Polylines, Complex Line Types And Multi-Lines
Primitive objects (arcs, circles, etc.) are the mainstay of drafting in CAD, but working with such objects on a regular basis is time consuming in a high production setting. To alleviate that, CAD software makes use of advanced "complex components" such as polylines to help simplify and speed up the drafting process. A polyline is a type of line made up of multiple segments instead of having a single start and end point. If you draw a large building out of lines, each wall is an individual component and when you need to edit the building you must select each one of those components. By creating the walls as a polyline, they look exactly the same but instead of being dozens of individual lines; the structure is now a single item that can be edited with a single click of the mouse. Polylines also have the ability to add radii, curves, and widths to each line segment, greatly simplifying the editing process.
Complex line types are lines with text, or symbols, embedded in them so that the object the line represents is readily identifiable. For example, an overhead electrical wire might have the letters "OHW" embedded in it so that it would appear like so on the drawing:
Likewise, a fence line might have an embedded "X" to designate post locations, or more complex symbols may be incorporated to make a line appear as a rock wall or a series of arcs to denote a tree line. These line types can be predefined by the software, or custom created content. These can be loaded into template drawings and associated to specific layers so the drafter doesn't need to assign line types to each item, just draw on the current layer and the lines are properly displayed.
Multi-Lines are used mostly in architectural drafting. They are single segment lines, but they can be customized to show multiple parallel offsets at differing widths and colors. For example, a multi-line can be defined to show the centerline of a wall, the foundation wall at a set width on either side, and the footing line on each side beyond that, each with different line types and colors. The multi-line still acts as a primitive line for editing purposes, despite its complex display parameters.
Hatch PatternsHatching--also referred to as poche-- is the act of filling empty spaces between primitives with repeating line patterns to highlight, or distinguish, those areas from the adjacent line work. For example, it is common practice to fill in the gaps between the interior and exterior of proposed walls in an architectural plan, while leaving the gap in existing walls empty. This makes a quick visual distinction between proposed/existing without needing to refer to notes or schedules.
CAD packages come pre-loaded with libraries of various industry standard hatch patterns such as sand, concrete, brick, etc. that can be used to highlight section of plans and elevations. Hatch patterns are created as a single object so that it can be easily edited but the pattern can be exploded back into its component parts (lines, dots, etc.) if so desired though that can create editing problems later on.
Blocks, Wblocks And Grouping
It can be very beneficial to join multiple primitives into a single entity so, they can be edited as a group, or even to keep others from accidentally modifying components of a design individually. For example, you may have a completed building that the civil engineer wants to use to locate your design on the site. You want to ensure that all components of the building get moved as a unit so that nothing is lost or changed when the engineer is moving the structure around. This is accomplished by means of the "block" command. Blocks are assemblies of multiple primitives, that aren't necessarily spatially connected, into a single -named- object. Because blocks are named and saved, you have the ability to insert them from a library, for multiple instances of the same symbol. Objects such as trees, catch basins, or steel I-beams are made up of multiple lines and arc segments. By creating these as named blocks, you can insert them into the drawing by name (i.e. "oak tree") and have each block be a single item that you can move about with a single pick.
You can also assemble individual blocks into larger blocks. Say you have a planting design of six oak trees that need to be in an identical pattern, in different locations on your site. You can create a new block called "oak planting layout" from the six oak tree blocks so that you can insert the layout into other locations as needed. Named blocks reside only within your current drawing. This is limiting because it is often useful to use defined blocks in many drawings. This is the function of the Wblock (Write Block) command, which allows you to save a block as a separate named file on the network that can then be inserted into other drawings as needed or even become the basis of a new design drawing in its own right. "Grouping" is similar to a block except that the group is not named, it is primarily a temporary assembly that allows you to move/edit a series of objects as a single unit but it can't be inserted into other locations.
External References (Xrefs) are a very powerful tool in the CAD environment. They are similar to blocks but with a few notable differences. Xrefs are an insertion of the graphic display of one drawing into a second drawing. In other words, if I have a drawing that shows all the existing information for my site, I can Xref it into my proposed layout plan and turn layers in the Xref on/off as needed, creating a "background" image that I can draft the new design on top of. Since the Xref is just a graphic image, it doesn't reside in memory of the current drawing; it just loads on start so that it doesn't increase the file size of the open drawing.
In addition, any changes that are made to the source file will automatically propagate to all drawings that have it inserted as an Xref. This ensures that changes to any plan in a set are correctly shown on all other plans that have been linked to it via Xrefs. Xrefs can be "nested" meaning that I can reference a drawing that already has Xref's inside it and those references will display in my current drawing as well. In other words, if I Xref file "A" into file "B" and then Xref file "B" into file "C", all the graphic information from both "A" & "B" will display on my screen in file "C". This process can be a dramatic time saver on large drawing sets, where a small change to the first drawing needs to show on all subsequent plans. If there are one hundred drawing in your set, a drafter would need a full day to update each plan manually, whereas if they're referenced together, the change need only be made in the first drawing and it is automatically updated in all other plans.
See, that's not so complicated after all, is it? How do you use complex objects to make your drafting simpler?