in View Extent.' Be sure to save the resulting shape file into a directory where you will be able to find it. I suggest making yourself a subdirectory in your local c:\temp directory. For obscure reasons, this directory should not have a name that is longer than 8 characters, and its name should not have any upper-case characters or spaces.

Convert the Shapefile to DXF

References

The conversion of your extracted shape file to dxf makes use of a utility that is part of the ArcInfo option of arcgis. YOu can find this utility in the ArcToolbox (click the little red toolbox icon on the arcmap toolbar). The tools are in the Conversion Tools toolbox in the toolset Convert to CAD. Here you will find the Add CAD Fields amd the Export to CAD wizards. If you don;t see these, then you didn'nt install the Complete ArcInfo option of ArcMap. The computers in room 516 all have this. Simply converting your shape file to dxf is a simple process of selecting the input shape file and the destination for the output.

Now, lets say that you want your contours to have the correct elevations in the resulting DXF, or you would like to have them put onto separate layers within the DXF, so that you can cut them on the laser cutter. This is possible, but requires more understanding of GIS than can be included in this document. For the spirited user, you probably want to right-click on some of these tools and read the documentation.

First, punch the help button on the Export to CAD dialog from ArcToolbox. See the note at the bottom that says that the presence of certain items in the attribute table will affect the elevations and/or layers in the resulting CAD file. The word 'Items' in this context relates to columns in the attribute table for your GIS layer. Chances are, these items do not already exist, although on all of the contour layers, there will be a column that contains the elevation -- it won't be named 'ELEVATION' or 'LAYER'. Therefore, if you can create a column with the right name, and populate it with the right data, you can be master of your DXF conversion!

YOu can use the Add CAD Fields wizard to add all of the apropriate fields to your shape files, and then you will need to calculate the value of the apropriate fields. Look in ArcMap's on-line help for:

  • Tables, Calculating Field Values

Thinking about Units and Coordinate Systems

References

CAD users and architects in general have some funny views concering scales for data. In the geographical world, there is no such thing as a scale for data that can be zoomed into and out of at will. A file of spatial data stores the coordinates for each vertex or point in some coordinate system that can usually be presumed orthogonal, with units in either feet or meters, and an origin that is very far away. Occaisionally you may run accross a GIS dataset that uses a Geographic Coordinate System (GCS), which means that the units are degrees of Longitude and Latitude. This is problematic, not only because the ground distance of a single degree is so big, that the precision of your CAD system may not be able to do anything useful with the data. Also, you should know that a degree of logitude is not equal in ground distance to a degree of latitude unless you are on the equator. This difference will be substantial in most parts of the world & so you will need to project your data before converting it. This idea of projection is much too deep to discuss here. See the references cited above.

Most CAD users like to scale and shift their CAD data so that the units are familiar to them. This can create an awkward situation if you ever want to bring more data in from GIS. But if you are going to do this, just remember the scale factors that you used, and always use 0,0,0 as your reference point.