topic Re: Raster Calculator to Map Algebra in Python Questions

Raster Calculator to Map Algebra

EricEagle — Sun, 12 Dec 2021 00:47:13 GMT

I have a model that does a really simple Raster Calculator operation:

"%Output accumulation raster%" >= 150‍

That's it. I have been racking my brain trying to figure out how do I get this into a python script? I have been looking at A Quick Tour of Using Map Algebra and frankly I'm overwhelmed because I don't have bundles of time on my hands, and I'm just trying to do something very straightforward somewhat quickly. I feel like I need to see a very specific, hand-holding, with reasons, example of taking this Raster Calculator expression and putting it into Python (3.6, ArcGIS Pro 2.2).

Part of the code in question looks like this (yes, I'm using Spatial Analyst hydrology tools here):

outfill = Fill(raster)

outFlowDirection = FlowDirection(outfill, "NORMAL")

outFlowAccum = FlowAccumulation(outFlowDirection)

stream_raster = Raster(outFlowAccum >= 150) #this is the part that fails
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I'm sure this is really 101 but nevertheless, I'd appreciate any guidance.

Re: Raster Calculator to Map Algebra

DarrenWiens2 — Wed, 24 Oct 2018 21:06:15 GMT

I'm not exactly sure what the non-tool equivalent is, but I default to Con, something like:

stream_raster = Con(outFlowAccum >= 150, outFlowAccum)‍

Con—Help | ArcGIS for Desktop

Re: Raster Calculator to Map Algebra

DanPatterson_Retired — Thu, 25 Oct 2018 02:01:17 GMT

If it fails, what is the error message.

If it were to work, then the you would probably want to add a stream_raster.save(… some path...)

It could be as simple as your model vs script not know the work environment and where files are coming from and going to.

Also the difference between .... outFlowAccum >= 150 and Darren's Con representation, is that yours will return a simple binary raster (0 or 1) and the Con variant will return 1 or the outFlowAccum. You could simply replace that portion with a 0 to get the binary raster that you want to represent stream vs non-stream

Re: Raster Calculator to Map Algebra

DarrenWiens2 — Thu, 25 Oct 2018 15:53:32 GMT

Hmmm, I think the Con will return the raster or nothing. If you want a full binary raster, use:

stream_raster = Con(outFlowAccum >= 150, 1, 0)

Re: Raster Calculator to Map Algebra

DanPatterson_Retired — Thu, 25 Oct 2018 16:02:02 GMT

used to numpy which gives binary automatically ( 0, 1 ) rather than whatever

Re: Raster Calculator to Map Algebra

Luke_Pinner — Fri, 02 Nov 2018 11:14:01 GMT

If you just want a binary 1/0 True/False output, use:

stream_raster = outFlowAccum >= 150