Sum Area Grouped By Attribute

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11-13-2017 01:18 PM
Occasional Contributor III

Hello everyone,

I have some data that looks like the attached pic. There's got to be an easy way to get a total area for each USE_CATEGORY and put the totals into a list or dictionary. Not sure if I need to maintain which total relates to which USE_CATEGORY. I don't need it for the current calculation I want to do, but may be worthwhile if I want to write it out to some external format later.

Thanks for any help/pointers.

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MVP Emeritus
MVP Esteemed Contributor

I agree with Dan, Summary Statistics is the first thing that comes to mind.  Since you mentioned putting "the totals into a list or dictionary," you could use a defaultdict:

from collections import defaultdict
use_areas = defaultdict(float)
fc = # path to feature class

with arcpy.da.SearchCursor(fc, ["USE_CATEGORY","SHAPE_Area"]) as cur:
    for use, area in cur:
        use_areas[use] += area
Occasional Contributor III

Yes, summary statistics. I'm not sure why that didn't pop into my head as I've already used it previously. I think I was focused more on the list/dictionary. I'm working on using summary statistics now and trying to use in_memory tables. Once that's working I think I'll do it using the defaultdict, just for the learning. I'll update the thread as things progress.

Thanks all.

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MVP Frequent Contributor

I'm surprised that Joshua didn't offer a pandas solution with its groupby function as well as a to_dict() --- the problem is with the limitations of FeatureClassToNumPyArray with some column types, but this may be something to consider.  This gets close but I don't think it's exactly what you are looking for.

fc = r'H:\MyfeatureClass'

#Create array
nparr = arcpy.da.FeatureClassToNumPyArray(fc, ['USE_CATEGORY','Shape_Area'])

#get array as pandas dataframe and populate the repeating values column
df = pandas.DataFrame(nparr,columns=['USE_CATEGORY','Shape_Area'])

#summarize as desired on shape area
df = df.groupby(by='USE_CATEGORY', as_index=True)['Shape_Area'].sum()

#make it a dictionary
myDictionary = df.to_dict()‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍
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MVP Emeritus

James... numpy... did a split and count of a featureclass of mostly equal polygons.  The process is like df.groupby

The big difference is you call the da.SearchCursor as usual, the line 5 is the key, it is what the ArcGIS module uses extensively in its code as I posted in my last blog.

in_fc = r"C:\no_space\path\testdata.gdb\Carp_5x5"   # full 25 polygons

sc = arcpy.da.SearchCursor(in_fc, ['OBJECTID', 'Shape', 'Class'], explode_to_points=True)

arr = sc._as_narray()

un, idx, cnts = np.unique(arr['Class'], return_index=True, return_counts=True)

array(['A01', 'A02', 'A03', ..., 'E03', 'E04', 'E05'], 

array([  0,   5,  10, ..., 109, 114, 119], dtype=int64)

array([5, 5, 5, ..., 5, 5, 5], dtype=int64)

result = np.asarray(list(zip(un, cnts)), dtype=[('Class', '<U5'), ('Counts', '<i4')])

array([('A01', 5), ('A02', 5), ('A03', 5), ..., ('E03', 5), ('E04', 5),
       ('E05', 5)], 
      dtype=[('Class', '<U5'), ('Counts', '<i4')])

[('A01', 5),
 ('A02', 5),
 ('A03', 5),
 ('A04', 5),
 ('A05', 5),
 ('B01', 5),
 ('B02', 5),
 ('B03', 5),
 ('B04', 5),
 ('B05', 5),
 ('C01', 5),
 ('C02', 5),
 ('C03', 4),
 ('C03E', 4),
 ('C03N', 4),
 ('C03S', 4),
 ('C04', 5),
 ('C05', 5),
 ('D01', 5),
 ('D02', 5),
 ('D03', 5),
 ('D04', 5),
 ('D05', 5),
 ('E01', 5),
 ('E02', 5),
 ('E03', 5),
 ('E04', 5),
 ('E05', 5)]
MVP Esteemed Contributor

I like pandas, and especially like that it is now bundled with ArcGIS, but one rub I have with it is the poor documentation compared to other Python documentation.  I find people new to Python struggle with the pandas documentation, unnecessarily so.  I know I could put my something where my mouth is and contribute to the docs myself, but it isn't a big enough priority to fit into my schedule for the time being.  In this case, a defaultdict works well and is a nice introduction to the extended data types in the collections module.

Occasional Contributor III

What version of arcmap is pandas bundled?

I’m learning pandas right now and find the documentation tough. At least there’s stackexchange.

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MVP Esteemed Contributor

Esri originally announced it would be 10.3.1, but it got pushed back to 10.4: What's new in ArcMap 10.4

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Occasional Contributor III

I've had no opportunity to date to look at doing this using dictionaries, but summary statistics is working great. So I think I'll just leave it at that.

Thanks everyone!

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