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posted 10/19/06 by Anthony Governanti Site Topology - Part One Anyway

Site Topology, what is it, and why should you care? I'm going to try to make this brief, and I'll address the details in more posts to come, but there is a lot of questions out there about site topology, and what its effect on Civil 3D is. I hope to clear some of it up, and educate those who care to know.

First off, what is a topology? Well, in mathematical terms, Topology (Greek topos, place and logos, study) is a branch of mathematics concerned with spatial properties preserved under bicontinuous deformation (stretching without tearing or gluing) as found in this Wiki.

For me, and as it applies to us Civil folks, I see it as a "network", or a mesh, of interconnected objects that are aware of themselves, and their neighbors. The easiest way to explain a topology is to see and example;


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Here we see a set of parcels. In a topology, these parcels are defined so that:
1.) They are aware of themselves - a parcel knows its perimeter, its area, and all the segments that are used to define itself

2.) They are aware of each other, if they are next to each other. This means that if parcel 25 is next to parcel 26, and they share a lot line, then the lot line only needs to be drawn once, and they both share its definition.

3.) There is a notion of the parcels as a whole or a group, so that if a lot line is removed, then what used to be two parcels is now turned into just one.

So in Civil 3D, we use a topology behind the scenes to get a lot of things to work and interact with each other. Parcels use this topology, but so do alignments and grading groups (feature lines included). This is how parcels do what they do; when you create a new lot line using the layout tools or converting a polyline, that line is added to the topology, and will react according to the rules that topology dictates.


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More on the rules later. This topology also explains how we can add labels to all of the parcels at the same time. The topology is aware of what lines are "shared" and will only label a lot line once, even if two parcels use that lot line.


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The next level of this topology is that it will allow for interaction between object types; an alignment will divide a parcel if it crosses through that parcel. Again the topology is what makes this happen, as the parcels are "aware" of the alignment, and one of the "rules" is that when an alignment crosses a closed parcel, it should subdivide it.


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A third level is how grading groups and feature lines fit in. Again the topology is aware of these objects, and if they cross other things in the topology, like parcel lines or alignments, there is interaction between them.

"OK Govna, riddle me this one; what if I DON"T want this stuff to interact?"

Ahh, there is hope for us after all; Sites. A site is a container that collects the "stuff" you do want to interact together. So if you don't want something to interact, or better put, you don't want them in the same topology, then you simply create the "stuff" in a different site.

So that's probably a good start, I'll be back with more soon, and we'll dig into some of these parcel rules....

Till next time,
Cheers,
AG

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