Editing the Stream Theme

Because GIS data are often created without the purpose of hydrologic modeling in mind, it is often the case that you will need to perform some editing to explicitly define topologic relationships before transferring the data to WMS. Most of the tools available in WMS-Hydro are for the purpose of preparing these data. WMS-Hydro includes several tools that can be used to clean-up or edit stream and basin layers so that problems described in the previous two sections can be overcome. The primary purpose of the editing tools is to make the stream network topology consistent so that a hydrologic model can be developed. Selecting the Stream Editing… command in the WMSHydro menu activates the stream editing tools dialog. The dialog is shown in Figure 1, and
includes four tools and three buttons for performing stream editing. A description of what each tool/button does when chosen follows.

1. Select Stream Tool

The three buttons and the extend to basin tool all require you to first select a stream arc. If you want to select two or more streams at the same time then hold the SHIFT key down while making your selections.
 
2. Split Stream Tool

The split stream tool is used to convert a single stream arc into two. You may want to split a stream so that you can begin a new arc at the location where the arc was split. To split a stream you first select the split stream tool and then you digitize (drag) a small line crossing the arc at the place where you wish to split it with the mouse cursor. The stream arc will be split at the location where the digitized line crosses the stream. The line digitized is a graphics and should be deleted when the operation is completed. If it is not you can select and delete it manually.
 
3. Snap Nodes Tool

The snap nodes tool can be used to snap the ends of two or more arcs together. This is important to make sure that stream topology is explicit at junction points. Existing data may appear correct until you zoom in and discover that a branching location does not connect to the main stream. After selecting the snap nodes tool you should drag a circle around the nodes you wish to snap. When all nodes are enclosed release the mouse button and the arcs will be snapped together. Figure 2 illustrates this process. A combination of split stream and snaps may be required to actually snap a branch to the main stream if there is not a node already present. The extend stream buttons can also be used to accomplish the same thing.


Figure 2 Snapping Streams Before and After.

4. Extend to Basin Tool

Just as stream branches may not be tied together, the location where the stream meets the edge of the watershed may not be topologically correct. The extend to basin tool can be used to force an intersection in the watershed polygon layer where the stream meets the boundary. To extend a stream basin you should first select the stream arc you wish to extend using the select stream tool, then select the extend to basin tool and select the end node of the stream you wish to extend. The stream will be extended along it's direction near the end of the arc until it intersects the basin. The end node of the stream will be extended to the intersection point and a new vertex will be inserted into the polygon layer. Figure 3 illustrates this process.


Figure 3 Extending a Stream Arc to a Watershed Boundary.

5. Merge Stream Arcs Button

This button allows you to combine two stream arcs that share a common end point. It is useful because it allows you to reduce the number of arcs required for processing and developing hydrologic models. However, it is not required to merge all such arcs. You should first select the consecutive arcs using the select stream tool and holding down the SHIFT key. With the arcs selected choose the Merge button and the arcs will be combined into a single arc.  The attributes of one of the arcs are lost (it is random which of the arc's attributes are used for the newly merged attribute so you likely will not want to do this unless the attributes are the same for each).
 
6. Extend-F (Forward) Button

The extend forward button allows you to extend a selected stream forward until it intersects the nearest stream arc. This is similar to the extend stream to basin tool except that it ties to stream arcs together rather than a stream arc and a basin polygon. The stream direction and connectivity tools described below may be useful in determining whether you will want to use this button or the extend backward button. The selected arc will be extended in the forward along the same direction defined by the end (to node) of the arc until it intersects the nearest arc.
 
7. Extend-B (Backward) Button

The extend backward button allows you to extend a selected stream backward until it intersects the nearest stream arc. This is similar to the extend stream to basin tool except that it ties to stream arcs together rather than a stream arc and a basin polygon. The stream direction and connectivity tools described below may be useful in determining whether you will want to use this button or the extend forward button. The selected arc will be extended in the backward along the same direction defined by the beginning (from node) of the arc until it intersects the nearest arc.

These materials may be used for study, research, and education, but please credit the authors and the Environmental Modeling Research Laboratory, Brigham Young University. All commercial rights reserved. Copyright 1999 Environmental Modeling Research Laboratory.