Geography 3251: Mountain Geography Assignment III: Natural hazards A Case Study of the 1980s Mt. St. Helens Eruption

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1 Name: Geography 3251: Mountain Geography Assignment III: Natural hazards A Case Study of the 1980s Mt. St. Helens Eruption Learning Objectives: Assigned: May 30, 2012 Due: June 1, 9 AM 1. Learn to characterize landscape change. 2. Learn about the 1980 eruption of Mt. St. Helens. 3. Learn to work with digital elevation models (DEMs) and remotely sensed data. 4. Gain insight into how 3D images can provide information about planning for natural hazards. Instructions: For this exercise we make use of digital elevation models and remotely sensed imagery from before and after the 1980 eruption of Mt. St. Helens. Note you do not need to have a background in remote sensing or GIS to complete this lab! Please Part I: 2D Analysis of Change 1. Navigate to the Z:\Geog Files\Hart\GEOG3251\Assign3 and copy it to the D: drive. 2. Open ArcMap 10 and open a blank map. Add the before and after DEMs by selecting the add data button and navigating to where you have your data on the D: drive. 3. Convert the post eruption DEM elevations to meters by opening the ArcToolbox, navigate to Spatial Analyst Tools, Map Algebra, and selecting Raster Calculator. Convert feet to meters. 4. Check to see if the spatial analyst toolbar is checked out. To do so go to the customize tab and then extensions. Click the box next to Spatial analyst. 5. From the ArcToolbox, choose Spatial Analyst Tools, Surface, Hillshade. Choose Mt. St. Helens before as the input. For the output raster type before_hs, leave all other values the same and click OK. Repeat the same procedure for Mt. St. Helens after, and name the output after_sh. Now you can see the hillshade. Turn on and off each layer to see the change before and after the eruption.

2 6. Now we want to subtract the before DEM from the after DEM to determine elevation changes for the region. Then we will be able too see which areas increased or decreased in elevation. In this step ArcMap will take each pixel and subtract the pixel value of the before DEM from the pixel value of the after DEM. Note that the pixel value equals the elevation. Within the ArcToolbox, navigate to Spatial Analyst Tools, Map Algebra, Raster Calculator. Type the following expression in the expression box: Mt. St. Helens after - Mt. St. Helens before Click OK. A new grid is created showing the overall elevation change after the eruption. Positive values represent an elevation gain, while negative values represent an elevation loss. 7. Next lets make the symbology a little clearer for the Change layer. Right click on change and select Properties then navigate to the symbology. On the symbology tab choose Classified on the left side. Next change the number of classes to 3, and click on the Classify button located on the left. Underneath the break values we want to change the number so they are a little more meaningful. Type -1, press enter, type 1, and leave the last value as 122. Click Ok. In the layer properties menu we want to change the label name so each range makes sense to the user. Click in the first range ( ) underneath the label column. Change the name to Loss, for the second column change the name to No Change, and for the last column type Gain. Now we have better classifications, everything with a negative loss will now be labeled loss, everything within -1 and 1 is considered no change, and everything which has positive change is a gain. Before closing this dialog, choose the display tab and type 40 in the transparent box. Click Ok. 8. Next we want to isolate the area that lost material. To do this we are going to modify the change DEM and create a new DEM which will just show the elevation loss. From the ArcToolbox,Spatial Analyst Tools, Reclass, choose Old Values New Values NoData NoData NoData NoData Reclassify. Make sure Change is chosen for the input raster. Leave 1 under New Values for the first row. In the next two columns change the value to NoData, therefore you will have a table like this: Save the output raster as Loss_dem. Click Ok. Now you can see the area of elevation loss only. Even though the DEM only has a value of 1, we can use this as a mask or clip area, as shown in the next step. 9. Next we want to calculate the volume of removed material. First we need to create new DEMs that only have the area that lost material. To do so use the spatial analyst extract by mask tool to create a new change dem. Select the input raster to be change, the input raster or feature mask to be the loss raster, and the call the output raster losschange

3 10. From the Spatial Analyst menu open the Raster Calculator and type in the following expression: Lossvolume = (([losschange] * - 1) * 30 * 30) note ArcMap is picky with spaces, so make sure you have them correct. We want volume to be a positive number therefore we multiply the Change dem by -1. To obtain the volume we then need to multiply by the area of each cell with is 30 x 30 meters. Click Evaluate. The Lossvolume dem is now added to the map. We can see the greatest amount of volume lost occurred in the crater which was left after the eruption. 11. Change the symbol of the LossVolume layer. Show stretched (on the left) and choose red bright for the color ramp. You can view the text by right clicking in the color ramp and check off graphics view. Feel free to use your own color here. Just choose another color other than gray. Next click on the display tab and make the layer 40% transparent. 12. The Lossvolume layer creates a DEM of the volume lost for each pixel; we now want to obtain the total amount of volume which was removed, essentially adding up all of the pixel volumes. Open the Raster Calculator and type the following expression: Total = zonalsum([loss_dem],[lossvolume]) Be sure to double click the layer in the layers column instead of typing it out for everything which appears in brackets. The zonal sum is taking the sum of all of the values from Lossvolume for the area of Loss_dem. Loss_dem is essentially our mask, to ensure we are only calculating values for the aerial extent of Loss_dem. The Total layer is now added and gives the total volume loss. Part II: 3D Analysis of Mt. St. Helens 1. Open ArcScene. This is located under the ArcGIS tab in programs. 2. Add the before and after DEM. Rename the layers to Mt. St. Helens Before and Mt. St. Helens After. 3. Open the Mt. St. Helens Before layer properties. Choose the base heights tab. Click on obtain heights for layer from surface, and from the dropdown choose the before dem. (The previous step now assigns the elevation data to the layer which will make the layer 3D. We can use this for vector data as well. For example if we had rivers available for this region we could assign the base heights from the dem, then the rivers will appear in 3D and will appear on top of the DEM.) Next choose the rendering tab and check Shade areal features relative to the scene s light position. In the symbology tab choose use hillshade effect. Choose a color ramp of your choice.

4 4. Repeat step 3 for the after DEM. Be sure to choose the after DEM in the base heights window. 5. Turn the before and after DEMs off and on to see the changes in 3D. You can use the navigate tool to move around the DEM by moving around it or up and down. You can also fly through the scene using the fly button which looks like a bird. The more you left click when flying the faster you will fly, to reduce your speed click on the right button and to stop click on the middle button.

5 1. Name: Geography 3251: Mountain Geography Assignment III: Natural hazards A Case Study of Mt. St. Helens Questions 1. What units are used for the DEM data? (2 pts) 2. What is the maximum elevation in the Mt. St. Helens before DEM? Where is this located? (3 pts) 3. What is the maximum elevation lost from Mt. St. Helens before and after the eruption? What is the maximum elevation gain from the eruption? (Include units) (3 pts)

6 4. What is the total volume loss? (Include units) (2 pts) 5. How can 3D data help with hazard planning? You may need another sheet of paper to answer this question. (15 pts)