Organizing CAD Data In ArcGIS: Part 2/7 Map Layer
My two year old has two distinct uses for her big bucket of blocks. One use is to dump out the contents of the blocks onto the floor and then meticulously stack them into a monolithic tower. The other use is to take the bucket of blocks as a whole dump it upside down and use it as a stepping stool to wreak havoc on my wife’s desk, and to retrieve snacks from the counter top!
ArcGIS has two distinct ways to represent CAD drawing data. One way is to treat the entire CAD file as a single cartographic layer. The other way is to treat the CAD file as a container of geographic features.
A CAD map product is often useful in its entirety as an information layer. The CAD document tells a story all by itself and can be useful as-is for its cartographic content. ArcGIS has the ability to represent an entire CAD file as a single cartographic layer where all of the vector objects and text are included on a single layer. The CAD graphic properties are used to draw the CAD file in ArcMap. In this way an ArcGIS CAD Drawing Layer is not unlike a raster image in that it is the picture of the entire CAD file that acts as a valuable data layer. The CAD Drawing layer mimics the CAD symbology and provides a single entry in the ArcMap table of contents for easy visibility control. The CAD object representation is still as vectors and objects can be selected and identified. However, spatial analysis and other advanced query operations are only possible with the GIS feature representation of a CAD file provided by the ArcGIS CAD Feature Dataset representation of a CAD file. I will discuss the ArcGIS CAD Feature Dataset representation in a future post.
We can simply navigate to a CAD file and select it to be added to a current ArcMap map. In previous posts I have discussed the affects and usefulness of defining a spatial reference. If a companion ArcGIS projection file is found in the directory with the selected CAD file a CAD file will be displayed on the map according to its location on the globe relative to other data already in the map frame. If no projection file is included the CAD file will be positioned according to the coordinates unmodified in the CAD file relative to whatever the coordinates are currently in the map frame. Since ArcGIS has no other information it assumes the CAD files coordinates should be displayed according to the map current frame’s coordinate system.
In a previous post I also discussed the use of an ESRI CAD world file, used to modify the coordinates in a CAD file to perform any need transformation of the coordinates that would relocate, scale and rotate data drawn in a relative coordinate system for use in a geographic coordinate space.
When browsing for CAD files you will notice two entries in the Add Data dialog or in the ArcCatalog data browsing pane. The white compass icon denotes the CAD Drawing Layer representation of an entire CAD file and the blue folder icon is used to access the CAD Feature Dataset, which include the five standard ArcGIS CAD feature classes.
Once the white drawing icon is selected all of the objects in the CAD file are displayed as a single layer in the table of contents. Using the layer properties dialog box for the CAD Drawing layer you can modify the transparency and some other display characteristics such as renaming the Map layer and controlling the visibility of individual CAD layers in the CAD file. The symbology however is fixed to that found in the CAD file. Otherwise the CAD Drawing Layer acts like any other ArcMap layer. You can move the layer up and down in the drawing order by dragging it and toggle its visibility for the entire layer.
Using the CAD Drawing Layer is most useful when the CAD file is itself a complete cartographic representation of the data useful as-is for plotting or browsing the data.