Clip_management to raster extent outputs larger raster than raster extent

Question

Using Python and ArcMap 10.0 SP5, I'm trying to reproject and clip a large input raster to exactly the same grid as a smaller target raster (same extent, pixel sizes and projection). The problem is that the script produces an output raster that has 1 more row than the target raster (e.g. 1324 rows instead of 1323 -- columns match perfectly, pixels appear to be on the same grid). There is an extra row of pixels that overhangs the target image - everything else appears to be okay.

The following is my code for projecting and clipping to a target raster. What's needed to ensure that the output extent is exactly the same as in target raster? I'm new to ArcMap so it could be something obvious.

import arcpy
import tempfile
import os
import shutil

sourceImage = "C:/data/WorldClim_alt/WorldClim_Alt.tif"
inputImage = "E:/subset/nCom_2000/nCom2000101to107.tif"
outputFilename = "E:/subset/test/nComTest_clipped.tif"

# Snap to source image and source image extent
arcpy.env.extent = sourceImage
arcpy.env.snapRaster = sourceImage

# Get cell size
cellx = arcpy.GetRasterProperties_management(sourceImage, "CELLSIZEX")

# Get temporary directory and filename
tempDir = tempfile.mkdtemp()
tempFilename = 'E:/subset/test/' + basename(outputFilename) +'_proj.tif'

# Reproject
arcpy.ProjectRaster_management(inputImage,tempFilename,sourceImage, "#", cellx)

#
# Clip to source file
#

# Get envelope for clipping
top = arcpy.GetRasterProperties_management(sourceImage, "TOP")
left = arcpy.GetRasterProperties_management(sourceImage, "LEFT")
bottom = arcpy.GetRasterProperties_management(sourceImage, "BOTTOM")
right = arcpy.GetRasterProperties_management(sourceImage, "RIGHT")
envelope = str(left) + ' ' + str(bottom) + ' '+ str(right) + ' ' + str(top)

# Clip 
arcpy.Clip_management(tempFilename, envelope, outputFilename, sourceImage,"#","NONE")

Edit in Response to Paul's comment on Clip help:

From the help on Clip, it mentions

If the clip extent specified is not aligned with the input raster dataset, this tool makes sure that the proper alignment is used. This may cause the output to have a slightly different extent than specified in the tool.

I think that the input to clip (the output from ProjectRaster) may be aligned with the in_template_dataset. The snapRaster environment variable was set to the in_template_dataset before ProjectRaster was called. Also, the output cell size of ProjectRaster was set to the in_template_dataset cell size. Visual inspection of the grids seems to indicate that they are aligned (except for an extra row and column in the ProjectRaster output).

The only effect of Clip seems to be the removal of a single column.

Answer 1

In my experience with clip management, these types of issues are often due to rounding errors. Basically it seems like actual image coordinates are often saved with greater precision than what is returned from, for instance, GetRasterProperties_management. In some tools, the output will be resampled based on the exact coordinates, but here, I believe it's just selecting input cells that are within the extent (the goodness or badness of this depends on what you're trying to do). For example, if the actual image coordinate of 'top' is

345109.3011115

And your clip extent specified is

345109.301112

an additional row will be selected, because the extent extends ever so slightly beyond that top row. To remedy this, I often select an extent that is a fraction of a cell inside of the actual clipping extent (for top and right, that's coordinates slightly smaller, while for bottom and left it's coordinates slightly larger). In your example (assuming the cells are greater than 1 unit resolution) I would do something like

top = float(arcpy.GetRasterProperties_management(sourceImage, "TOP")) - 1.0
left = float(arcpy.GetRasterProperties_management(sourceImage, "LEFT")) + 1.0
bottom = float(arcpy.GetRasterProperties_management(sourceImage, "BOTTOM")) + 1.0
right = float(arcpy.GetRasterProperties_management(sourceImage, "RIGHT")) - 1.0

Good luck, ArcGIS loves to play games with these types of things.

Answer 2

From the help on Clip, it mentions

If the clip extent specified is not aligned with the input raster dataset, this tool makes sure that the proper alignment is used. This may cause the output to have a slightly different extent than specified in the tool.

Have you tried running the tool without using the optional parameter in_template_dataset?

Also, you might find this way easier to create the envelope. The numbers in the brackets refer to index numbers for the format function, and the values are auto casted to string. The spaces between brackets are important.

envelope = "{0} {1} {2} {3}".format(left, bottom, right, top)

I notice that you are creating a temporary folder for your projected image. You might be interested in using the in_memory workspace, which is designed to store intermediate steps of workflows. Assuming you have enough memory (RAM), this route is usually faster than saving a temp file to disk. Just remember to Delete afterwards!

Answer 3

I implemented almost the same workflow and I had one more row than the target raster after clip. I solved the problem by clearing the snapRaster before the clip function:

arcpy.ClearEnvironment("snapRaster")

Perhaps, you have to clear the extent, too:

arcpy.ClearEnvironment("extent")

Furthermore, I have not specified an “in_template_dataset”, since the envelope comes from the sourceImage:

arcpy.Clip_management(tempFilename, envelope, outputFilename)