Creating NDVI from raster using ArcPy

Question

I have to create NDVI's for multiple raster images, and so want to find out the most efficient way to do it.

The code below works, but I wondered if there was a more efficient way to do it?

# Strip out the red and near infrared bands    
arcpy.CompositeBands_management(input_raster + r"\Band_3", "raster_red")
arcpy.CompositeBands_management(input_raster + r"\Band_4", "raster_nearir")

# Create the ndvi numerator and denominators 
arcpy.Minus_3d("raster_nearir", "raster_red", "numerator")

arcpy.Plus_3d("raster_nearir", "raster_red", "denominator")

# Set local variables
top = arcpy.sa.Raster("numerator")
bottom = arcpy.sa.Raster("denominator")

# Execute Divide
raster_ndvi = top / bottom

# Save the output
raster_ndvi.save("ndvi_result")```

Answer 1

I would do something like this:

from arcpy.sa import *

input_raster = 'path/to/raster'

red = Float(Raster(input_raster + r"\Band_3"))  
nir = Float(Raster(input_raster + r"\Band_4"))

ndvi = (nir - red) / (nir + red)

ndvi.save("ndvi_result")

You could also use GenerateRasterFromRasterFunction_management, but that seems to take much the same processing time

import time

from arcpy.sa import *

input_raster = r"c:\temp\LC80900812014223LGN00.TIF"  # Some Landsat 8 data

s = time.time()
red = Float(Raster(input_raster + r"\Band_4"))    # Band names are 1 indexed
nir = Float(Raster(input_raster + r"\Band_5"))

ndvi = (nir - red) / (nir + red)

ndvi.save("c:/temp/ndvitest1.tif")

print("Map algebra:", time.time() - s)

s = time.time()
arcpy.GenerateRasterFromRasterFunction_management(
    raster_function="NDVI",
    out_raster_dataset=r"c:\temp\ndvitest2.tif", 
    raster_function_arguments=r"Raster c:\temp\LC80900812014223LGN00.TIF; VisibleBandID 3;InfraredBandID 4;ScientificOutput 1",  # Bands are 0 indexed
    format="TIFF")
print("GenerateRasterFromRasterFunction:", time.time() - s)

Map algebra: 17.283000230789185
GenerateRasterFromRasterFunction: 17.730000257492065