I have a polygon shapefile that consists of many different shaped features. I would like to identify only those shapes that are circles. Is there a way to do this? Is it a calculation I could do in the field calculator?
Thanks…
[GIS] How to determine if an object is a circle
geometry
Related Solutions
I had the epiphany. It is rather mundane. I was leaving out one essential piece of information for PostGIS to calculate the right angle.
What I was calculating was the angle between only the two points intersecting the small buffer exterior. To calculate the angle of the intersection, I need to calculate both angles between both points on the buffer exterior and the intersection point of the two line features and subtract them.
I updated the full SQL, but here's the salient bit:
SELECT
...
abs
(
round
(
degrees
(
ST_Azimuth
(
points.point2,
points.intersection
)
-
ST_Azimuth
(
points.point1,
points.intersection
)
)::decimal % 180.0
,2
)
)
AS angle
...
FROM
points
For me your shapefile with polygons seems to be okay. I checked it with the "Check validity".
Here is the recreated problem:
It appeared because basic usage-requirements were not met when working with the "Split Features".
So, there are several ideas on how to overcome it:
Either the splitting line has to be drown beyond the geometry
or the splitting line has to snap vertices/segments of the geometry
Best Answer
I would calculate the thinness ratio, which for a perfect circle will equal 1.0. The formula for thinness ratio is: T = 4pi(A/(P*P))