Visual Analytics: Get to the Point with a Geo Coordinate Map
Perhaps you’ve heard people talk about tornado alley – it’s an area down the middle of the US where tornadoes occur. Tornadoes are powerful storms – capable of sending a 2×4 wood board through a car door – scary. In the first post of this series, I noted that successful geospatial data visualizations have geography as a character in the data story. If we wanted to test or locate tornado alley, then a Geo Coordinate objects is perfect.
Telling a Geospatial Story with SAS Visual Analytics
If your data story is about where violent storms occur, then you can imagine users wondering where tornado touched down in the past. Geo Coordinate maps are excellent at showing exactly where an event occurred. In the following figure, the markers indicate where tornadoes with EF5/F5 strength (230 mph+ winds) occurred in the past 65 years. This data visualization shows the alley down the middle of the country.
The data shows that a F5/EF5 is not that common. To make this geo coordinate map a little more interesting, let’s add the EF4/F4 tornado touchdown points. This helps the user understand how much more likely a EF4/F4 event is. I used the Display Rules to turn the EF5/F5 ones blue. There’s not a legend for the user but if the user hovers over the markers they can see more details. Also I have a date range slider that focuses on a 10 year period of 1967-1977.
While the data points are a little messy – it’s very clear where a severe tornado occurred in that timeframe. This data would not have had the same impression if I had plotted it as a line chart or even a pie chart. The touchdown points help you realize why those particular states may need to have a higher disaster recovery budget.
So here’s how I created a custom datapoint and added some filters to help keep the data manageable.
Adding a Custom Datapoint based on Tornado Touchdown
The National Climatic Data Center website has a database of storm event data available for download. The data are largely location-based data with the state, county and the coordinates of where the tornado originated (BEGIN_LAT and BEGIN_LON) and ended (END_LAT and END_LON). The BEGIN_LAT and BEGIN_LON coordinates are perfect for plotting and even more perfect for a geo coordinate object. I renamed the BEGIN_LAT and BEGIN_LON to Latitude and Longitude for SAS Visual Analytics.
What few users realize is that when you create a custom geographic data item you can actually use any data item that makes sense. When reviewing my data, I see that a Tornado F/EF Scale data item that has the storm intensity from (F1- EF5). We can combine this data item with the geographic coordinates to create a new data item.
To create a new Geography data item, right-click the Tornado F/EF Scale item and select Geography > Custom. In the pop-up, select your values for Latitude and Longitude. Ah, the magical latitude and longitude allowed me to create a custom item from a categorical variable.
Adding a Slider Object based on Event Year
When you have so many data items you might get a warning from SAS Visual Analytics that it cannot display all the items. With custom data items – it is more likely to happen. You can add some of the filter objects to let the viewer control the story. For instance, I added a slider at the top and added the Event Year data item. Filters placed in the Section area filters the map automatically.
Then I added a List object to the side along with the Tornado F/EF Scale data item. You cannot place a List object in the section filter area (next to the Slider object) but you can place it next to the map. [You have to set up an interaction for this object – it doesn’t happen automatically.] Also, it might be interesting to see the other tornadoes by color so I updated the display rules. In retrospect, I probably should have made the F5/EF5 red since they are more severe and the F1/EF1 purple as a cue to the viewer.
When to Use these Geo Coordinate Objects
Geo Coordinate objects are really useful when you need to pinpoint a location. It shows where something has happened so the viewer can understand the event frequency. Since we have the ability to create a custom data point – our story is especially engaging. However using the filters helps the user control how much data is shown at one time. In part 3 of this series, you will learn more about Geo Region maps. Review Part 1 to learn about how to use geospatial data.
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