Computation methods

Polygons have a few more useful methods. They can be applied to a collection of polygons (multi-polygon)too.

Besides area you can calculate as well perimeter of surfaces. The perimeter of a polygon includes the length of all rings (exterior and interior).

SELECT "SHAPE".ST_asWKT() AS "WKT",
"SHAPE".ST_Area() AS "AREA",
ROUND("SHAPE".ST_Perimeter(), 4) AS "PERIMETER"
FROM "TUTORIAL_GEO"."SPATIALSHAPES"
WHERE "SHAPE".ST_GeometryType() in ('ST_Polygon');

As mentioned, you can calculate area and perimeter of multi-polygons as well. Below is the example of computations of the multi-polygon, which is a result of union aggregation of individual polygons in the table.

SELECT
"AGGRSHAPE".ST_asWKT() AS "WKT",
"AGGRSHAPE".ST_Area() AS "AREA",
ROUND("AGGRSHAPE".ST_Perimeter(), 2) AS "PERIMETER"
FROM (
SELECT ST_UnionAggr("SHAPE") as "AGGRSHAPE"
FROM "TUTORIAL_GEO"."SPATIALSHAPES"
WHERE "SHAPE".ST_GeometryType() in ('ST_Polygon')
)

The method ST_Centroid() returns the point that is the mathematical centroid of a polygon or multi-polygon.

SELECT
"SHAPE".ST_asWKT() AS "WKT",
"SHAPE".ST_Centroid().ST_asWKT() AS "CENTROID_WKT"
FROM "TUTORIAL_GEO"."SPATIALSHAPES"
WHERE "SHAPE".ST_GeometryType() IN ('ST_Polygon');

And another example where all individual polygons are combined into one multi-polygon.

SELECT
"AGGRSHAPE".ST_asWKT() AS "WKT",
"AGGRSHAPE".ST_Centroid().ST_asWKT() AS "CENTROID_WKT"
FROM (
SELECT ST_UnionAggr("SHAPE") as "AGGRSHAPE"
FROM "TUTORIAL_GEO"."SPATIALSHAPES"
WHERE "SHAPE".ST_GeometryType() in ('ST_Polygon')
);

Centroid of a polygon is not always at the surface of that polygon. Consider the following example.

SELECT
"PolygonWithInnerRing".ST_asSVG() AS "SVG",
"PolygonWithInnerRing".ST_Centroid().ST_asSVG() AS "CENTROID_SVG"
FROM (
SELECT NEW ST_Polygon('Polygon ((-5 -5, 5 -5, 0 5, -5 -5), (-2 -2, -2 0, 2 0, 2 -2, -2 -2))') as "PolygonWithInnerRing"
FROM "DUMMY"
);

Slightly modified SVG for better visualization, like the one below:

<svg xmlns="http://www.w3.org/2000/svg" version="1.1" viewBox="-5.01 -5.01 10.02 10.02">
<path fill="yellow" stroke="black" stroke-width="0.1%" d="
M -5,5 l 10,0 -5,-10 Z
M -2,2 l 0,-2 4,0 0,2 Z
"/>
<rect width="1%" height="1%" stroke="red" stroke-width="1%" x="0" y="1.79365"/>
</svg>

shows that the centroid (the red dot) is located inside of the inner ring, and therefore not f the surface of the polygon.

In some cases you will need to have a point (like some sort of marker on the map) that lays somewhere on the surface of a given polygon. That’s the purpose of a method ST_PointOnSurface().

Update the example above to include the calculation of the point-on-surface.

SELECT
"PolygonWithInnerRing".ST_asSVG() AS "SVG",
"PolygonWithInnerRing".ST_PointOnSurface().ST_asSVG() AS "CENTROID_SVG"
FROM (
SELECT NEW ST_Polygon('Polygon ((-5 -5, 5 -5, 0 5, -5 -5), (-2 -2, -2 0, 2 0, 2 -2, -2 -2))') as "PolygonWithInnerRing"
FROM "DUMMY"
);

Again slightly modify styling of SVG objects, to have the centroid in red and the point-on-surface in blue.

<svg xmlns="http://www.w3.org/2000/svg" version="1.1" viewBox="-5.01 -5.01 10.02 10.02">
<path fill="yellow" stroke="black" stroke-width="0.1%" d="
M -5,5 l 10,0 -5,-10 Z
M -2,2 l 0,-2 4,0 0,2 Z
"/>
<rect width="1%" height="1%" stroke="red" stroke-width="1%" x="0" y="1.79365"/>
<rect width="1%" height="1%" stroke="blue" stroke-width="1%"  x="-2.25" y="0"/>
</svg>

You can see on the visualization that the blue dot is indeed located on the yellow surface.

The ST_Distance method computes the shortest distance between two geometries.

In a round-Earth spatial reference system this method can only be used with geometries for point to point calculations.

The statement below will return five geometries from the table SPATIALSHAPES that are closest to the point (0,0).

SELECT TOP 5
"SHAPEID",
"SHAPE".ST_asWKT() AS "WKT",
ROUND("SHAPE".ST_Distance(NEW ST_Point('POINT (0 0)')), 4) AS "DISTANCE"
FROM "TUTORIAL_GEO"."SPATIALSHAPES"
WHERE "SHAPE".ST_isEmpty() = 0
ORDER BY 3 ASC;