Default Polygon Shape Length

Question

Hi, I just discovered the shape length calculated for a polygon feature is the perimeter of the feature and not the feature length. Is this always the case or is there a way to make it so the shape length field automatically calculates and shows the feature length?

SiqingChen · Answer

Thanks Johannes. I wonder if the scripts work for irregularly shaped polygons?

JohannesLindner · Answer

Is this always the case or is there a way to make it so the shape length field automatically calculates and shows the feature length?

The Shape_Length field always shows the polygon's perimeter. Most areas represented by polygons are not regular shapes like rectangles, where the concepts of "length" and "width" make sense. They are mostly irregular, so the perimeter is the only sensible choice for Shape_Length.

If you want to calculate "Length" as "The longest side of this polygon", you can do it with code in Calculate Field.

Arcade (copied from my answer here😞

var distances = []
// get the polygon parts
var rings = Geometry($feature).rings
// loop over the parts
for(var r in rings) {
    var ring = rings[r]
    // loop over the vertices (skip first)
    for(var v = 1; v < Count(ring); v++) {
        Push(distances, Distance(ring[v - 1], ring[v]))
    }
}
return Max(distances)

Python:

# LengthField = 
get_longest_side(!Shape!)

# Code Block
def get_longest_side(polygon):
    sr = polygon.spatialReference
    distances = []
    for p in range(polygon.partCount):
        ring = polygon.getPart(p)
        for i in range(1, len(ring)):
            c1 = ring.getObject(i-1)
            p1 = arcpy.PointGeometry(arcpy.Point(c1.X, c1.Y, c1.Z), spatial_reference=sr)
            c2 = ring.getObject(i)
            p2 = arcpy.PointGeometry(arcpy.Point(c2.X, c2.Y, c2.Z), spatial_reference=sr)
            distances.append(p1.distanceTo(p2))
    return max(distances)

You can also do it in the Python Window:

with arcpy.da.UpdateCursor("TestPolygons", ["SHAPE@", "DoubleField1"]) as cursor:
    for polygon, length in cursor:
        sr = polygon.spatialReference
        distances = []
        for ring in polygon:
            for i in range(1, len(ring)):
                p1 = arcpy.PointGeometry(arcpy.Point(ring[i-1].X, ring[i-1].Y, ring[i-1].Z))
                p2 = arcpy.PointGeometry(arcpy.Point(ring[i].X, ring[i].Y, ring[i].Z))
                distances.append(p1.distanceTo(p2))
        cursor.updateRow([polygon, max(distances)])

RhettZufelt · Answer

Not sure if it meets your needs, but the post here by @JohannesLindner are working for me with attribute rules.

R_