WMS:ArcView Data Guidelines

Because the data structures used for the three primary methods of hydrologic data development in WMS parallel data types found in GIS software such as ARC/INFO® and ArcView®, there are several possible scenarios for importing and using data developed by GIS applications that support the ArcView® file formats.

Shapefiles

There are nearly as many different ways to have stored watershed data in a GIS as there are watersheds stored. Therefore, what is done with basin polygons or stream networks in WMS will likely be somewhat different. This section outlines some of the key issues involved in importing shapefiles (ARC/INFO® or ArcView® data) and provides examples of common problems.

Fundamental to importing any vector data layer is the ability to map attributes associated with the shapefile to corresponding parameters used by WMS. The same dialog is used for each of the three basic layer types. A set of key words can be used to define the item names of attributes in the shapefile so that mapping to corresponding variables in WMS occurs automatically. In the event that the attribute name is different, the fields can be manually mapped. Regardless of the way the data is stored in the GIS, take advantage of as much of the predefined and stored hydrologically related parameters as possible.

Ideally the project will have three data layers when importing watershed related shapefiles: 1) a polygon layer representing basin boundaries, 2) a line (arc) layer representing the stream network, and 3) a point layer representing the outlet points (these should be the intersection points of the basin polygons and stream network layers). If these three layers have been properly defined, import them and automatically create the topologic model used for hydrologic modeling in WMS. If one or more of these layers are not present, either create it in ArcView® or ARC/INFO® or define it in WMS after importing what is available. For example, if there is only a layer defining basin boundaries then construct a network using feature objects in WMS which properly connects the basin polygons together. This stream network may or may not actually represent the conveyance channel in the actual watershed, but must be present at least to the point that connectivity between sub-basins is defined.

Another problem that may occur is the ordering of vertices/nodes in a stream network may not be consistent with what WMS expects. WMS expects that the first node (or from node) in an arc be the downstream node, while the second node (or to node) is the most upstream node. If the stream vector data is not defined in this fashion it will have to be reordered prior to generating a correct topological watershed representation. The Reorder Streams command can be used to accomplish this.

A special extension for ArcView® has been developed by EMRL to allow some of these editing procedures to be taken care of on the GIS side. This extension also allows to package the data in a superfile and then start WMS and pass the necessary data directly. See the WMS home page for more information about downloading the ArcView® extension with accompanying documentation.

Grid (DEM) Files

All ARC/INFO® or ArcView® grid data imported to WMS must be in the ASCII grid format. Grid files can be used as DEMs in WMS. Also, flow direction and flow accumulation grids can be used to define their respective attributes for DEM points. Once having imported the elevation and flow direction ASCII grids, all of the remaining watershed parameters can be developed directly within WMS. The elevation DEM may also be used as the background elevation map used when creating a TIN.

TIN Files

ARC/INFO® TIN files must be in ASCII-NET format in order to import them and use for watershed characterization. The TIN can be used directly to create streams and basin boundaries, or as a background elevation map used in conjunction with feature objects for TIN creation.

Related Topics

• Importing Shapefiles
• Importing DEMs