Key Procedures
SECTION 5.2: DEM S ETUP
1. The DEM Setup section is shown in Figure 5.2.
Figure 5.2
One button loads the DEM grid map used to calculate all subbasin/reach topographic parameters. Two check boxes (options) load or create a mask grid and/or load a “BurnIn” stream dataset. A “BurnIn” stream dataset is used to force the SWAT subbasin reaches to follow known stream locations.
2. To load or select the DEM grid, click the file browse button beside the text box labeled ”Open DEM grid"
3. A dialog box is opened to specify which DEM map grid to use (Figure 5.3).
Figure 5.3
You may choose Select from Map to choose a DEM grid that previously has been added to the current ArcMap document.
Click OK after the selection. If the first option was selected, the list of the grid layers in the current map is shown (Figure 5.4) otherwise a grid dataset file browser (Figure 5.5) will appear to allow you to specify which DEM will be used.
Figure 5.4
Figure 5.5
Select the name of the DEM map grid and click OK if the DEM was chosen from a layer in the current map. Otherwise click Add if the DEM was selected from disk.
4. The DEM is loaded into the current ArcSWAT project “Watershed\Grid”
folder, and the new path to the source DEM is shown the text box. If the DEM chosen is in a geographic coordinated system defined, the message in Figure 5.6 will appear. This will indicate that you must go back and project your DEM into a projected coordinate system before proceeding. If the DEM chosen has no coordinated system defined, the
message in Figure 5.7 will appear. This will indicate that you must go back and define a proper projected coordinate system for you DEM before proceeding.
Figure 5.6
Figure 5.7
5. Once your DEM has been properly loaded, click the DEM projection setup button (Figure 5.8) to define the properties of the DEM.
Figure 5.8
6. The DEM Properties dialog box will open and allow the DEM vertical and horizontal units of measure and the projection to be verified (Figure 5.9).
Figure 5.9
The DEM X-Y units and the spatial reference cannot be edited from this interface. You must define these parameters of your DEM during projection definition prior to using ArcSWAT. The Z unit can be changed using the drop-down box provided.
Note: Careful!! The DEM properties dialog should correctly report the horizontal and vertical units. Incorrect settings will affect the results of the watershed geomorphic parameterization. If the user does not select Z-units, the interface will use z-units of meters by default.
7. Once the DEM properties have been set, click OK. This will close the DEM Properties dialog box.
8. Define Mask (optional)
The first option in the DEM setup section allows you to import or create a dataset that masks out a part of the DEM grid. Only the portion of the DEM covered by the mask will be processed by the interface. This map is not required but will reduce the processing time of the GIS functions.
Click the check box beside Mask, then, click on the file browse button next to the Mask text box. A prompt dialog will open (Figure 5.10)
Figure 5.10
The user has three options for masking an area of the DEM map. To activate one option, highlight the option and click OK.
a. The first option, Load from Disk, allows the user to import a grid map from a disk drive. If this option is selected, a grid data set browser is opened (Figure 5.11).
Figure 5.11
Select the name of the mask grid and click Add. The mask is
loaded into the ArcSWAT project “Watershed\Grid” folder and the new path to the Mask grid is shown the text box.
b. The second option, Select from Map, allows a raster dataset already loaded in the current map document to be selected as the mask.
A prompt box appears with a list of all raster loaded in the Watershed View (Figure 5.12).
Figure 5.12
Select the name of the mask raster and click OK. The mask is loaded into the ArcSWAT project “Watershed\Grid” folder and the new path to the Mask grid is shown the text box.
c. The third option, Manually Delineate, allows the user to draw and edit a polygon mask using the manual delineation tool. (Figure 5.13)
Figure 5.13
While delineating the mask, the standard ArcGIS zoom-in and zoom-out tools can be used without closing the dialog.
To begin delineating a mask, click the draw button . A message box will appear, letting you know that you can begin drawing the mask.
Figure 5.14
Click on the map to begin defining the polygon boundary. Click at each corner boundary, or vertex, of the polygon. Double-click the final vertex. The shape of the polygon will be displayed (Figure 5.15)
Figure 5.15
To add a new vertex to a polygon or move a vertex, click the Edit button . Move the cursor over the mask polygon and double click. This will highlight the vertices for the polygon. To add a new vertex, move the cursor to the position on the line where the new vertex will be located.
Right-click and select Insert Vertex (Figure 5.16).
Figure 5.16
To delete a vertex, move the cursor over the vertex to be deleted. The cursor will become a crosshair. Next, right-click and select Delete Vertex (Figure 5.17).
Figure 5.17
To move an existing vertex, place the cursor on the vertex to be moved.
When the cursor symbol changes to a crosshair, hold down the left mouse button and drag the vertex to the new position (Figure 5.18).
Figure 5.18
To stop editing the mask polygon, right click outside the polygon and select Stop Editing.
Figure 5.19
To delete a mask polygon, click the Delete button . The current mask polygon will be removed from the map.
Once all drawing and editing on the grid mask is completed, click the Apply button. This will convert the mask polygon to a grid dataset that will be stored in the project “Watershed\Grid” folder.
d. Once the mask grid is loaded, the grid data set path will be shown in the text box labeled “Mask” in the Watershed Delineation dialog box and a layer called “Mask” will be added to the map.
Note: The Analysis Mask of Spatial Analyst Properties is now set. Applications of Spatial Analyst commands will be limited to the mask zone.
9. Burn in a stream network (optional)
A stream network dataset can be superimposed onto the DEM to define the location of the stream network. This feature is most useful in situations where the DEM does not provide enough detail to allow the interface to accurately predict the location of the stream network. Burning in a stream network improves hydrographic segmentation and sub-watershed boundary delineation. The theme must be a polyline shapefile or feature class.
Tip: Prior to loading Burn In streams, you should edit your stream dataset to provide a continuous set of stream lines (e.g. draw lines through lakes and ponds, remove isolated reaches). For this task, you can start an editing session in ArcMap and property modify the stream network if necessary
Note: Other than outlet lines, the stream lines should not cross the edge of the DEM (or the Mask Area if a mask was set). Lines crossing the edge can affect the resulting flow direction.
To load a streams dataset, click the check box beside Burn In, then, click on the file browse button next to the Burn In text box. A prompt dialog will open (Figure 5.20).
Figure 5.20
You may select a polyline layer that previously has been added to the current map or load a polyline dataset from disk.
Click OK after the selection. If the first option was selected, the list of the polyline layers in the map is shown (Figure 5.21) otherwise, a polyline dataset file browser (Figure 5.22) will appear to allow you to specify which dataset will be used.
Figure 5.21
Figure 5.22
Select the name of the stream network dataset (hold the Shift key for multiple selections) and click OK. The burn in streams dataset will be converted to a raster and imported into the project “Watershed\Grid”
folder. When importing is completed, the new dataset path will be shown in the text box labeled Burn In. The new stream raster will be added to the current map and will be named “DigitStream”.