PostGIS Polygon – How Can I Split a Polygon into Two Equal Parts Along a N-S Axis?

polygon-creationpostgissplitting

I have a PostGIS database with a table called 'sites' that includes a field sitename and a field geometry.

If I select, e.g. 

select sitename, ST_AsGeoJSON(geometry) from sites limit 1;

I get:

Plot 7 | {"type":"Polygon","coordinates":
[[[-111.974951718315,33.0745275800293,353],
[-111.974935360975,33.0745276003588,353],
[-111.974935370263,33.0745546649121,353],
[-111.974951727608,33.0745546445826,353],[-111.974951718315,33.0745275800293,353]]]}

These are rectangles oriented very close to N-S. I would like to divide each plot in half along (or close to) its N-S axis. Is there an easy way to do this?

Solutions in PostGIS-SQL preferred. Ultimately I plan to write insert statements like

insert into sites (name, geometry) 
   values ('Site 1 E', (select ??east half of geometry?? from sites where name = 'Site 1'))

The closest I've gotten is ST_Split, which requires the polygon as well as the 'blade' (line used to split), but it is not clear how I can compute the blade from the geometry.

Best Answer

I would like to divide each plot in half along (or close to) its N-S axis. Is there an easy way to do this? [...] can compute the blade from the geometry.

Something like?

xmin+((xmax-xmin)/2)

Here we create a sample data set foo. In it we create a polygon, we calculate the xaxis to bisect the polygon with, and we create the blade.

Then we do the bisection and display the results with ST_AsText

WITH foo AS (
  SELECT geom, blade
  FROM ST_MakeEnvelope(-10,-10,10,10) AS geom
  CROSS JOIN LATERAL ( SELECT ST_xMin(geom) + (ST_xMax(geom) - ST_xMin(geom)) / 2 ) AS axis(x)
  CROSS JOIN LATERAL ST_MakeLine(
    ST_MakePoint(axis.x, ST_yMin(geom)),
    ST_MakePoint(axis.x, ST_yMax(geom))
  ) AS blade
)
SELECT ST_AsText(geom),
  ST_AsText(blade),
  ST_AsText( ST_Split(geom, blade) )
FROM foo;
                   st_astext                    |       st_astext        |                                                st_astext                                                 
------------------------------------------------+------------------------+----------------------------------------------------------------------------------------------------------
 POLYGON((-10 -10,-10 10,10 10,10 -10,-10 -10)) | LINESTRING(0 -10,0 10) | GEOMETRYCOLLECTION(POLYGON((-10 -10,-10 10,0 10,0 -10,-10 -10)),POLYGON((0 10,10 10,10 -10,0 -10,0 10)))
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