0

I am using rasterio to open image files related to 2 specific bands:

b4 = rio.open(r)
b8 = rio.open(poly)

I then read the bands into a python array, I think it's a numpy array but not sure?:

#Read Red(b4) and NIR(b8) as arrays
red = b4.read()
nir = b8.read()

Then I perform band based calculation against them via the following:

#Calculate NDVI
ndvi = (nir.astype(float) - red.astype(float)) / (nir + red)

Now, this of course can have the denominator (nir + red) be equal to zero, which will cause a divide by zero issue that I believe is messing with the results.

Is there a way to set the instances where the denominator is equal to zero to have a value of -1 or something? How are you handling those in your own raster functions?

Improve this question
1

I suggest using numpy to build your raster function, which can handle situations where the denominator is 0. 

red = np.arange(0, 15).reshape(4,4)
ndvi = np.arange(-10, 6).reshape(4,4)
ndvi = np.divide(np.subtract(red, nir), np.add(red, nir))
ndvi = np.where(ndvi == np.inf, -99, ndvi)

numpy returns np.inf when dividing by zero, which you can later on replace by any arbitrary number to indicate an invalid NDVI value; in the example -99.

Improve this answer
6
  • That's exactly the type of thing I was looking for. What is your Arrange and Reshape doing there in your example? – Nathan Raley Apr 22 '20 at 20:50
  • 1
    np.arange gives a 1-D array starting-stopping at the numbers provided. reshape reshapes the 1-D array (shape (16,)) to a 2-Dimensional array (shape(4,4)). See arange and reshape documentation. If this answers helped you out, please tick 'answered'. – Hans Roelofsen Apr 23 '20 at 13:58
  • What was the reasoning behind the numbers you supplied for the parameters in the arrange and reshape calls? – Nathan Raley Apr 23 '20 at 16:01
  • The reason i ask is b/c just using the divide and where pieces, you still get a RuntimeWarning: invalid value encountered in true_divide. So I was trying to figure out the reasoning behind choosing the 0,15 and -10, 6 for the arrange calls, and 4,4 for the reshapes? – Nathan Raley Apr 23 '20 at 16:16
  • I designed the value ranges so that a zero would occur once in the denominator. The (4, 4) shape is just to create a 2D array that would easily fit in my python window. – Hans Roelofsen Apr 28 '20 at 7:07

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.