How to detect/select/identify island polygons

Question

I am generalizing geological maps and as a part of the process I am dealing with "Island Polygons". For that, I first need to identify them or select them.

I already tried Select by Location, by creating another layer from the same layer and used intersect the source layer, which select not only island polygons but polygons which touch the polygon also.

How do I select only Island polygons which are surrounded only by one polygon?

Answer 1

If you have an advanced licence, you can use "polygon to line" and the "IDENTIFY_NEIGHBORS" option. Each contour will then be split in as many lines as you have neighbours. This will also store the left and right ID's (-1 if no neighbour) for each line. You can then summarize the resulting table and find which polyon have only one neighbour (select by attribute on the "count" field)

Answer 2

This script will interrogate the WKT makeup of the geometry and if the geometry has a hole will fill it in. The output is a feature class with filled-in polygons only, only those that had holes in the first place are copied to the new feature class. You can then use Select By Location to find polygons in your original dataset that are completely within the new filled feature class. This has worked for me but I knew my data had no multipolygons, each feature was a single polygon (you could remove multiparts or explode to achieve this), also if a polygon was within another polygon it was an island and it didn't just sit on top of another polygon.

import arcpy

# this is where your dataset resides, change path between ""
arcpy.env.workspace = r"C:\Users\******\Documents\ArcGIS\Default.gdb"

# this is the name of your feature class from the above path
fc = "your_fc"

# grab the WKT values for each feature
# this is a Python dictionary
shape_dict = {}

# for each unique feature we will get the WKT and store in the dictionary
# OBJECTID is a unique id but you can replace this with any unique id field name
with arcpy.da.SearchCursor(fc, ["OBJECTID", "SHAPE@WKT"]) as cursor:
    for row in cursor:
        shape_dict[row[0]] = row[1]

# this is an empty Python list, we will populate it witt WKT to make new features 
feature_list = []

# for each unique feature in the dictionary
for key, value in shape_dict.iteritems():

    # get an index list of all open parentheses 
    po_index = [pos for pos, char in enumerate(value) if char == "("]

    # get an index list of all closing parentheses index list
    pc_index = [pos for pos, char in enumerate(value) if char == ")"]

    # I have found that WKT for polygons seems to always print as
    # MULTIPOLYGON (((
    # even if a simple polygon. This rebuilds the WKT to remove holes
    if len(po_index) > 3:
        new_wkt = value[0:pc_index[0]+1] + "))"
        new_geom = arcpy.FromWKT(new_wkt)
        # and appends the new geom to a list
        feature_list.append(new_geom)

# creates a feature class based on the list and call it filled
# this will be put in the same path at the top of the script
arcpy.CopyFeatures_management(feature_list, "filled")

# set this to match the EPSG of the original polygons
# 27700 is for British National Grid, change to suit your projection
arcpy.DefineProjection_management("filled", "27700")

Answer 3

Without any test data I'm just thinking out aloud here, so may be a red herring. Assuming you have an Advance licence level I can't but help think the solution using existing tools is some sort of fusion of the ideas posted so far. I was wondering if it was worth the effort to explore the following idea:

• Convert your polygons to lines as described by @radouxju
• Load all that up into a geodatabase and then set up a geodatabase typology to test for the line rule Must Not Overlap.

This would identify and select up lines that overlap, so your outer boundary of your island overlapping your inner boundary of the surrounding polygon?

Answer 4

You can try to select objects in layer which have no intersection.