Distribution of Data - Spatial Mathematics: Theory and Practice through Mapping - page 186

8.7.3 Applying measures of distribution to tornado data
Earlier in this chapter, you used ArcGIS Online to examine the spatial and
temporal distribution of tornadoes in the USA. Now, you can apply your new
skills of measuring distribution to the same type of tornado data you were
examining earlier. To do this, you will need to download the tornado data
and examine it in ArcGIS for Desktop. First visit the ArcLessons library to
obtain the data: http://edcommunity.esri.com/arclessons/lesson.cfm?id = 565.
Download and unzip the data, and open the MXD (map document) in ArcGIS
for Desktop. Perform the following tasks:
1. Analyze the tornado attribute table. How many tornadoes are in this
data set, and what is the range in years for this data set? The range is
1950 through 2004, and there are 46,931 tornadoes in this data set.
2. Where and when did the tornado that caused the most injuries occur?
Where and when did the tornado that caused the most fatalities occur?
The most injurious tornado occurred in northern Texas in 1979, injur-
ing 1740 people, while the most fatal tornado occurred in Michigan in
1953, killing 116 people.
3. Perform a spatial join to join the tornado points to the state boundar-
ies. To do this, right click on the tornado touchdowns data layer, use
Joins and Relates, and choose “join data from another layer based on
spatial location.” In the dialog box, join states to the tornado points,
saving your results in a shapefile named “tornadoes_with_state_
names.shp.” Following the join, a state name will be associated with
each tornado.
4. In your joined data set, access the attribute table for the tornado
points. Right click on the state_name field and select “summarize”
to obtain the number of tornadoes by state. Place your result in a
.dbf table with a logical name, such as “tornadoes_by_state.dbf.” Sort
this new table on the number of tornadoes in each state. Which state
contained the most tornadoes according to this data set? Texas by far
experienced the most tornadoes, followed by Oklahoma.
5. Which states contained no tornadoes, according to this data set?
According to this data set, every state experienced a tornado.
6. Create a definition query on the data and select out the January tor-
nadoes, and examine the pattern. Repeat for the April tornadoes, and
then the July tornadoes. Does the pattern support the discussion ear-
lier in this chapter about the seasonal pattern of tornadoes? Yes, tor-
nadoes seem to be most numerous in January and February near the
Gulf Coast, and expand in number and in areal extent to the Dakotas
by late summer, and ebb back down to the Gulf by the end of the year.
Some exceptions exist, but the ebb and flow of the annual pattern is
persistent, consistent with our discussion on air masses and fronts.
7. Clear your definition query. Generate a mean center for the tornadoes.
In what state is the mean center? It is in Missouri. Create a definition