几何网络是有向网络，是用来模拟现实中的如排水管，天然气管，电力等的网络。可是这个有向到底体现在哪里，原来以为以为集几何网络的方向是数字化的方向，但是我错了。我一般用几何网络做路径分析，我们知道，如果不强加限制，从A点到B点，那么，逆着原路走，也可以从B到A,这似乎体现不到方向，但是追中分析中，这种是很明显的。我们现在从几何网络的特点说起：
网络要素只存在于几何网络中
三种类型:
简单节点（Simple junction）
简单边 （Simple edge）
复杂边 （Complex edge）
独立节点类
维护网络的完整性
网络边的端点必须和网络节点连接
系统维护
网络连通性的物理表现 – 索引
高性能图形引擎
快速的网络搜索
紧凑高效
连通关系、权重（属性）存储在BLOB中
各种网络分析功能基于逻辑网络进行
简单边
要素（feature）与图形元素（element）一一对应
在简单边中间插入简单节点，简单边将被打断
复杂边
要素（feature）与图形元素（element）一对多
复杂边的中间增加网络节点不会打断边
会在复杂边的几何图形中增加一个节点
会产生新的逻辑要素 – 如：边元素（edge element）
可以使用“分割”工具打断线
在创建几何网络的时候 指定了包含源和汇的要素类，那么在这些里面会多出一个字段 ancillary role，这个有三个阈值 Nnone，Sink，Source。
而几何网络中的方向是由连通性，源和汇，可用性决定的，所以在建立几何网络后，我们就要做这些设置，下面的是我设置好的一个，这个比较简单，只是为了说明问题，在设置这些之后，我们可以看到流向。
Create a Geometric Network and Set Flow Direction
Start with a stream layer that is completely connected with no breaks. There cannot be any braided stream channels either. This layer can be in shapefile or Geodatabase format.
Next you need to export all of the nodes from the stream layer into a point shapefile. However, you only want to export nodes for stream junctions and dangles (end points). You don’t want pseudo nodes (arc mid-points). The best way to get the nodes is using ET Geowizards. ET Geowizards > Polyline > Export Nodes > Dangling and Regular.
1) Use ArcCatalog to create a File Personal Geodatabase. ArcCatalog > Right click > New > File Geodatabase. Create a Feature Dataset inside the Geodatabase. Inside Geodatabase > Right click > New > Feature Dataset.
2) Import your stream layer into the Feature Dataset. ArcCatalog > Inside Feature Dataset > Right Click > Import > Feature Class Single. Also import your node point layer into the Feature Dataset. ArcCatalog > Inside Feature Dataset > Right Click > Import > Feature Class Single.
3) Next create the Geometric Network. Right click the right pane in ArcCatalog inside the Feature Dataset. Select New > Geometric Network.
When running through the dialog, select both the lines and points to participate in the network. Tell the wizard that you want to set sources and sinks. Finish the wizard.
4) Add the geometric network to ArcMap. Be sure the original point layer and line layers are displayed.
5) Next edit the point layer to set the sink. Editor > Start Editing. Make the point layer the editable layer. Select the point that is the pour point for your network using the editor select tool. This would be the very end point where all water would flow out of the network.
6) Open the attribute table and look at the selected record. There should be a field called AncillaryRole. Left click the value in this field and set it to Sink. All of the other records should have a value of None. Save edits and stop editing. (Note that the very end point may not work. You may have to select the next one upstream in the network.)
7) Display the stream lines, junctions, and points in ArcMap. Add the Utility Network Analyst toolbar to ArcMap.
8) Editor > Start Editing – I don’t know which layer should be the active one for editing, so do all three layers. Select each layer in the Editor dropdown and then hit the Set Flow Direction icon on the Network toolbar. That should set the flow. Save edits and stop editing.
Flipping Arcs
Scenario 1 – No geometric network with a few arcs to flip.
If you have not set up a geometric network and want to see the direction that your arcs are digitized, you can symbolize your arcs using the arrow symbol that is in the standard symbol set called “Arrow at End”.
If you need to flip just a few arcs, you can begin an edit session. Then double click the arc so that it becomes a sketch and you can view the vertices as green squares. Then right click a green square and click Flip. The arc direction gets flipped.
Scenario 2 – No geometric network with lots of arcs to flip.
If you have lots of arcs to flip you need to find a good way to determine which ones need to be flipped. You can use a DEM along with your stream line data to do this. Go to the ESRI Support page > Downloads > ArcScripts. Search “Flip Lines”. You should see a Python script called “Calculate polyline endpoint elevations – ensure streams flow downhill”.
http://arcscripts.esri.com/details.asp?dbid=15527
Download and run this script. The script will not flip the arcs, but can help you determine which ones to flip using the from node and to node elevations.
To actually flip the arcs, download and run this script:
http://arcscripts.esri.com/details.asp?dbid=13023. This is called “Flip Lines” and it’s the version written by Michele Lundeen.
Scenario 3 – Geometric network is created with lots of arcs to flip.
If your network is already set up, you can use a VBA program to flip all arcs so that their digitized direction points downstream. The program is on the ESRI web site here:
http://arcscripts.esri.com/details.asp?dbid=13023. This is called “Flip Lines” and it’s the version written by Michele Lundeen.