Take Minimum value of multiple raster using ArcPy

[ShouvikJha]

Hi all, 

I have monthly raster for different parameters in three different folders. I am trying to take the minimum value of raster from each raster in two different parameters and solve the below equation. For that, I have written a code, but the code is producing an error. 

It is giving an error in line no 35. 

Sample data attached below 

Code: 

def main():
    import arcpy
    import os
    arcpy.env.overwriteOutput = True
    # Checkout extension
    arcpy.CheckOutExtension("Spatial")

    ws_in_APAR = r'E:\APAR'

    ws_in_TSCALAR = r'E:\T_SCALAR'

    ws_in_WSCALAR = r'E:\W_SCALAR'

    ws_out_GPP = r'E:\GPP_1982'


    # list "mean" rasters (r001_APAR so on)
    arcpy.env.workspace = ws_in_APAR
    lst_ras_APAR = arcpy.ListRasters()
    print "lst_ras_APAR", lst_ras_APAR

    # list "mean" rasters (r001_TSCALAR so on)
    arcpy.env.workspace = ws_in_TSCALAR
    lst_ras_TSCALAR = arcpy.ListRasters()
    print "lst_ras_TSCALAR", lst_ras_TSCALAR

    # list "mean" rasters (r001_WSCALAR so on)
    arcpy.env.workspace = ws_in_WSCALAR
    lst_ras_WSCALAR = arcpy.ListRasters()
    print "lst_ras_WSCALAR", lst_ras_WSCALAR




    for ras_name in lst_ras_APAR, lst_ras_TSCALAR, lst_ras_WSCALAR   :
        ras_APAR = arcpy.Raster(os.path.join(ws_in_APAR, ras_APAR))
        ras_TSCALAR = arcpy.Raster(os.path.join(ws_in_TSCALAR, ras_TSCALAR))
        ras_WSCALAR = arcpy.Raster(os.path.join(ws_in_WSCALAR, ras_WSCALAR))



        # calculate (ras_TSCALAR.minimum wil take the minimum value of perticular raster)
        ras_GPP = (ras_APAR * 1.48) * (ras_TSCALAR.minimum * ras_WSCALAR.minimum)

        # save raster
        ras_num = ras_name[:3]
        out_name_GPP = os.path.join(ws_out_GPP, 'r{0}_GPP'.format(ras_num))
        ras_GPP.save(out_name_GPP)



if __name__ == '__main__':
    main()‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

Error message:

Traceback (most recent call last):
  File "E:\GPP.py", line 64, in <module>
    main()
  File "E:\GPP.py", line 47, in main
    ras_APAR = arcpy.Raster(os.path.join(ws_in_APAR, ras_APAR))
UnboundLocalError: local variable 'ras_APAR' referenced before assignment‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

[DanPatterson_Retired]

to use …. for in …. syntax you need to provide an iterable like a list

for ras_name in [lst_ras_APAR, lst_ras_TSCALAR, lst_ras_WSCALAR]: # note the encompasing [ ]

[ShouvikJha]

Dan Patterson‌, please can I get an example, it will help me to understand further 

[DanPatterson_Retired]

It is safer to cast the iterable as a list

a_list = [1, 2, 4, 3, 7, 6, 5]

counter = 0

for a_value in a_list:
    print("number {} has a value of {}".format(counter, a_value))
    counter += 1
    
number 0 has a value of 1
number 1 has a value of 2
number 2 has a value of 4
number 3 has a value of 3
number 4 has a value of 7
number 5 has a value of 6
number 6 has a value of 5‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

the way you have it you are passing a list as an iterable, so the whole list is passed. 

If you want individual values from those lists then you can enclose them then unpack

for a_value in [*a_list, *a_list]:
    print(a_value)
    
1
2
4
3
7
6
5
1
2
4
3
7
6
5‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

Or if you need to pair objects, then you can use zip (I think it works in python 2, but it certainly works in python 3.

for a_value in list(zip(a_list, a_list)):
    print(a_value)
    
(1, 1)
(2, 2)
(4, 4)
(3, 3)
(7, 7)
(6, 6)
(5, 5)‍‍‍‍‍‍‍‍‍‍

In short... make your iterable explicit and don't rely on providing the iterable as you have done

[ShouvikJha]

Hi Dan Patterson‌, Thanks, I am trying to follow your suggestion 

[ShouvikJha]

Hi Xander Bakker‌, please can you look at the code. Thanks 

[XanderBakker · Accepted solution]

Have a try with this code (change the paths on lines 8 to 11):

def main():
    import arcpy
    import os
    arcpy.env.overwriteOutput = True
    # Checkout extension
    arcpy.CheckOutExtension("Spatial")

    ws_in_APAR = r'C:\GeoNet\MinMultiRas\APAR'
    ws_in_TSCALAR = r'C:\GeoNet\MinMultiRas\T_SCALAR'
    ws_in_WSCALAR = r'C:\GeoNet\MinMultiRas\W_SCALAR'
    ws_out_GPP = r'C:\GeoNet\MinMultiRas\GPP_1982'

    # list "mean" rasters (r001_APAR so on)
    arcpy.env.workspace = ws_in_APAR
    lst_ras_APAR = arcpy.ListRasters()
    print "lst_ras_APAR", lst_ras_APAR

    # list "mean" rasters (r001_TSCALAR so on)
    arcpy.env.workspace = ws_in_TSCALAR
    lst_ras_TSCALAR = arcpy.ListRasters()
    print "lst_ras_TSCALAR", lst_ras_TSCALAR

    # list "mean" rasters (r001_WSCALAR so on)
    arcpy.env.workspace = ws_in_WSCALAR
    lst_ras_WSCALAR = arcpy.ListRasters()
    print "lst_ras_WSCALAR", lst_ras_WSCALAR

    if len(lst_ras_APAR) == len(lst_ras_TSCALAR) == len(lst_ras_WSCALAR):
        print("each list has equal amount of rasters, let's assume they match...")

        for i in range(len(lst_ras_APAR)):
            ras_APAR = arcpy.Raster(os.path.join(ws_in_APAR, lst_ras_APAR[i]))
            ras_TSCALAR = arcpy.Raster(os.path.join(ws_in_TSCALAR, lst_ras_TSCALAR[i]))
            ras_WSCALAR = arcpy.Raster(os.path.join(ws_in_WSCALAR, lst_ras_WSCALAR[i]))

            for ras_path in [os.path.join(ws_in_APAR, lst_ras_APAR[i]),
                             os.path.join(ws_in_TSCALAR, lst_ras_TSCALAR[i]),
                             os.path.join(ws_in_WSCALAR, lst_ras_WSCALAR[i])]:
                arcpy.CalculateStatistics_management(ras_path)

            # calculate (ras_TSCALAR.minimum wil take the minimum value of perticular raster)
            print("ras_TSCALAR.minimum: {}".format(ras_TSCALAR.minimum))
            print("ras_WSCALAR.minimum: {}".format(ras_WSCALAR.minimum))

            ras_GPP = (ras_APAR * 1.48) * (ras_TSCALAR.minimum * ras_WSCALAR.minimum)

            # save raster
            ras_name = lst_ras_APAR[i]
            ras_num = ras_name[1:4]
            out_name_GPP = os.path.join(ws_out_GPP, 'r{0}_GPP'.format(ras_num))
            print out_name_GPP
            ras_GPP.save(out_name_GPP)

    else:
        print("raster list have unequal number of items, aborting process...")

    arcpy.CheckInExtension("Spatial")


if __name__ == '__main__':
    main()‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

What I changed was the way the rasters are extracted from the lists and I included a calculate statistics since for some reason they are not present and that caused the minimum value to be None.

[ShouvikJha]

Xander Bakker‌, Thanks. It's working fine