Exercise 12 Spatial Analysis on Antarctica

Transcription

1 Exercise 12 Spatial Analysis on Antarctica Due: Tuesday, March 6 Goal: Using ArcMap s Spatial Analyst tools for digital elevation models and rasters. Datasets: Bed elevation Ice thickness Surface elevation Coastline RockOutcrop Southpole raster dataset of orthometric elevations raster dataset of ice thickness raster dataset of orthometric surface elevations a polygon of Antarctica and the permanent ice shelves a polygon of rock outcroppings a point shapefile with the location of the south pole These data came from two sources: The Antarctic BEDMAP project and The Scientific Committee on Antarctic Research (SCAR) Antarctic Digital Database (ADD). Data can be found in our class folder under Antarctica.zip: Assignment: Antarctica has all of the same complexities as the other continents on the globe: mountain ranges, streams, glaciers, valleys, plains, inland seas and islands. However, much of this topography is hidden from our view by the vast glaciers and ice sheets that cover the continent. With recent advances in remote sensing technology, we now have data available to us to make maps of what lies below the ice in Antarctica. These maps can help us answer a wide range of lingering questions about Antarctica. Exactly how much ice is there? What would it look like if all of the ice melts off? Due to isostacy, how high would the land mass rise if the ice were gone? Part 1 Spatial Reference If we look at the metadata as it is, we see that all of the data are currently in a Projected Coordinate System (PCS): PCS type: South Pole Stereographic Linear Unit: meter Latitude of Origin: Central Meridian: Standard Parallel: False Easting: False Northing: Datum: WGS84 1

2 As of ArcGIS 9 (we are using ArcGIS 10) there is a predefined PCS that matches this. However, for practice we will modify an existing PCS. Open a new empty map Right click on Layers in the Table of Contents Select Properties to open the Data Frame Properties window Select the Coordinate Systems Tab From here Select a Predefined Coordinate system following the path: Predefined > Projected Coordinate Systems > Polar > South Pole Stereographic Notice how the parameters do not match those of the PCS above? We will now modify this existing PCS: Double click the coordinate system's name. Enter a new name (we will use SCAR Antarctic) In the projection Box, under name, use the drop down menu to select Stereographic_South_Pole Change the value for the Standard Parallel to match the above value ( ) Click ok Click the Add to Favorites button so that this PCS can be used again 2

3 If this doesn t work for you, remember that ArcMap can project on-the-fly. By adding any of the data into ArcMap, the PCS will be automatically set, and any other data that are added will take on that PCS. (In ArcMap, the Data Frame always uses the PCS from the first file added.) Part 2 Organizing and exploring the data Add all the files from the Exercise Data. Add the following files first: Coastline, South Pole, Rock Outcrop. Note that these are all shapefiles. Next, add the Surface Elevation (surfel), Bed Elevation (bedel) and Ice Thickness (icet) files. For the time being, toggle off the Bed Elevation and Ice Thickness files. Because the current display is stretched the result is a map with little range in the grayscale. o To change the stretch right click on the file SurfaceElevation to navigate to the Layer Properties window. o Select the Symbology tab, scroll down, and in the statistics drop down menu select From Current Display Extent and click OK. What do you see now? 3

4 Questions to be handed in!! 1. Right click on the surfel layer, hit source tab, scroll down...what is the resolution or cell size (this value is in meters), pixel type (integer or floating point), pixel depth (in bits) and number of bands of this raster data set. Make a chart like the one below: Raster Layer Resolution (km) Pixel Type # of bands Pixel Depth (bits) 2. What are the Mean, Maximum and Minimum elevations of the raster dataset? 3. What is the Default Stretch Type of the Surface Elevation raster? Back to the Layer Properties window Changing Symbology: In the Symbology tab, on the left choose Show: Classified. (Arc may ask if you want to compute a histogram, if so, choose yes) Then, click on the Classify button to the right to bring up a new window. o Classification Method: Defined Interval; to group the interval by a setting that you choose o Interval Size: 200; this is what your contour interval will be o Click Apply 4

5 Once back at the Layer Properties window, choose a color ramp. o Left clicking once on the Symbol bar will allow you to flip the color ramp o Click on the Label bar to see the formatting for the labels Now change the symbology of the Coastline file by double clicking on the colored square. Choose Hollow. o Now move the coastline layer above the elevation file in the table of contents. Your resulting map shows the elevation at 200 m intervals. It should look like the one below: Make a screen shot of this map, and save it to your word document. Then, to make the map look nicer, we will remove the boxes around the continent 5

6 Part 3 Spatial Analyst Extension All of the work that we will be doing in this section will be in the Spatial Analyst Extension First you ll need to turn on the Spatial Analyst extension. Do this by clicking on Customize on the main tool bar. Then choose extensions. In the Extension window, make sure that the Spatial Analyst box is checked. You can also right click on the top of the screen and add the Spatial Analyst Toolbox. Analysis Mask In the Geoprocessing menu at the top of the page, select Environment settings (see image on the next page). Scroll down and extend the Processing Extent tab o Extent: Default; the extent is what the area of analysis will be. This allows for an area to be analyzed without being clipped. Scroll further down to the Raster Analysis tab. o o Cell Size: Maximum of Inputs; this means that when two or more rasters are being used, the new raster will not have any cells that are smaller than the cells from any of the input rasters. Mask: Coastline; defines the cells within the analysis extent. For us, this means that the only cells that will be analyzed will be within the coastline of Antarctica. Click OK Extract by Mask By clipping the elevation file to match the coastline of Antarctica, we will restrict the symbology to just the continent and its ice shelves. So now, the map will match the Analysis mask that we just made. Open ArcToolbox and follow the path Spatial Analyst > Extraction > Extract by Mask Input Raster: SurfaceElevation; this is the raster that is going to be clipped Input Raster or feature mask data: Coastline; this is the layer that defines where to clip the above raster Output Raster: **Browse to the folder that you want this stored in and name the file SurfClip 6

7 Creating A Hillshade Using a hillshade in conjunction with a semi-transparent elevation grid creates a nice effect. In the Spatial Analyst toolbox, expand Surface and select Hillshade o Input Raster: SurfClip; this is the layer that the hillshade is drawing its information from. o Output Raster: ***Browse to the folder that you want this stored in and name the file HillshadeS. o Leave everything else at its default setting, see Tool Help if you d like to learn more about the different parameters. Once you have your hillshade made, go to Layer Properties > Symbology, and set the Stretch Type to Standard Deviations, with two or less deviations. (In the window this will be the box that indicates n: and feel free to leave it at two). Now, drag your SurfClip layer on top of HillshadeS, and give it 50% transparency. Import the symbology of Surface Elevation layer and then turn off the Surface Elevation layer. You might want to turn on the Effects Toolbar and adjust the brightness/contrast of your hillshade to get a spiffier (less drab) result, or play with the type of stretch in symbology. Your map should now look something like this (remember to leave your Coastline, SouthPole and RockOutcrop files toggled on so that they appear on the map): Make a screenshot and include in your write-up. 7

8 Questions to be handed in!!! 4. The highest point in Antarctica is the Vinson Massif (a.k.a. Mount Vinson), in the Ellsworth Mountains. a. Using the SurfaceElevation raster, find the cell that contains the top of Mt. Vinson and give its latitude and longitude, in decimal degrees. Hint: the selection tools in the Selection menu do not work with raster data. Change the symbology to highlight cells over 4080 meters. Make that class of cells red for ease of identification. Zoom in and use the Identity tool from the toolbar (blue circle around an I ) to get the highest elevation value. To get locations in decimal degrees, you can set the display units of the Data Frame in the Data Frame Properties window or the dropdown arrow in the Identify tool results box. b. Find the height of Mt. Vinson on the internet. What is it? Give a plausible reason why the known height doesn t match the height in our DEM. c. What is the elevation of the cell that contains the South Pole? Continuing on with Part 3 We will now be working with the bedrock data The "bedel" raster shows the topography beneath the ice on the continent, and also shows the bathymetry of the sea floor. You ll notice this layer forms a giant circle- the layer extends to 60 South latitude in all directions. Follow the above directions, using the BedElevation file. So in the end, you should have a map that looks just like your SurfaceElevation map does now. o When you extract by mask, name your new file BedClip o When you make your hillshade, name your new file HillshadeB o Create a similar layout, with HillshadeB being a lower layer, BedClip should be on top of that layer, with transparency set to 50% o Again, don t forget to include the Coastline, SouthPole, and RockOutcrop files. 8

9 Questions to be handed in!!! 5. What are the mean, maximum and minimum elevations for the continent? Hint: Check the source tab in the file s properties. More Spatial Analysis! Creating a Binary Raster In the map above, some parts are above sea level, and some are below. We can t tell the difference by just looking. By creating a binary raster, we will have only two values. In this case we will set the values as above sea level, or below sea level. Then we ll use symbology to create a map that can show us exactly what parts of Antarctica would be below sea level. Open the Raster Calculator in ArcToolbox by following the path Spatial Analyst > Map Algebra > Raster Calculator We will be using a conditional statement, so begin by double clicking Con listed in the box on the right Then, in the box on the left, double click on BedElevation Using the keys in the pad above, your statement should read: o Con( BedElevation <= 0,1) Output Raster: **Browse to the folder you want to store this file in, and label it BinaryWater Now, make the water areas blue and slightly transparent, and move it above the HillshadeB and BedClip layers in the Table of Contents. Keep the Coastline, SouthPole, and RockOutcrop layers on. Questions to be handed in!! 9

10 6. Explain, in words that include if then, the meaning of the conditional statement used to generate the binary raster above. Hint: Use ArcGIS destop help and search conditional statement for explanations of similar examples Creating a Contour Line Looking at the map you just made, wouldn t it be nice to add a black line around the water? In this situation, the easiest way to do that is by making a contour line. From the Spatial Analyst tool set, select Surface > Contour o Input raster: BedClip; this is the raster that will provide the data on which the contour lines will be set. o Output polyline features: **Browse to your folder, name the file BedContour o Contour Interval: 4440 (this will mean we only have one contour, because the highest elevation of the bed is 4364m). o Base Contour: 0 o Z factor: 1 Set the contour line to Black in color, with a width of.5 Your map should now look something like this (see image below): Make a screenshot and include in your write-up! 10

11 Questions to be handed in!!! 7. Although I downloaded this ice thickness raster, we could have created one from the files we ve worked with so far. How? Give your answer in a list of steps (general steps and also specific steps for doing this in ArcGIS). 8. How thick is the ice at the South Pole? What is the maximum ice thickness and where is it located (in lat/lon decimal degrees)? Calculating Ice Volume and Area For this, we will be using the 3D Analyst Extension. To turn this on, be sure to go to Customize on the main menu, select Extensions, and toggle on 3D Analyst. Open ArcToolbox > 3D Analyst > Functional Surface > Surface Volume Then use these settings: o Input Surface: IceThickness; this is the layer that Arc will use to make the calculations. o Output Text File: **Browse to your folder and title this file IceTable o Reference Plane: ABOVE; this tells Arc whether to calculate above or below a certain height o Plane Height: 0 o Z Factor: 1 Click OK 11

12 Open your table once the calculations are complete (you ll have to go to your folder to open the file to Excel). o Do this by opening Excel, o select open from File on the main menu, browse to your folder o Files of type: All files o Select IceTable o Now you ll be in the Text Import Wizard window Original Data Type: delimited; click Next Delimiters: Commas; click next Column Data Format: General; click Finish Now your data is in a pretty Excel graph The statistics given give the 2D area (plan view area), the 3D area (area of the irregular surface defined by the top of the ice-thickness raster when the base is assumed to be level) and the Ice Volume (2D area x sum of all cell values). The units for the results are the same as the spatial reference, in our case meters. Questions to be handed in!!! 9. What is the volume of the Antarctic ice sheet and ice shelves/tongues, in cubic kilometers? 10. What is the surface area, in square kilometers, of Antarctic ice? Isostatic Rebound- Spatial Analyst Isostasy refers to a process that restores equilibrium after a load is either added to or removed from a tectonic plate. As ice melts off of Antarctica, it will lighten the load of the plate, and the underlying continent will rebound, rising upward. The total amount of rise can be modeled as being directly proportional to the thickness of the ice and the ratio of the density of the underlying mantle to that of the ice. Specifically, for individual raster cells: (Density of Ice/Density of Mantle) X (Ice Thickness) = Elevation Change o Average density of ice: 0.98 g/cm 3 o Average density of mantle: 3.34 g/cm 3 o For today, use the density ratio of ice to mantle equal to.2825 To obtain an elevation raster for the continent that includes this elevation difference we will: o Step 1: Multiply an ice thickness raster by to obtain elevation change o Step 2: Add the resulting raster to the bedrock elevation raster to obtain isostatically compensated elevations for the Antarctic continent 12

13 Step 1 multiplies an integer raster (ice thickness) by a floating point value, resulting in a floating point raster. Open the Raster Calculator (Spatial Analyst > Map Algebra) Enter the expression IceThickness * , using the buttons given Browse to your folder and save this file as ElevChange Click Okay (this may take a moment, be patient) Now, open the Raster Calculator again, and load the new file (ElevChange) and add it (use the + sign from the buttons) to the BedElevation raster. Browse to your folder and save this file as BedIsostasy (again, give this a minute to execute) Use some symbology to dress up your map: Symbolize your new raster Create and save a Hillshade Create another zero elevation contour Questions to be handed in!! 11. How do the mean, maximum and minimum elevations for the continent after isostatic rebound differ from those of the sub-ice topography before rebound (see question 5). Greenhouse Antarctica The south polar ice cap accounts about 91.5% of the ice in the world. If it were to melt, the sea level would rise by about 73 meters. If all of the ice on the planet were to melt, sea level would rise by 80.5 meters. Now, we ll produce a map that shows higher sea level. SAVE OFTEN! Use Raster Calculator to subtract 80.5 meters from the cells of BedIsostasy. o Name this one yourself, but be sure to remember the file name Make a Hillshade of this new raster. Create a binary raster of the regions above and below sea level. o Again, be sure to keep track of your file names Create a shoreline o Think about what contour interval you want to use here! Symbolize the results and make an attractive map. Questions to be handed in!!! 12. Make a screen shot of this final map and paste it into your word document. Be sure to include a grid and a scalebar. 13

14 13. Explain why, for this map in particular, it would not make sense to put in a North arrow. 14. The USGS has calculated volumes of ice for Antarctica that are substantially larger than those you calculated. The USGS calculated 30.1 million km 3. Why do you think your results are different? Your final map should resemble something like this 14

15 Part 4: Final Bits! Your professor periodically goes to the southern Transantarctic Mountains to search for meteorites. Do a bit of analysis using the trip itinerary from one of these planned excursions. Download the ANSMET.zip file from the Data folder on our website, unzip it, and add both the PolyData.shp and PointData.shp to your map. The two points are McMurdo Station on the coast of Antarctica and the Shackleton Glacier Airfield. (Right-click on the point layer name and click Label Features ) The polygons are the search areas while camped out on the ice. Question to be handed in!! 15. To get to the field site, researchers fly from McMurdo Station to the Shackleton Glacier Airfield on a Twin Otter plane, with a cruising speed of 280 km/hr. How long of a trip will this be in a non-pressurized, non- climate-controlled (COLD!) airplane? 16. Now, how long of a trip will it be from there to the field camp site (the middle of the four polygons)? 17. What is the southernmost latitude to be searched in decimal degrees? (Hint, under View>Data Frame Properties>Units>Display you can select decimal degrees and then scroll on the map with the coordinates appearing in the lower right.) 18. We hope to get an invite to the South Pole Station s Christmas party (ha, unlikely!). If we can navigate crevasses and other hazards at 30 km/hr on our snowmobiles, when should we leave camp to get to Christmas Dinner? Dinner is at 6 pm on December 25, but we should probably show up at 5:30 for a shower, to wash away the weeks of stank. Now we want to determine how much area this adventurous team of 4 will explore in their quest for space rocks. To calculate area of polygons, you need to first create a new field in the table and then use the Calculate Geometry function, as described below. Open the polygon attribute table. Add a Field called area sq_m that is data type: float with a value of 10 for both precision and scale. Once created, right click on the column heading and select Calculate Geometry. You should be able to leave all the defaults (Property Area, Coordinate System PCS: South Pole Stereographic and Units Square Meters ). Final question to be handed in!! 19. Given weather and transportation logistics, we will probably be actively searching for meteorites for a total of 30 days. How many square kilometers must we search per day to cover the total area within the polygons? 20. Finally, make a table reporting the maximum, minimum, and average values of Elevation and Ice Thickness within the search areas (polygons). (Hint: You used tools to do this type of operation earlier in the lab.) 15