I want to run a custom tool created with the Model Builder in ArcGIS. It needs to output to the SAME file each time it runs, just adding the data. Basically I have a custom Dissolve tool that will dissolve (in this case) counties together based on user selection. I need a single file containing ALL the dissolved final shapes, but the Model Builder complains unless each output file is unique. Is there anyway around this or do I have to create dissolve_1, dissolve_2, etc... (to 500 or so) and then combine them afterwards?


2 Answers 2


It seems to me like the easiest way to accomplish this task is to

  1. create a new empty output layer/feature class,
  2. dissolve the multiple inputs to a scratch (or better yet, an in_memory layer) and,
  3. APPEND or MERGE them together into your designated output file.
  • Append was what I was looking for. Missed it somehow. Thanks.
    – Josh
    Jul 18, 2013 at 17:40

One method to create unique names in Model Builder is to add %n% into the output parameter. In your case, the proper usage would be dissolve_%n%

For example

enter image description here

Or if you would like to create unique names by list position, use %i%

enter image description here

Reference: Examples of in-line variable substitution with ModelBuilder system variables

  • My understanding of this is that it only works if you use a Loop. Running the tool multiple unique times would create an error as it resets to 0 (and therefore the name is Buffer0 each time).
    – Josh
    Jul 18, 2013 at 17:19
  • I'm not sure how to get access to this in model builder (it may not be possible), but Create Unique Name would solve this issue.
    – Paul
    Jul 18, 2013 at 17:28
  • @Josh, so you need a tool that searches for the last unique name in the run (e.g. dissolve_4) and names the new output "dissolve_5"...? How about list position %i%?
    – Aaron
    Jul 18, 2013 at 17:38
  • That might have worked. The simplest way to deal with my problem was in the answer by Ryan - append to an already existing file.
    – Josh
    Jul 18, 2013 at 17:41

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