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I had UK area divided into six different zones. I wanted to dissolve my feature class by zone just so I have six features in it. I used Dissolve tool which worked fine for five zones. One of the zones rather than being dissolved, got split into something that looks tiles.

I used the tool in ModelBuilder and also independently. And makes not difference if my output is stored in geodatabase or in the folder. I also run repair geometry on my input.

What would be the reason for that and how to make Dissolve tool work correctly?

Please see the image below.

enter image description here

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I think you are encountering a limitation of the Dissolve tool which is described in its documentation:

• The availability of physical memory may limit the amount (and complexity) of input features that can be processed and dissolved into a single output feature. This limitation could cause an error to occur, as the dissolve process may require more memory than is available. To prevent this, Dissolve may divide and process the input features using an adaptive tiling algorithm. To determine the features that have been tiled, run the Frequency tool on the result of this tool, specifying the same fields used in the dissolve process for the Frequency Field(s) parameter. Any record with a frequency value of 2 has been tiled. Tile boundaries are preserved in the output features to prevent the creation of features that are too large to be used by ArcGIS.

Caution:

Running Dissolve on the output of a previous dissolve run will rarely reduce the number of features in the output when the original processing divided and processed the inputs using adaptive tiling. The maximum size of any output feature is determined by the amount of available memory at run time; therefore, output containing tiles is an indicator that dissolving any further with the available resources would cause an out-of-memory situation or result in a feature that is unusable. Additionally, running the Dissolve tool a second time on output that was created this way may experience very slow performance for little to no gain and may cause an unexpected failure.

You could try running the same Dissolve on a machine with more RAM.

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    @geometeome Another resource with further explanation is this blog post: blogs.esri.com/esri/arcgis/2010/07/23/… From your screenshot that is an extremely complex polygon (the blog post specifically mentions coastlines). The blog post also mentions several errors and warnings (you need to check the geoprocessing results window) you may see with such data, specifically with the dissolve tool. If you can't throw more RAM at it, the alternative is to simplify/generalize your polygon before dissolving. – Chris W May 5 '15 at 23:35

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