Obtaining a DEM and Pour Point for VELMA 2.0 (Condensed Version)
Written by Dr. James Graham, Melissa Collin, & Sean Fleming
Obtaining an Original DEM
- Download a DEM that has the best possible quality for your area
- Projected into a meter based coordinate system. An integer cell size is preferred. Do not project your raster in ArcMap, instead use BlueSpray to reproject
Preparing a DEM and Pour Point for VELMA using BlueSpray
The following steps assume that you have downloaded the latest release of BlueSpray from http://gsp.humboldt.edu/Websites/BlueSpray/STUsersGuide/Download.html. Note that BlueSpray requires a 64-bit installation of Java.
Obtaining a High Quality DEM
- Obtain the Correct Area:
- Load the DEM by dragging the DEM file into a view panel in BlueSpray
- Use the Marquee tool to select the area to crop
- Right click on the DEM layer and select ”Transforms General -> Crop Raster” Click OK. This will create a new layer.
- Right click on the new DEM layer, select “Export to File…” to save the DEM
- Note that BlueSpray keeps everything in memory, not on disk. This makes processing very fast but dangerous! Remember to save any layers you need.
- Projecting:
- If the DEM is not in the desired Spatial Reference System, right click on the “Scene” and select “Change SRS”. If this is greyed out, it is because you don’t have a defined SRS and you’ll need to “Define” one first.
- Select your desired SRS. BlueSpray will project all the data in the scene to the new SRS. Save the new DEM if desired.
- Resampling:
- If the DEM does not have a desired cell size, change it by right clicking on the layer-> transforms->sample->change width and height to an integer.
- If this is not working in BlueSpray resample in ArcMap with interpolated or bilinear but not nearest neighbor to the desired cell size/resolution.
- This can also be used to down-sample to change the resolution of the DEM. VELMA needs to be in meters, so always project into a meter based coordinate system and change the cell size into meters.
Finding the Direction Raster, Accumulation Raster, & Stream Network in BlueSpray
- Finding the Direction Raster
- Right click on the DEM and select “Transforms: Water -> 2. Find Flow Direction”. Make sure the DEM is selected as the first input and click OK. This will create the Flow Direction layer.
- Finding the Pour Points
- Right click on the Original DEM and select “Transforms: Water -> 3. Find Pour Points” and click OK. This will create a Pour point layer.
- Finding the Initial Watershed Raster
- Right click on the Pour Point raster and select “Transforms: Water -> 4. Find initial watershed raster”.
- Make sure you have the correct Pour Point raster and Flow Direction raster selected and click OK. This will create a watershed raster.
- Finding the rest of the Watershed
- Right click on the Watershed raster and select “Transforms: Water -> 5. Add Pixels to Watershed Raster”.
- Use the watershed raster that was created above and the same direction and elevation raster and click “OK”. This will create a new direction raster.
- Find the Accumulation Raster
- Right click on the New Direction raster and select “Transforms: Water -> 6. Find accumulation”. This will create an accumulation raster.
- (Optional) Find the Stream Network
- Right click on the Accumulation raster and select “Transforms: Water -> 7. Find stream network”.
- Select the new direction raster for the “Directions”.
- Set the “Minimum Accumulation” value. If unsure, leave at the default of 100.
- Click “OK”. A vector layer showing each of the streams while be created.
Obtaining output for VELMA
The process below will provide a raster and pour point for VELMA.
- Making a DEM with all the pixels flowing to Pour Points
- Right click on the original DEM and select “Transforms: Water -> 8. Make pixels flow to pour point”
- Select the DEM and the new direction raster and click OK. This will create a New Elevation Raster.
- Create the Pour Point for the desired
- Right click on the “Scene”, Select New Layer, and select “Point Layer…” leave all default settings, click OK.
- Right click on the eyeball/Black cursor icon and select “Editable”. A pencil icon will appear. Make sure the pencil is selected in the drop down box in the toolbar.
- Move the Flow Accumulation layer so it is visible just below the new point layer.
- Zoom to where the pour point should be, ideally it will be at or near a gauge station. Click in the desired accumulation pixel to add the pour point.
- Create the new Watershed
- Right click on the Newly Created Pour Point layer and select “Transforms: Water
- Find initial watershed raster”. Select the New Direction raster for the direction input layer. This will create a watershed layer.
- Use the Marquee tool to crop the Flowing Elevations layer to the desired area. Make sure you have at least a 10% buffer around the entire watershed.
- Export the newly cropped layer as an ASCII file (see instruction below)
- Select the information (“I”) tool and click on the pixel that overlaps with the pour point. Note the X & Y pixel location for VELMA.
- Right click on the cropped layer and select Setting > Raster tab > and note the Column, Rows, and Cell Size for VELMA.
Exporting files in ASCII format from BlueSpray
- Right click on your file and select Export to File.
- Select the file type ASCII Grid (.asc)
- Save in your outputs folder
Changing Cell values of a Raster to Export
The sets below are to create “test” data maps that can be used as place holders when running a VELMA simulation for Tree Age, Tree Cover, and Soil. Keeping in mind using the test data will not produce highly accurate VELMA results.
- Open your final DEM ASCII file and select, Transform General
- Click Classify, under the New Value box type the Test Data value number.
- Test Data Values:
- Soil : 3
- Tree Age : 80
- Tree Type: 1
- Click Okay
- Test Data Values:
- Ensure that you are exporting integers not float rasters for all ASCIIs except for the DEM
- Transforms General, Data type, and the desired bit as an integer
- Export your raster using the instruction from above.
- Repeat for each test parameter you need to create (Age, Cover, Soil).