I'm using ArcGIS Pro 2.9.3, and I want to use the Mosaic to New Raster tool to merge groups of rasters. I have many days that each have multiple rasters within them. For example, I have










So in this case, I'd want to merge July3a.tif-July3c.tif into one raster, July3.tif, then July10a.tif-July10c.tif into July10.tif, and so on.

I've scratched out a basic while loop, but it only works for one day at a time, and it would be awfully time consuming to adjust the arguments for each and every day.

i = July3a.tif

while i = July3*.tif

arcpy.management.MosaicToNewRaster(i, output_location, July3.tif, {coordinate_system_for_the_raster}, {pixel_type}, {cellsize}, number_of_bands, {mosaic_method}, {mosaic_colormap_mode})

i += 1

Is it possible to make a loop that merges multiple days at once?

  • For each day do you only ever have the a, b and c versions or do they randomly go to z?
    – Hornbydd
    May 18, 2022 at 19:23
  • @Hornbydd The latter. My rasters are satellite images that have been filtered for clouds, so some dates have more or less images than others.
    – Beck
    May 18, 2022 at 19:26

1 Answer 1


Here is the code you can use, obviously I don't have your data so I spoofed it up with text files, so you need to edit the code and replace the txt with tif.

Here is my sample data:

Spoofed data

# Import modules
import arcpy
import glob
import os
import collections

# Read tiff files in folder into a list
folder = r"C:\temp"
lstTIFFS = glob.glob(folder + r"\*.txt") # replace txt with tif

# Create special dictionary where the items will be inserted into a set
dicFiles = collections.defaultdict(set)

# Build dictionary, key is day and item is a set of characters, e.g. june6,{a,b,c}
for fp in lstTIFFS:
    fn = os.path.basename(fp)[:-4]
    character = fn[-1] # returns something like a
    day = fn[:-1] # returns something like june6

# Mosaic data for each day
for k,v in dicFiles.iteritems():
    # create empty list of files for a single day
    lstInputs = list()

    # Build full path and insert into list
    for character in v:
        fp = folder + "\\" + k + character + ".txt"

    # Mosaic data
    outputname = "Merged_" + k + ".tif"
    arcpy.MosaicToNewRaster_management(lstInputs,r"C:\temp",outputname) # Important! set other parameters

This code is developed for ArcMap so if you want to run it in ArcGIS Pro you need to change this line:

for k,v in dicFiles.iteritems():


for k,v in dicFiles.items():

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