Geometric Network 基本概念和连接规则

Geometric Networks（几何网络）可以在ArcInfo和ArcEditor中创建，ArcView只能浏览。

Geometric Networks一般用于自来水管道流向、电线、燃气管道、电话网络等，它由一组相互连接的edges和junctions组成，并且具备相应的连接规则来代表真实世界中相应实体的行为。Geodatabase feature classes作为数据源用以定义geometric network。

几何网络一般建立于geodatabase的feature dataset中，feature dataset存储的是几何网络的edges和junctions数据源，几何网络的两个主要组成部分就是Edges和Junctions。

下面是对这些基本要素的定义：

Edges - An edge is a feature which has a length through which some commodity flows. Edges are created from line feature classes in a feature dataset and correspond to edge elements in a logical network.

Examples of edges: water mains, electrical transmission lines, gas pipelines, telephone lines, etc...

Junctions - A junction is a feature that allows two or more edges to connect and facilitates the transfer of flow between edges. Junctions are created from point feature classes in a feature dataset and correspond to junction elements in the logical network.

Examples of junctions: fuses, switches, service taps, valves, etc...

几何网络中的Edges和Junctions在拓扑关系上是相互连接的，edges必须通过junctions和其他edges连接。关于edges和junctions有更细的划分：

There are two types of edges in a geometric network:

Simple Edges - Simple edges are always connected to exactly two junctions, one at each end.

An example of a simple edge would be a water lateral in a water network. The water lateral connects at one end to a junction along the main distribution line and, at the other end, to a service point junction (such as a tap or pump).

Simple edges have no mid-span connectivity. If a new junction is snapped mid-span on a simple edge, thereby establishing connectivity, then that simple edge is physically split into two features.

A simple edge corresponds to a single edge element in the logical network.

Complex Edges - Complex edges are always connected to at least two junctions at their endpoints but can be connected to additional junctions along their length.

An example of a complex edge would be a water main in a water network. The main water distribution line is a single complex edge with multiple lateral lines connected to junctions along its length. The water main is not split at the junction where each lateral connects to the main.

Complex edges have mid-span connectivity. If a new junction is snapped mid-span on a complex edge, that complex edge remains a single feature. Snapping the junction does cause the complex edge to be split logically—for example, if it corresponded to one edge element in the logical network before the junction was connected, it now corresponds to two edge elements.

Complex edges correspond to one or more edge elements in the logical network.

There are two types of junctions in a geometric network:

User defined junctions - Junctions that are created based upon a users source data (point feature classes) when the geometric network is first established. Junctions correspond to a single junction element in the logical network.

Orphan junctions - When the first edge feature class is added to the geometric network, a simple junction feature class is created, called the orphan junction feature class. The name of the orphan junction feature class corresponds to the name of the geometric network appended with '_Junction'. For example, a geometric network named 'Electric_Net' would have a corresponding orphan junction feature class named 'Electric_Net_Junctions'. The orphan junction feature class is used by the geometric network to maintain network integrity.

During the creation of the geometric network, an orphan junction is inserted at the endpoint of any edge at which a geometrically coincident junction does not already exist in your source data. Orphan junction features can be removed from the geometric network by subsuming them with other junction features. To subsume an orphan junction is to incorporate it into the network by deleting it and replacing it with a user defined junction while still maintaining network connectivity.

The orphan junction feature class is deleted when its geometric network is deleted. For this reason, the schema of the orphan junction feature class should not be modified.

Geometric Network有几个比较关键的概念，直接影响实际业务模型的建立：Sources和sinks决定几何网络的流向，比如水流方向，输电方向；Network weights代表了网络传输过程中每段路程所需付出的代价，可以是费用、时间等，它可以存在于几何网络所有要素中；Enabled和disabled features是网络中的障碍物。

Geometric Network连接规则（全为帖图，如有疑问，请参考文档或留言）：

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