II. DEM Differencing

Once the 1954 and 2001 DEMs are aligned the DEM differencing analysis can begin. If two DEMs need to be subtracted a simple DEM subtraction can be done using the raster calculator (discussed later), but in this case I wanted to analyze not only elevation change, but volume change as well. For this study I chose to use analysis masks to better define the area of interest and then performed the differencing on the analysis mask, which also reduced processing time.  Analysis masks are essentially clips out of the DEM.  The analysis masks/clips are drawn based on field data, aerial photographs, and satellite imagery. In this case analysis masks were created for the Santiaguito dome complex, a proximal and medial region south of the dome, and a single mask for Pacaya.  Analysis mask are used to clip out the same extent from each DEM ensuring only the areas of interest are analyzed.  The Santiaguito masks were chosen based on previous work done by Harris et al., 2006. In creating the analysis masks you want to confine the activity related only to the volcano, therefore the more defined analysis mask the better.

Creation of Analysis Masks: To create an analysis mask/clip I first created a new shapefile. This was done using ArcCatalog which is the file management system for ArcGIS.  ArcCatalog is found under the ArcGIS tab in the start menu.  ArcCatalog is shown below:

       

To create a new shapefile simply click on the folder which the file should be save in, choose new, and choose shapefile or personal geodatabase.  If all of the data is located in a personal geodatabase, save the file in here, otherwise save as a shapefile.  Next the Create New Shapefile menu is shown (above) which requires the file name to be specified.  Note that ArcGIS does not like special characters, spaces, or names longer then 13 letters.  Name the shapefile to represent the area you that will be clipped, in this case I am creating a clip of the dome. Next the feature type is chosen; the options are point, line, and polygon.  For this study I used polygon.   Next I chose edit to specify the coordinate system. Note that if you have elevation data you would check the box stating the coordinates will contain Z values.  This does not apply to this case. This procedure would be done anytime you created a new shapefile for any dataset. 

Editing Analysis Mask:
Once the analysis masks are created the shapefile is added into ArcMap.  Next the shapefile has to be edited to create the polygon for the analysis mask. This is done using the editor toolbar. I am only going to go over basic steps for editing the analysis masks, if you need more help please refer to ArcGIS Help.



In the above image I have added our analysis mask, Dome_clip, and I have also added the hillshade for the 2001 DEM for Santiaguito.  To show the Editor toolbar, go into the gray space at the top of the window and right-click. Choose Editor, now the Editor toolbar is visible. On the editor toolbar choose start editing from the dropdown menu.  Choose the file you which to edit, in this case Dome_clip. Make sure this is in the target dropdown and create new feature is the task.Click on the pencil tool, which will be used to draw the polygon. To start drawing the polygon, single click, until the polygon is finished, double-click to end it. If the polygon needs to be modified, first select the polygon with the black arrow, and then choose Modify feature under the task menu.  I would highly suggest using the ArcGIS help to become familiar with the Editor toolbar.  When finished with your edits, save them under the Editor drop down menu. The analysis mask is shown below that was used for the Santiaguito Dome:


The analysis mask was created from known previous activity, field maps, and satellite data. Once the analysis masks are created they can be used to clip the existing DEMs, which is done in model maker.  

Using Model Maker - Analysis Mask Clipping:
ArcToolbox has many tools which is used in this analysis. Instead of using each of these tools separately for each DEM, I created a model using all of the necessary tools at once, therefore reducing processing time. Models were used in this research to clip the 1954 and 2001 DEMs, and to subtract the DEMs.  The first step is to create a new toolbox, which is shown below. Once a new toolbox is created it will automatically be added to the other toolboxes. Next the toolbox should be renamed; in the example below I name the toolbox Volume. Next right-click on the new toolbox and choose New>Model.  



Next the model window is brought up as shown below:



To add tools into the model maker, choose the tool in ArcToolbox on the left. In this case I have chosen extract by mask.  Click on the tool with the mouse and while holding the mouse button down, drag the tool into the model window. The extract by mask tool and output raster is now located in the model.  The tools show up white when no data has been assigned to the tool.  The tool can be accessed by double-clicking on the graphic in the model window and the necessary data is then added.  Most tools are rather self explanatory. In this case the Extract by Mask dialog uses an input raster, a clip raster or feature class, and an output raster as shown below:

                   

The Extract by Mask parameters are shown below in the model window. The above information represents the top portion of the model, which takes the 1954 DEM (oldgr54) as the input raster, the analysis mask (caliutm.shp) as the mask data, and the output raster which is the clipped 1954 DEM (caliente54).   The model which was used for this research to clip each DEM is shown below:



For this model the extract by mask parameters would have to be set for both the 1954 and 2001 DEMs.  Each model is saved using the model menu.  After the model has been saved, the model can then be run by pressing the play arrow (the button to the far right on the toolbar).  When the process is complete the clipped DEM for 54 and 01 will be saved in the directory chosen (in the extract by mask menu).  The analysis mask for the model is shown below:


Creating analysis masks and extracting those masks from each DEM ensures the same area is being analyzed for each dataset. Therefore an analysis mask would be created for each area of interest and the mask would then be used to extract that portion from the DEM.  Note to open the model dialog again in ArcToolbox the model should be selected, right-clicked, and edit should be chosen.  If the model is double-clicked or open is chosen on the context menu the model will not open.

Using Model Maker- DEM Differencing
For this research I have created a Volume toolbox which has the clip model and the differencing model.  This will be available on the network. Instead of creating a new toolbox, as shown above, you can add a toolbox instead. Using the tools in ArcToolbox, I created a DEM differencing model as shown below:



Each process step by step is shown below:
1.  First I added the clipped DEMs, in this case 54 and 01.  These are input into the single output map algebra dialog.   Use the folder to input each file and place a subtraction sign between them. If the expression is not in the correct format a red X will show up, meaning the expression is not correct.
2. The map algebra expression will create a difference DEM, in this case the diff DEM is diffcal.  For this model I was focusing on the caliente region which was masked in the previous step.
3. Next I wanted to determine the positive (>0 m) and negative elevation (<0 m) changes. Using the extract by attributes dialog the input raster is diffcal in both cases. The clause is VALUE>= 0 or VALUE <0, for the positive and negative elevation differences respectively.  Value is the elevation value for each pixel. The output raster I created in the dialog is a DEM of either the positive or negative elevation changes.
4.  Once two DEMs are created with positive and negative elevation change, the DEMs are input into the surface volume dialog. This dialog calculates the volume above or below a specified plane height and is output to a textfile. For the positive elevation values we are determining a surface volume above a plane height of 1.  For the negative elevation values we are determining a surface volume below a plane height of 0.  This results in two text files which include the area and surface volume for the positive and negative elevation difference data sets.
5.  The volume for each text file is found and the positive volume is then subtracted from the negative volume giving the overall volume change for the masked region.

Using the above process any user should be able to create a shapefile and use model maker to clip and subtract DEMs.  
We now have the elevation and volume difference for each masked area. Next I will determine the DEM error through kriging which can be found here.