Using Polygon Neighbors for boundary polygons to find boundary lengths in ArcMap?

Question

I am working in ArcMap. I have a map of districts in South Carolina, and I'm trying to find the boundary length that every district has with every other district, and the boundary of South Carolina itself. I understand that the GeoProcessing tool Polygon Neighbors can find the boundary lengths between polygons, but I also need the length that a district has with the edge of the state.

I am trying to verify that whatever values I get here correspond accurately with the perimeter of each district, so I don't simply want to use the perimeter tool and subtract off the sum of other neighbor lengths.

I have considered creating a buffer polygon around the state to act as an "outside" district, but I'm not sure how to do that.

Answer 1

The trick here is counting number of neighbors that share polygon edges. Workflow for that using spatial join one to many:

arcpy.Integrate_management(in_features="COUNTIES #", cluster_tolerance="0.01 Meters")
arcpy.FeatureToLine_management(in_features="COUNTIES", out_feature_class="C:/SCRATCH/to_dissolve.shp", cluster_tolerance="", attributes="NO_ATTRIBUTES")
arcpy.Dissolve_management(in_features="to_dissolve", out_feature_class="C:/SCRATCH/edges.shp", dissolve_field="", statistics_fields="", multi_part="SINGLE_PART", unsplit_lines="DISSOLVE_LINES")
arcpy.SpatialJoin_analysis(target_features="edges", join_features="COUNTIES", out_feature_class="C:/SCRATCH/SCRATCH.gdb/SJ_12M", join_operation="JOIN_ONE_TO_MANY", join_type="KEEP_ALL", field_mapping="", match_option="SHARE_A_LINE_SEGMENT_WITH", search_radius="", distance_field_name="")

Add field NEIGHBOURS, compute count of polygons per TARGET_FID in spatial join output, transfer it back to the same table:

Visually inspect spatial joins and make sure outer (1 neighbour) and inside edges (2 neighbours) located where expected (if this is not the case, fix topology of polygons):

• Select outer (1 neighbour) spatial joins (edges) and transfer their lengths to relevant field in counties table using JOIN_FID for join.
• Select inside (2 neighbour) spatial joins, summarise by JOIN_FID and transfer their total length to relevant field in counties table using JOIN_FID for join: