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Tuesday
Aug022011

Visual crossplotting

To clarify, add detail
Edward Tufte

Pyroclastic flow on Nabro, Eritrea. Click for a larger image. NASA.Recently, the prolific geoblogger Brian Romans posted a pair of satellite images of a pyroclastic flow on Nabro in Eritrea. One image was in the visible spectrum, the other was a thermal image. Correlating them by looking back and forth at the images is unsatisying, so I spent 10 minutes merging the data into a single view, making the correlation immediate and intuitive. 

Maps like this are always better than abstractions of data like graphs or crossplots (or scatter plots, if you prefer). Plots get unwieldy with more than three dimensions, and there are almost always more dimensions to the data, especially in geoscience. In the image above there are at least half a dozen dimensions to the data: x and y position, elevation, slope, rugosity, vegetation (none!), heat intensity, heat distribution,... And these other dimensions, however tenuous or qualitative, might actually be important—they provide context, circumstantial evidence, if you will.

When I review papers, one of the comments I almost always make is: get all your data into one view—help your reader make the comparison. Instead of two maps showing slightly different seismic attributes, make one view and force the comparison. Be careful with colours: don't use them all up for one of the attributes, leaving nothing for the other. Using greys and blues for one leaves reds and yellows for the other. This approach is much more effective than a polygon around your anomaly, say, because then you have indelibly overlain your interpretation too early in the story: wait until you have unequivocally demonstrated the uncanny correlation.

If you're still not convinced that the richer image conveys more information, see how long it takes you to do this Spot The Difference. Come back tomorrow for the answer (and the point!)...

Creative Commons licensed image from Wikimedia Commons, work of User Muband (Japan)

GIMP is your friend!

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Reader Comments (3)

In my architectural design studios one of the greatest lessons I learned was to have your images speak for themselves. It's definitely an art and it takes practice, but having a really "communicative" image/photo/diagram/doodle is extremely valuable.

As for the differences in the right image (spoiler alert):

- Cat has a lollipop instead of a spoon
- Floppy disk instead of the square cookie
- Smiley face on the plate over the cat's head
- Armless doll things instead of blue bowl on shelf
- Different mountains in picture on wall
- Different time on the clock (2:45 instead of 2:30?)
- Heart knobs on the cabinet instead of round
- Kid's tongue is not sticking out
- Kid is wearing socks now
- One pink squid thing is missing from green thing in the bottom corner (or "Missing one flower from plant"?)
- 3 Bananas instead of 2
- Grandma's string necktie is missing
- Grandma's hairpin is missing

Did I get them all?

August 3, 2011 | Unregistered CommenterReid

@Reid: What a sport! But, as you know now I think, you missed a couple.

I like what you said about figures 'speaking for themselves'. Lots of writers of papers take this to mean 'make sure the caption has a long explanation of the figure'. But it really means 'make sure your figure doesn't need a caption'.

August 3, 2011 | Registered CommenterMatt Hall

'make sure your figure doesn't need a caption' - exactly; ironically I've never been able to say that so succinctly. closest I've come is "let the image speak for itself", which is too much of a cliche for my taste.

August 4, 2011 | Unregistered CommenterReid

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