Nodata issues in map algebra

Question

A dummy question. I need to change all values in my rasters into ones. I've been able to do this fine in the Rater Calculator just by multiplying the layer by zero and adding one (my_layer*0+1), but when I'm doing the same in arcpy, for some reason it works for a part of my raster but for some it changes all the nodata cells to ones as well. The nodata values in different rasters are different because my rasters have been generated from different source data. Does it not recognize some nodata values? If so, this is odd as it displays them correctly when I just import them on the mxd. Any ideas on why this works sometimes, but sometimes not?

Here's the exact script:

bin_test = arcpy.Raster("F:/datasets/map1.img")*0+1

Answer 1

My favourite way to deal with this would be to use a Con(IsNull) statement...

Basically, using a Con statement allows you to define a condition; if it is true, do this, if it is false, do this - including leaving the values as they were.

So in this case, I would be tempted to do something like

Con(IsNull(myRaster), myRaster, 1)

Which basically states:

• Set up a condition to test cells in myRaster for NoData (IsNull)
• If it is NoData, leave it as NoData
• Otherwise, convert the cell value to 1

Some documentation:

• Con
• List item

...I don't know, however, why you're NoData cells are being multiplied into something besides NoData...

Answer 2

NoData is defined different ways depending on the raster source, including numerically. (You can look it up in raster properties if you are curious what the actual value is for your particular datasets.)

I suggest instead using Reclassify instead of Raster Calculator, which can either explicitly ignore NoData values (leaving them NoData) or explicitly include them (changing them to 1). It isn't entirely clear to me whether you do or don't want those NoData values respected, but Reclassify works for either scenario.

Answer 3

Just so you know there are a million ways to do this. But, that said, multiplying is a very indirect method, these methods are more straightforward and efficient, even using the raster calculator. Going at it this way instead of multiplying by zero is probably safer with different bit depths / data formats...

These three calculations should give you the same answer:

dataraster = Con(IsNull(myRaster), myRaster, 1)
dataraster = SetNull(IsNull(myRaster), 1)
dataraster = Con(~ IsNull(myRaster), 1)