Is there any way to differentiate and select polygons according to their shape on SpatiaLite/PostGIS?

Question

I have this small SpatiaLite database and it looks like the following:

It's made of polygons and I'd like to find a way of excluding all polygons that look more like a line. Something like excluding the following ones:

Is there any PostGIS function that I can use to make a query and filter polygons according to their shape (not filtering by area size)? In a way that the ones that look more like a line are excluded?

I understand that the result may not be perfect but I'd like to know if there's any way of differentiating the shape of polygons with SpatiaLite/PostGIS...

Answer 1

Spatialite 5.0 has a function "Circularity" http://www.gaia-gis.it/gaia-sins/spatialite-sql-latest.html.

computes the Circularity Index from the given Geometry by applying the following formula:

index = ( 4 * PI * Sum(area) ) / ( Sum(perimeter) * Sum(perimeter) )

it only applies to Polygons or MultiPolygons with the following
interpretation:
    1.0 corresponds to a perfectly circular shape.
    very low values (near zero) correspond to a threadlike shape.
    intermediate values correspond to a more or less flattened
    shape; lower index values means a stronger flattening effect.
if the given Geometry does not contains any Polygon but contains
at least a Linestring the index will always assume a 0.0 value.
if the given Geometry only contains one or more Points the index
will always assume a NULL value.

SQL for your dataset:

select circularity(geometry) as shapeindex,*
from dataset
order by shapeindex asc;

The first selected feature in the BLOB explorer

Answer 2

You could use ST_MaximumInscribedCircle to find polygons which are "narrower" than a given distance. This will also find small polygons which are not elongated, so to filter those out you could use ST_Perimeter.

Answer 3

You can use a compactness measure to determine the "circularity" of a polygon. See this question.

Answer 4

How about using a QGIS function to create an "Oriented minimum bounding box"? The process creates a field called height and another width for each feature. You could then calculate the ratio of the two fields and set a threshold to what you consider elongated or not.

Answer 5

A very simple approach: calculate the length of the boundary as well as the area of the polygon, than use this to get the boundary/area ratio. Long boundary divided by small area results in a high value: thus you could define a threshold above which your polygons tend to be more or less like a line.

You could directly use Select by expression with an expression like the following. Replace x by a certain threshold that fits your needs. Start e.g. with 0.02 and than increase the value until you're staisfied with the result:

length (boundary($geometry)) / area($geometry) > x