Question about aggregating polygons

[AndrewNarwold]

I have a shapefile that has numerous polygons.  I would like to combine polygons that have the same values for some characteristic into larger polygons.  I don't think the dissolve function will work because the polygons are not necessarily contiguous (Think parcels of land with streets in between.  I want to get a district that contains multiple parcels that may or may not be contiguous.)  I don't think aggregate polygons would work either as there are potentially hundreds of these districts in my data set.  Any ideas?  Thanks.

[DanPatterson_Retired]

Yes but...

• ArcInfo license required
• you still lose the attribute information like dissolve
• useful for select by attributes if you want to split manually

[DarrenWiens2]

Yes, but...

- if he's already using the Aggregate tool, he must have ArcInfo

- if you're automating the aggregate by attribute, then you may as well carry over the attribute at the same time (using Python)

[DanPatterson_Retired]

​I suspect what he is referring to is that the polygons aren't aggregated completely in space.  If they were then you still have the problem of the aggregation of the attributes as you get with dissolve.  Unless I am wrong, he wants to split the data into bits based upon the district.  Each district will consist of many polygons, but at least you can retain the attributes.  All he want to do is skip the select by attributes step prior to doing what he wants in the geoprocessing environment.  Of course, this can be done on individual files as I suggested, or it can be automated in a script whereby, a select by attributes, is performed first, then geoprocessing function next, on to the next.  No mention was made of scripting otherwise I would have offered up some alternates that exist or that I have in NumPy world.  We await...

[AndrewNarwold]

Hi All:

So as you can probably tell by my questions and familiarity with some of these topics, I am an advanced beginner at best.  I have not used scripts to date.  What I am trying to do is to derive the boundaries of special tax assessment districts.  I do not have shapefiles for these.  What I have are individual parcels owned by different individuals that have a field that indicates which, if any, special tax assessment they fall under.  I was thinking that if I could identify all of the individual parcels, then I could in essence create the polygon that contains all of those parcels.  I would like to have attributes of the newly created polygon such as the centroid and area.  This is why neither aggregate polygons or dissolve don't work for me.  Dan, your Split layers by attribute should do this if I can figure out how to run it?

Thanks

Andrew

[DanPatterson_Retired]

• unzip the zip file into a folder
• launch arcmap and open arctoolbox the little red tool box
• at the top of the arctoolbox tree, right-click on the word arctoolbox and select add toolbox
• navigate to the folder where the toolbox is
• make sure you have the shapefile loaded into arcmap and run the toolbox, specifying the parameters (ie the input layer, the field to query etc

that should do it

oh yeah

Using the ArcToolbox window—Help | ArcGIS for Desktop

[AndrewNarwold]

I ran the split layers tool and I now have separate layers for the parcels for each assessment zone.  But I am still stuck with trying to figure out how I can generalize the polygon that encompasses all of the individual lots within the zone.

[DanPatterson_Retired]

screen grab time and a list of geoprocessing functions to perform

[AndrewNarwold]

I am sorry, I am not sure what this means.

[DanPatterson_Retired]

a screen grab of your districts so that the layout of the polygons can be seen

a list of functions that you want to use/do

Both of these will have some definite control over what is the most appropriate suggestion and/or alternates

For example, if you want area...do you just want the area of a particular grouping of parcels associated with a district? or do you want to include the area of the roads that separate the houses as well. (some suggestions on produceing the boundary plus a concave hull could be used...keeping your data intact but allowing you to answer the paper. or do you want population density...many caveats as to how that can be expressed)  In short there is no one simple answer until the questions and spatial framework are finalized.

[RebeccaStrauch__GISP]

just to throw something completely different into the mix, in case the suggestions from the others don't do what you need, have you looked into subtypes?  You would want to first import you shape file into a FGDB, but then you could use ​A quick tour of subtypes—Help | ArcGIS for Desktop   to help categorize a common group of attributes without actually changing the data and polygons. I would suggest adding a field, calculating a unique value based on the attributes you are are wanting to group, and using that field as your subtype.  That makes it quick to visualize/symbolize the various grouping you want and you can always use that field then to extract to a temp file for calculation....if you can't use the file as a whole (with the new field).

Anyway, just wanted to throw that out since sometimes I know I get fixated on one line of thought (dissolve, aggregate, etc.) and forget that adding a field and calculating can sometime do what I need just as fast.