Refining urban area highlighting: the previous method didn’t work around the World!

18 07 2008

So the previous method with the lovely flowchart and everything proved to be great for London but when I tried to use exactly the same method in New York it took far too much urban area away leaving us with a rather dull image when it should be very bright to indicate the NYC metropolis. Take a look at the difference between New York and London! So I essentially went back to the drawing board and thought, “okay what are the main features to a LandSat image?” answer: urban areas, rural areas and water! So that means I need to distinguish between urban and rural and then distinguish between urban and water, then somehow combine the two. Thats exactly what I did.

The method is very similar to before, except I’m now using both hue and saturation from the 742 false colour image instead of simply hue. The pretty flowchart for the process is shown below and for those you you who thought the last one was a mess wait ’til you see this one!

The method essentially works due to the different properties of hue and saturation. There is a great difference between the hue value for urban and rural areas, therefore hue is used to distinguish between rural and urban. Here is the hue component of the 742 image with a colour threshold to highlight urban areas (notice that that water is also heavily highlighted).Now we apply a colour threshold to the saturation layer and take advantage of the great difference in value for land and sea here. So now we have two layers one highlighting urban and sea and one highlighting urban and rural, we need to pick out the urban. This is done by using a clever layer mode algorithm in GIMP called ‘darken only’. This compares the pixel value in the two layers (hue and saturation) and displays the lower value pixel, in this way water and rural areas are removed from the image. Result!

Here are the resulting finished images for London and New York.

You can compare the previous method and the new one here:

Old London
Old New York
New London
New New York

By the way if anyone is really interested in this stuff and would like more guidance or some high resolution images just get in touch.

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