Issue 65

Visualizing the Path and Intensity of the Joplin Tornado in MapViewer 7

On May 22, 2011, a devastating EF-5 multi-vortex tornado descended upon the city of Joplin Missouri. The tornado reached a maximum width in excess of 1 mile as it traveled eastward across the city. The most deadly tornado in the United States since 1947, the tragedy claimed the lives of 155 people. This marks the first EF-5 in southwest Missouri since records have been maintained for such events.

Since the tornado, there have been updated aerial images of the destruction left in the wake of this massive tornado. Using MapViewer 7, Didger 4 and some basic parameters of tornado destruction classifications, we can create a map to visualize the path, width, and intensity of the tornado.

Tornado classification

The first part of the project is to gather the imagery and the necessary information for being able to quantify the intensity based on the aerial images. An understanding of how the Enhanced Fujita Scale is used to quantify the magnitude of the tornado can be beneficial in assigning a value. By examining the image, you can observe the level of damage and create a point value.

This diagram shows the Enhanced Fujita Scale, which is used to evaluate tornado damage.

Creating a map

The first step is to find aerial imagery posted of the city after the tornado.

1. NOAA has posted high resolution imagery of the damage on their website (www.crh.noaa.gov/sgf/?n=event_2011may22_summary) that can be exported as KML.
2. From Google Earth, the image can be saved and loaded into Didger 4. By using the Image | Download TerraServer Images command in Didger, we can easily georeference the image and save as a TIF file.
3. Once georeferenced, load the satellite imagery of southern Joplin as a base map in MapViewer by clicking Map | Base Map command.
4. Select the TIF image and click Open.
5. In the Bitmap Import Options dialog, set the Spatial Reference to Embedded GeoTIFF Parameter and click OK. The image will appear in the plot window.

This aerial image shows the area around Joplin, after the tornado struck.

Analyzing the images

Once the image is successfully loaded into MapViewer, we can start examining the image to assess the level of damage. Because you would have to manually enter the value for each point, it is easier to gather a collection of points of like value, and repeat this for each level (1 through 5).

1. Use the Boundary | Digitize command to mark several locations throughout the map to help identify the intensity of the tornado at the given location. This is mainly a visual interpolation based on the level of damage in the aerial photos along with the survey map from NOAA. The best workflow is to digitize all points for one classification of intensity.
2. The digitize command will automatically open a dialog box titled “digit.dat”. Once finished digitizing, save the DAT file. You will use MapViewer’s worksheet to edit the data.


This image shows the points digitized, marking the damage area near Joplin.

3. Open the new DAT file in the worksheet by using the File | Worksheet command. In the worksheet window, choose File | Open command. Select the DAT file and click Open.
4. Click on column A to highlight the column. Click the Edit | Insert command to insert a new column at the beginning of the data, creating a blank Column A.

Insert an empty column, allowing for the primary identification number to be automatically entered.

5. Use the Data | Transform command to add a Primary Identification (PID). In the Transform dialog, type A =Row()in the Transform equation box and click OK. This uses the row number as the PID for each digitized data point.


The Transform dialog is used to automatically assign a
PID based on the row number.

6. Use the Data | Transform command to add intensity values to your XY data. The intensity values come from how intense the damage was at each digitized location. In the Transform dialog, type the formula D=x in the Transform equation box. X is the intensity of the tornado at the location.

Use the Transform dialog to easily assign intensity values to each point.

Intensity values are added to Column D.

7. Click the File | Save command to save the data.
8. Click the File | Close command and click Keep to retain the link from the data to the map.

Assessing the damage

Now that the data has been processed, it is very easy to create a contour map in MapViewer that will represent the intensity of the tornado based on the level of destruction seen.

1. Once you’ve collected data for all 5 zones, create a new layer by clicking the New Layer button in the Layer Manager.
2. In the new layer, create a pin map using the DAT file by clicking Map | Pin Map.
3. Select the DAT file and click Open.
4. In the Pin Map dialog, assign the PID (Column A) the X coordinate (Column B), and the Y coordinate (Column C) and click OK.
5. Convert the pin map to a contour map by clicking Map | Thematic Maps | Contour Map.
6. In the Contour Map dialog, set the Variable to match the Column D value for intensity.
7. Click on the Levels tab.
8. Set the Number of Levels to 5 and set the Classification method to “User Defined”.


Set the contour map levels on the Levels tab of the Contour Map dialog.

9. Click on the Fill header to open the Fill dialog box.
10. Change the Fill Pattern to Diagonal Cross (top row – 3rd column) and set the foreground color as desired.

Create a semi-transparent contour fill by using a hatch pattern and setting the Background Mode to Transparent.

11. Set the Background Mode to Transparent and click OK.

The final map of partially transparent colors indicates the intensity of EF-5 tornado in Joplin.

Conclusion

By creating a map showing the levels of destruction based on the aerial imagery, we get a better understanding of not only how a tornado travels, but also a better understanding of the sheer magnitude of destruction that comes with the most powerful EF-5 tornado. This catastrophe shows there are no ways to prevent tornadoes, only study their patterns to become better prepared for future events.