topic Re: Question about aggregating polygons in Geoprocessing Questions

Question about aggregating polygons

AndrewNarwold — Tue, 08 Sep 2015 14:24:02 GMT

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.

Re: Question about aggregating polygons

WesMiller — Tue, 08 Sep 2015 14:28:50 GMT

If i understand what your after correctly i think dissolve will get you want you want it will combine all the polygons contiguous or not and create a single polygon.

Re: Question about aggregating polygons

ChrisSmith7 — Tue, 08 Sep 2015 15:55:14 GMT

Dissolve should work for you:

Dissolve—Help | ArcGIS for Desktop

It will create multi-part polygons for you by default, which means you can dissolve by a field, or multiple fields, with the same attribute(s) - the polygons are not required to be contiguous.

Re: Question about aggregating polygons

DanPatterson_Retired — Tue, 08 Sep 2015 16:43:31 GMT

Dissolve is fine if there is some need to adjoin adjacent land parcels...for instance, same landuse and/or owner etc etc. I generally recommend against this since I is far simpler to have singlepart poly* features than multipart. In fact, many geoprocessing functions only work properly when you have singlepart features (ie determining the centre of a polygon or simply counting the number of one type.

So do you have a particular reason to do this. Is it to save file size? Have you considered that a layer could be split based upon its attributes rather than aggregating geometry? (ie group residential land use into one layer...). Any insight into why you need/want to do this might provide some suggestions on how it affects your workflow and how you might benefit from alternates yet maintain the integrity of the geometry and its attributes.

Re: Question about aggregating polygons

AndrewNarwold — Tue, 08 Sep 2015 16:54:28 GMT

Thanks for the advice. What I am trying to do is create polygons for different special tax assessment zones. These are not currently geo-coded. So I am trying to aggregate separate land parcels (not contiguous) into one assessment zone. I then want to do some geoprocessing with the larger assessment zones. Does this make sense?

Re: Question about aggregating polygons

WesMiller — Tue, 08 Sep 2015 17:05:31 GMT

When you run the dissolve you will be asked where to save the new feature class, so you wont be hurting anything to test. Give it a test and see what happens.

Re: Question about aggregating polygons

DanPatterson_Retired — Tue, 08 Sep 2015 17:09:57 GMT

Why not simply query for the assessment zone attribute, which I assume you have. Once queries, you can save the selection to its own layer and not have to worry about that criterion again since any operations will apply to it. When you are done, you can simply delete it. The advantage is that you don't have to worry about going to the trouble of limiting the operation to that class. Currently, I can't see any geoprocessing function which would be advantaged by consolidating the geometry. Also what do you do about the cases that the primary class is one type but the secondary, tertiary.... are different types? How do you plan to accommodate for that when you geoprocessing operation changes to something else?

Re: Question about aggregating polygons

AndrewNarwold — Tue, 08 Sep 2015 21:01:30 GMT

Hi Dan:

Bear with me here if you will. I tried using dissolve, and got the multipart polygon. I can see why that is an issue and it does not solve my question. So I could certainly split the data into multiple layers based on each tax assessment district. But what I would get is hundreds of parcels in each layer with the same tax assessment district number, but as distinct polygons. How would I go about constructing the district itself. For instance, I would like the centroid of the district or the area of the district.

Thanks,

Andrew

Re: Question about aggregating polygons

DanPatterson_Retired — Tue, 08 Sep 2015 21:16:19 GMT

Andrew... try my Split Layers by attribute

http://www.arcgis.com/home/item.html?id=15ca63aebb4647a4b07bc94f3d051da5

It is a convention toolbox, which you load in ArcToolbox and run it from there, not ArcCatalog.

Porting it over to ArcGIS Pro and File Geodatabases is ongoing. Now let me know if you can get to that link...it isn't visible from ArcScripts 2.0 Beta for some reasons that are too difficult to explain.

In any event, you can split your big file into subfiles, based upon a common attribute...which I presume is a District field in your table. If you read carefully, you will get outputs that are separated. Cavaets, the inputs must be a shapefile (long story) so export any featureclass to a shapefile and process all your results in a folder. This will produce a number of unique shapefiles which you can bring in as feature classes as you need them, or simply create a file geodatabase solely for this purpose and load the stuff up...do your processing and the rest of your projects are intact and undisturbed. Let me know

Re: Question about aggregating polygons

DarrenWiens2 — Tue, 08 Sep 2015 21:16:44 GMT

Since you mention Aggregating polygons and that Dissolve doesn't work, I assume you've tried the Aggregate tool...? Does it produce the results you're after, and you're looking for a way to automate cycling through each district, or does it not produce the results you want at all?

Is this the situation you're describing?:

Re: Question about aggregating polygons

DanPatterson_Retired — Tue, 08 Sep 2015 22:13:39 GMT

Yes but...

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

Re: Question about aggregating polygons

DarrenWiens2 — Tue, 08 Sep 2015 22:18:51 GMT

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)

Re: Question about aggregating polygons

DanPatterson_Retired — Tue, 08 Sep 2015 22:52:18 GMT

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...

Re: Question about aggregating polygons

AndrewNarwold — Wed, 09 Sep 2015 01:57:54 GMT

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

Re: Question about aggregating polygons

DanPatterson_Retired — Wed, 09 Sep 2015 02:38:46 GMT

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

Re: Question about aggregating polygons

RebeccaStrauch__GISP — Wed, 09 Sep 2015 12:40:08 GMT

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.

Re: Question about aggregating polygons

AndrewNarwold — Wed, 09 Sep 2015 17:42:04 GMT

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.

Re: Question about aggregating polygons

DanPatterson_Retired — Wed, 09 Sep 2015 18:23:47 GMT

screen grab time and a list of geoprocessing functions to perform

Re: Question about aggregating polygons

AndrewNarwold — Wed, 09 Sep 2015 18:25:58 GMT

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

Re: Question about aggregating polygons

DanPatterson_Retired — Wed, 09 Sep 2015 18:30:56 GMT

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.