Archive for Lake of Menteith

from down-under, Lake Menteith upside-down

Posted in Books, R, Statistics with tags , , , , on January 23, 2013 by xi'an

Lake Menteith Landsat image, as printed in Bayesian Core (2007)The dataset used in Bayesian Core for the chapter on image processing is a Landsat picture of Lake of Menteith in Scotland (close to Loch Lomond). (Yes, Lake of Menteith, not Loch Menteith!) Here is the image produced in the book. I just got an email from Matt Moores at QUT that the image is both rotated and flipped:

The image of Lake Mentieth in figure 8.6 of Bayesian Core is upside-down and back-to-front, so to speak. Also, I recently read a paper by Lionel Cucala & J-M Marin that has the same error.

This is due to the difference between matrix indices and image coordinates: matrices in R are indexed by [row,column] but image coordinates are [x,y]. Also, y=1 is the first row of the matrix, but the bottom row of pixels in an image.

Only a one line change to the R code is required to display the image in the correct orientation:

image(1:100,1:100,t(as.matrix(lm3)[100:1,]),col=gray(256:1/256),xlab="",ylab="")

As can be checked on Googlemap, the picture is indeed rotated by a -90⁰ angle and the transpose correction does the job!

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