ArcPy tips and observations


When I started using ArcGIS a few weeks ago, I quickly became frustrated when operations failed after pointing and clicking several times. This motivated me start scripting in ArcPy. Now, I only need to load the script file once to make it crash. Thus, I am crashing more efficiently :). Overall, my experience with ArcPy has been alright. I discover two blog posts about hating ArcGIS 10 and ArcGIS sucking that are well worth the read. Despite the titles, the author actually likes ArcGIS. I understand why. ArcPy and ArcGIS are powerful. Plus, the documentation is usually well done and easy to find online (although, including data and reproducible examples like R’s help files would be a great addition).  However, ArcPy does have some problems/idiosyncrasies. Here’s what I have discovered:

1) Non-basic error message are often cryptic and some require hours of using Google. The archives on GIS Stack Exchange has become a new favorite page of mine. As an example, I got this error message (I removed and shortened my file paths in the code snip):

Layer (Folder) exists, please delete output if you wish to recreate layer
done with extraction for us_asp
Could not create tile. name: z:\my documents\...\slopeclipped, adepth: 32, type: 1, iomode: 3, version: 3, compression: 0, eval case: 1 (GetOpenTileChannel)
Traceback (most recent call last):
File "", line 1, in
File "z:/My Documents/.../", line 77, in
mF.RasterClipper(Slope, grid, SlopeClipped, arcpy.env.workspace)
File "", line 37, in RasterClipper
arcpy.Clip_management(raster, clipExt, out, "", "ClippingGeometry")
File "C:\ArcGIS\Desktop10.1\arcpy\arcpy\", line 12015, in Clip
raise e
arcgisscripting.ExecuteError: ERROR 999999: Error executing function.
Failed to copy raster dataset
Failed to execute (Clip).

This error leads me to my second tip.

2) The workspace should not include a space. Sometimes, but not all the time, this causes problems. This took me a while to figure out and I only found this by chance online when looking up another problem. For example, this code is okay:
arcpy.env.workspace = "C:/Folder/"

is okay, but
arcpy.env.workspace = "C:/My Documents/"

caused my example problem.

3) Be prepared to use libraries such as NymPy. ArcPy does not always play well with USA wide data. This seems to be the case for ArcPy’s wrapper functions more so than the base functions (My guess is the best performing functions are GDAL functions). I plan on posting this function once I have written it and tested it.

4) “Python Scripting for ArcGIS” is still a great reference!

I plan to post more of my project as I work through it.