topic Re: Dissolve/merge features without creating new feature class python in Python Questions

Dissolve/merge features without creating new feature class python

2Quiker — Thu, 09 Jan 2020 16:35:05 GMT

I have a feature class that can have many features within it i would like to merge all the features in this feature class into just one based on the "BUFF_DIST" like you would in ArcMap when you start editing, select all the features and go to edit, merge and you get the popup "Chose the feature with which other features will be merged:" i am trying to avoid creating a new feature class. Anyone done this or have an example of some python code?

Re: Dissolve/merge features without creating new feature class python

XanderBakker — Thu, 09 Jan 2020 17:26:24 GMT

Hi 2CQuiker ,

During an editing session in Pro, you can select multiple polygon features and merge them:

...resulting in a single feature:

Is this what you are looking for?

Re: Dissolve/merge features without creating new feature class python

2Quiker — Sun, 12 Dec 2021 06:53:32 GMT

yes i can do it on pro, i am trying to do it in a python script.

I have the following to start with but i am not quite here but my approach may not be the correct one.

BD = str('BUFF_DIST')
array = arcpy.Array()
with arcpy.da.SearchCursor(lyr, BD) as rows:
    for r in rows:
        for part in r[0]:
            array.append(part)
             
poly = arcpy.Polygon(array, sr)
arcpy.management.CopyFeatures(poly, r'in_memory\multipart_poly_merge')‍‍‍‍‍‍‍‍‍

Re: Dissolve/merge features without creating new feature class python

XanderBakker — Thu, 09 Jan 2020 19:25:12 GMT

Hi 2 Quiker ,

Do you just want to merge the geometries or do you need merge rules on the attributes too? Doing this on the same featureclass could lead to loss of data if something in the script goes wrong. It is possible though.

When I come to think of it a little more... you have a field BUFF_DIST, so you buffered on an attribute and you could dissolve the result during buffering directly and no need to do any additional scripting.

Re: Dissolve/merge features without creating new feature class python

DanPatterson_Retired — Thu, 09 Jan 2020 19:34:51 GMT

The data access editor provides some limited capabilities, but It isn't clear whether geometry changes are fully supported

Editor—Data Access module | ArcGIS Desktop

Re: Dissolve/merge features without creating new feature class python

2Quiker — Sun, 12 Dec 2021 06:53:34 GMT

The lyr just happened to be one that I was testing try to see how I can accomplish the task. The real layer i have is a different layer.

I tried the following with SHAPE@ and get the following. The other issue with this code is that none of the attributes get transferred, there is only OID, Shape in the new temp layer.

array = arcpy.Array()
with arcpy.da.SearchCursor(lyr, ['SHAPE@']) as rows:
    for r in rows:
        for part in r[0]:
            array.append(part)
             
poly = arcpy.Polygon(array, sr)
arcpy.management.CopyFeatures(poly, r'in_memory\multipart_poly_merge')‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

Re: Dissolve/merge features without creating new feature class python

DanPatterson_Retired — Sun, 12 Dec 2021 06:53:38 GMT

If the shape (eg. row) has multiple parts, you will effectively just add all the parts back into the array and end up with the same polygon.

If you truncate the appending, you will end up with less than the whole. So to demonstrate... let's cut your 'cursor' at the first part skipping the rest.

type(p_0)
arcpy.arcobjects.geometries.Polygon
p_0.partCount
2
type(p_0[0])
arcpy.arcobjects.arcobjects.Array‍‍‍‍‍‍

Re: Dissolve/merge features without creating new feature class python

2Quiker — Fri, 10 Jan 2020 16:14:51 GMT

sorry but i am not sure how i can incorporate what you posted into my code to do what i need it to do.

Re: Dissolve/merge features without creating new feature class python

DanPatterson_Retired — Fri, 10 Jan 2020 16:49:38 GMT

What I was pointing out that your approach of merging/appending/unioning a multipart shape is just going to leave you with a multipart shape and won't give you the desired result. Notice, you only search for the geometry then cycled through the geometry bits and appended the parts before copying all the parts back together to recreate the polygon. My demo was to show you what happens during the process when you terminate the append bit component.

arcpy.da.SearchCursor(lyr, ['SHAPE@']) as rows:

If you want to merge on some other field, you should be using that field in the search cursor, then you can append the polygons.

But... using the appropriate tool in arctoolbox is a lot better since it will check the geometries and perform the necessary intersections, and cleaning up the geometry as it goes. The reluctance to create a new file is a bit confusing... you can always add Delete_management to any script you want after you have done a Union_analysis

two layers. union geometry

inFeatures = your features

arcpy.Union_analysis(inFeatures, outFeatures,.... blah blah)

arcpy.Delete_management(inFeatures) # delete one or both

Delete—Data Management toolbox | ArcGIS Desktop

one layer, union geometry

arcpy.Dissolve_management(tempLayer, outFeatureClass, dissolveFields, "", "SINGLE_PART", "DISSOLVE_LINES")

How Dissolve (Data Management) works—Data Management toolbox | ArcGIS Desktop

Then delete the original if needed

Re: Dissolve/merge features without creating new feature class python

2Quiker — Mon, 13 Jan 2020 18:08:54 GMT

I appreciate the info but I was trying to find a way to do this with out so many process.

Re: Dissolve/merge features without creating new feature class python

XanderBakker — Mon, 13 Jan 2020 18:53:31 GMT

Hi 2 Quiker ,

So, if I understand it correctly, your featureclass is not necessarily the result of buffering (so you cannot apply dissolve during buffering) and you don't want to create a new output (it should overwrite the featureclass) and you want to apply merge rules on the attributes. Sounds to me that you are in for a bit of coding. Do you consider that it is worth it? How large is your featureclass (how many features, how complex are they and how many features can be merged together)? Doing this in memory on a large and complex featureclass may run into limitations.

Re: Dissolve/merge features without creating new feature class python

2Quiker — Mon, 13 Jan 2020 21:49:10 GMT

Correct this is would not be a result of buffering, i don't wan to recreate a new feature class and I want to merge the attributes similar to the what you do in Arcmap when you select features,start editing, click editor tool bar- merge. This would all take place in ArcMap's Python not stand alone. I came close to this with the code i posted but i can't get it correct.

Thanks for the reply.

Re: Dissolve/merge features without creating new feature class python

JamesCrandall — Sun, 12 Dec 2021 06:53:40 GMT

Since you want to overwrite the features that were included in the dissolve/union, you will always have the potential of data loss should something go wrong. If it were me I'd look to convert the searchCursor to JSON (there's a SHAPE@JSON property available), run the dissolve on that to create a new output, remove the original features then use the JSON of dissolved features to update the original feature class.

You'd have to look into the Reading geometries—ArcPy Get Started | ArcGIS Desktop and Writing geometries—ArcPy Get Started | ArcGIS Desktop to get full context. Plus you'd need to come up with the union process -- I just use the GeometryServer/union task on one of our ArcGIS Servers to perform that process on input features like so,

def runUnion(geomEntity):

    queryURL = "https://YourArcGISServerSite/rest/services/Utilities/Geometry/GeometryServer/union"
    params = urllib.urlencode({'f': 'json',
                               'geometries': geomEntity,
                               'geometryType': 'esriGeometryPolygon',
                               'sr': 3857
                               })

    req = urllib2.Request(queryURL, params)
    response = urllib2.urlopen(req)
    jsonResult = json.load(response)

    return jsonResult‍‍‍‍‍‍‍‍‍‍‍‍‍‍

I think in the end someone mentioned it will take a bit of coding and question if it's worth the effort, which is always the case I guess. For me, I've simply made the switch from localized data to working 100% with REST services which took a lot of effort to get migrate all of our python implementations off of SDE/FGDB/Local data. However, in the end it's worth it for us because it opens up the amount of data available to a global scale.

Re: Dissolve/merge features without creating new feature class python

2Quiker — Tue, 21 Jan 2020 17:26:19 GMT

At this point i would like to just merge the features without the attributes.

Re: Dissolve/merge features without creating new feature class python

XanderBakker — Sun, 12 Dec 2021 06:53:43 GMT

Hi 2 Quiker ,

See the example below:

def main():
    import arcpy

    fc = r'C:\GeoNet\DissolvePy\data.gdb\pols_test'
    fld_dissolve = 'Class'

    # get unique values from field
    unique_vals = list(set([r[0] for r in arcpy.da.SearchCursor(fc, (fld_dissolve))]))

    for unique_val in unique_vals:
        geoms = [r[0] for r in arcpy.da.SearchCursor(fc, ('SHAPE@', fld_dissolve)) if r[1] == unique_val]
        if len(geoms) > 1:
            diss_geom = DissolveGeoms(geoms)

            # update the first feature with new geometry and delete the others
            where = "{} = '{}'".format(fld_dissolve, unique_val)
            cnt = 0
            with arcpy.da.UpdateCursor(fc, ('SHAPE@'), where) as curs:
                for row in curs:
                    cnt += 1
                    if cnt == 1:
                        row[0] = diss_geom
                        curs.updateRow(row)
                    else:
                        curs.deleteRow()


def DissolveGeoms(geoms):
    sr = geoms[0].spatialReference
    arr = arcpy.Array()
    for geom in geoms:
        for part in geom:
            arr.add(part)
    diss_geom = arcpy.Polygon(arr, sr)
    return diss_geom

if __name__ == '__main__':
    main()‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

A couple of thing to keep in mind:

Try this on a copy of your data!
It will only works on polygons in this case
It assumes that your field to dissolve on is a text field
check the where clause to have it match the syntax of your workspace
It will put the dissolved polygon in the first of the features for each list of features to dissolve, so any additional fields will have the first value. No intelligence is applied to mimic any merge rules of the fields

Input polygons for testing:

resulting polygons:

Re: Dissolve/merge features without creating new feature class python

2Quiker — Wed, 22 Jan 2020 16:11:41 GMT

Xander,

Thanks for the reply and code, after running your example i noticed that the result is the same as my script. I thought i posted pics of my code results but i guess i didn't. It seems as both your and my code cuts out the over lapping polygons. I forgot to mention that there are some over lapping poygons.

Re: Dissolve/merge features without creating new feature class python

XanderBakker — Sun, 12 Dec 2021 06:53:46 GMT

Hi 2CQuiker ,

Can you change the function "DissolveGeoms" for this one below and try again (on a copy of your data)?:

def DissolveGeoms(geoms):
    cnt = 0
    for geom in geoms:
        cnt += 1
        if cnt == 1:
            diss_geom = geom
        else:
            diss_geom = diss_geom.union(geom)
    return diss_geom‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

Re: Dissolve/merge features without creating new feature class python

2Quiker — Wed, 22 Jan 2020 17:56:44 GMT

Xander that worked thank you for sharing!

Re: Dissolve/merge features without creating new feature class python

JoshuaBixby — Sun, 12 Dec 2021 06:53:49 GMT

Here is some code I have used in the past, similar but a bit different to Xander's. My code does two passes through the data set with the same cursor, the first pass gets the union-ed geometries, and the second pass updates and deletes rows.

from arcpy import da
from itertools import groupby
from operator import itemgetter
from functools import reduce

fc = # path to feature class or table
case_fields = ["field1", "field2"] # field(s) to group records for merging
sort_field, sort_order = "OBJECTID", "ASC"
shape_field = "SHAPE@"

fields = case_fields + [sort_field, shape_field]
sql_orderby = "ORDER BY {}, {} {}".format(", ".join(case_fields), sort_field, sort_order)

with da.UpdateCursor(fc, fields, sql_clause=(None, sql_orderby)) as cur:
    case_func = itemgetter(*range(len(case_fields)))
    merged_polys = {
        key:reduce(arcpy.Geometry.union, (row[-1] for row in group))
        for key, group
        in groupby(cur, case_func)
    }
    cur.reset()
    for key, group in groupby(cur, case_func):
        row = next(group)
        cur.updateRow(row[:-1] + [merged_polys[key]])
        for row in group:
            cur.deleteRow()
‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

If the data set is extremely large, where building a dict of merged polygons might not be practical, there is another approach using an update and search cursor in parallel.

from arcpy import da
from itertools import groupby
from operator import itemgetter
from functools import reduce

fc = # path to feature class or table
case_fields = ["field1", "field2"] # field(s) to group records for merging
sort_field, sort_order = "OBJECTID", "ASC"
shape_field = "SHAPE@"

fields = case_fields + [sort_field, shape_field]
sql_orderby = "ORDER BY {}, {} {}".format(", ".join(case_fields), sort_field, sort_order)

kwargs = {"in_table":fc, "field_names":fields, "sql_clause":(None, sql_orderby)}
with da.UpdateCursor(**kwargs) as ucur, da.SearchCursor(**kwargs) as scur:
    case_func = itemgetter(*range(len(case_fields)))
    ugroupby, sgroupby = groupby(ucur, case_func), groupby(scur, case_func)
    for (ukey,ugroup),(skey,sgroup) in zip(ugroupby,sgroupby):
        poly = reduce(arcpy.Geometry.union, (row[-1] for row in sgroup))
        row = next(ugroup)
        ucur.updateRow(row[:-1] + [poly])
        for row in ugroup:
            ucur.deleteRow()
‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

Re: Dissolve/merge features without creating new feature class python

2Quiker — Tue, 11 Feb 2020 22:24:06 GMT

I am getting an error and " The field BUFF_DIST is a double, file is a shapefile.

Runtime error
Traceback (most recent call last):
File "<string>", line 24, in <module>
File "<string>", line 22, in <dictcomp>
RuntimeError: A column was specified that does not exist

Never mind I forgot to change OBJECTID to FID, since it's a shapefile.

Everything works, thanks for sharing.