topic Re: Performing if else condition on two raster layers in ArcGIS Spatial Analyst Questions

Performing if else condition on two raster layers

LawrenceSekaluvu — Tue, 26 Aug 2014 17:57:57 GMT

Hi,

I am working on a project and i am writing my scripts in python. However, i am experiencing an obstacle that might make me quit this project for good. Here is is the problem. I want to perform a if else condition on two raster layers that have constant values across all there grid. Initially, i create two raster layers, each with a constant value within each cell. However, when i perform a loop on these two raster layers, one of them at the end will have cells with different values and i need to loop in again to compare it with the other raster with a constant value. I tried using the "Con" tool but things have not worked at all. Any clues please?

Re: Performing if else condition on two raster layers

IanMurray — Tue, 26 Aug 2014 19:15:37 GMT

Could you be a little more concise in explaining what you are trying to do?

Re: Performing if else condition on two raster layers

DanPatterson_Retired — Tue, 26 Aug 2014 20:28:41 GMT

Definitely include what you have tried either within the Con statement or within your script

Re: Performing if else condition on two raster layers

LawrenceSekaluvu — Tue, 26 Aug 2014 20:58:46 GMT

Hi @Ian,

I want basically to perform a "if else" condition on each cell within a raster in python window environment. At first, i thought that i could get two raster layers (one with a constant value for all cells and the other with varying values within each cell) and insert the "if else" statement right away within the loop. This seems not to work out all. Lastly, I have some other three parameters that need to be calculated for each cell depending on whether the "if else" condition returns true or false.

Re: Performing if else condition on two raster layers

DanPatterson_Retired — Tue, 26 Aug 2014 21:07:38 GMT

Please post your script

Re: Performing if else condition on two raster layers

curtvprice — Tue, 26 Aug 2014 21:12:14 GMT

As Dan said, Con is the tool you want. A Python if/else statement will not process raster cell values cell by cell like you want to.

Re: Performing if else condition on two raster layers

LawrenceSekaluvu — Tue, 26 Aug 2014 21:18:27 GMT

@Dan,

I did that but when I use the Con statement, there is one moment when i have to compare a raster layer with all cells having the same value against a constant value, and In this case i get an error message. However, if i perform the same Con condition on a raster layer with a range of cell values and specify the condition within the value range, it works out quite well with out bring any errors. May be there is some else hidden that i don't understand!!

Re: Performing if else condition on two raster layers

IanMurray — Tue, 26 Aug 2014 21:21:32 GMT

Actually, you could convert a raster to an ascii, and compare cell values between the two ascii files, assuming they have the same extent. You could even generate a new ascii based on if the values are the same, and convert it back to a raster by adding the header file from the original ascii to the output ascii(I actually have a project I'm working on that is based on this).

But yea, a conditional statement would be easier.

Re: Performing if else condition on two raster layers

LawrenceSekaluvu — Tue, 26 Aug 2014 22:14:15 GMT

here is my script

>>> import arcpy

... from arcpy.sa import *

...

... #calculating slope

... S = Slope("DEM","PERCENT_RISE","")

... H = S/100

...

... #initialize parameters

... T = 10

... Temp = Times("DEM",0)

... M0 = Plus(Temp,0.2)

... fb = 0.345

... D = 2

... Cellsize = 2

... d1 = 0.21

... d2 = 0.01

... d3 = 0.31

... d4 = 0.4

... d5 = 0.395

... Kr = 2.0

... Kh = 0.004

... for t in range (0, T):

... t += 1

...

... #first conditional statement

... C01 = Con("M0","M0","","VALUE >= fb")

... C1 = Times(C01, 0)

... Kv = (d1*(d2/d3)) + Kh

... Qv = Kv * H * Cellsize * D

... K1 = Plus(C1,Kv)

... Qh1 = Plus(C1,Qv)

...

... #second conditional statement

... C02 = Con("M0","M0","","VALUE < fb")

... C2 = Times(C02, 0)

... Ep = Exp((-13*(d4/d5)))

... Kf = Kr * Ep

... Qf = Kf * H * Cellsize * D

... K2 = Plus(C2,Kf)

... Qh2 = Plus(C2,Qf)

...

... K = K1 + K2

... QT = Qh1 + Qh2

... Mt = K + QT

... M0 = Mt

... print K, QT, Mt

Re: Performing if else condition on two raster layers

DanPatterson_Retired — Tue, 26 Aug 2014 22:36:26 GMT

I would still like to see the Con statements you used AND the error message, which is probably the key.

Re: Performing if else condition on two raster layers

DarrenWiens2 — Tue, 26 Aug 2014 23:32:51 GMT

Your statement:

C01 = Con("M0","M0","","VALUE >= fb")

should be:

C01 = Con("M0","M0","","VALUE >= " + str(fb))

and similarly for the second Con statement.

Re: Performing if else condition on two raster layers

curtvprice — Wed, 27 Aug 2014 06:24:38 GMT

Why aren't you using real Python map algebra? For example, you're doing this:

Temp = Times("DEM",0)
M0 = Plus(Temp,0.2)

Python Map algebra takes advantage of the Raster object and overtyping of operators to write straightforward expressions like this:

M0 = Raster("DEM") * 0 + 0.2

Which if course, begs the question, why are you zeroing out every cell of your DEM before you add 0.2 to it?

Map Algebra is a lot easier to debug than using the tools directly (Plus, Times, etc)

Here's another example:

C01 = Con("M0","M0","","VALUE >= " + str(fb))

Using map algebra on the raster object m0 created above:

C01 = Con(m0 >= fb, m0)

Re: Performing if else condition on two raster layers

LawrenceSekaluvu — Wed, 27 Aug 2014 15:12:32 GMT

@Ian,

Thanks, let me try out that and see whether it will work out quite well.

Re: Performing if else condition on two raster layers

LawrenceSekaluvu — Wed, 27 Aug 2014 15:14:33 GMT

Here is the error message i get

ERROR 999999: Error executing function.

The table was not found. [VAT_Con_plus_ras1]

A column was specified that does not exist.

The table was not found. [VAT_Con_plus_ras1]

The operation was attempted on an empty geometry.

ERROR 010416: Error in setting raster table filter for D:\Codetrial\Cond.gdb\plus_ras2.

ERROR 010213: Error in reading raster M0.

ERROR 010325: Run-time error - ***.

ERROR 010067: Error in executing grid expression.

Failed to execute (Con).

"""

Re: Performing if else condition on two raster layers

DanPatterson_Retired — Wed, 27 Aug 2014 16:42:19 GMT

hmmmm quick guess...it is a floating point raster, hence it has no table, hence no field named value so you can't query a value field

Re: Performing if else condition on two raster layers

DarrenWiens2 — Wed, 27 Aug 2014 16:51:25 GMT

Just checked it out, Dan. I get Lawrence's exact error message by leaving the variable name inside the where clause:

>>> val = 580
>>> C = arcpy.sa.Con("block_dem_1m.tif","block_dem_1m.tif","","VALUE > val")

... but not like so:

>>> C = arcpy.sa.Con("block_dem_1m.tif","block_dem_1m.tif","","VALUE > " + str(val))

The above is a floating raster, with no table.

Re: Performing if else condition on two raster layers

DanPatterson_Retired — Wed, 27 Aug 2014 17:10:52 GMT

Rules that out then...thanks for the update and you appear to be correct about the " locations

Re: Performing if else condition on two raster layers

LawrenceSekaluvu — Wed, 27 Aug 2014 17:29:45 GMT

@Darren, i have tried out that and it works well with a raster having a range of values a cross the grid cells. However, it doesn't work out for rasters with a constant value across the cells.

Re: Performing if else condition on two raster layers

DarrenWiens2 — Wed, 27 Aug 2014 17:38:12 GMT

Do you mean something like this doesn't work, or something else entirely?

C01 = Con("M0","M0","","VALUE == " + str(fb))

As always, please post your script so we can better see what you're talking about.

Re: Performing if else condition on two raster layers

SteveLynch — Wed, 27 Aug 2014 22:33:25 GMT

If I understand the problem correctly I would do the following:

Multiply each raster by say 1000 (or whatever you decide on) because COMBINE will INT the values
Use COMBINE

Have a look at the COMBINE tool and decide if this will do the job.

-Steve