How to get the correct centroid of a bigquery polygon with st_centroid

问题

I'm having some trouble with the ST_CENTROID function in bigquery. There is a difference between getting the centroid of a GEOGRAPHY column and from the same WKT version of the column. The table is generated using a bq load with a geography column and a newline_delimited_json file containing the polygon as wkt text.

Example:

select st_centroid(polygon) loc, st_centroid(ST_GEOGFROMTEXT(st_astext(polygon))) loc2,polygon from table_with_polygon

Result:

POINT(-174.333247842246 -51.6549479435566)
POINT(5.66675215775447 51.6549479435566)
POLYGON((5.666771 51.654721, 5.666679 51.655027, 5.666597 51.655017, 5.666556 51.655154, 5.666702 51.655171, 5.666742 51.655037, 5.666824 51.655046, 5.666917 51.654737, 5.666771 51.654721))

POINT(-174.367214581541 -51.645030856473)
POINT(5.63278541845948 51.645030856473)
POLYGON((5.632691 51.644997, 5.63269 51.644999, 5.63273 51.645003, 5.632718 51.645049, 5.632843 51.645061, 5.632855 51.645014, 5.632691 51.644997))

POINT(-174.37100400049 -51.6434992715399)
POINT(5.62899599950984 51.6434992715399)
POLYGON((5.629063 51.643523, 5.629084 51.643465, 5.629088 51.643454, 5.628957 51.643436, 5.628915 51.643558, 5.629003 51.64357, 5.629021 51.643518, 5.629063 51.643523))

POINT(-174.293340001044 -51.6424190026157)
POINT(5.70665999895557 51.6424190026157)
POLYGON((5.706608 51.642414, 5.706624 51.642443, 5.706712 51.642424, 5.706696 51.642395, 5.706608 51.642414))

POINT(-174.306209997018 -51.6603530009923)
POINT(5.69379000298176 51.6603530009923)
POLYGON((5.693801 51.660361, 5.693802 51.660346, 5.693779 51.660345, 5.693778 51.66036, 5.693801 51.660361))

POINT(-174.291766437718 -51.6499633041183)
POINT(5.70823356228228 51.6499633041183)
POLYGON((5.708187 51.649858, 5.708091 51.650027, 5.70828 51.650069, 5.708376 51.649899, 5.708187 51.649858))

POINT(-174.369405698681 -51.653769846544)
POINT(5.63059430131924 51.653769846544)
POLYGON((5.630653 51.653531, 5.630462 51.653605, 5.630579 51.653722, 5.630574 51.65373, 5.630566 51.653729, 5.630551 51.653759, 5.630559 51.65376, 5.630555 51.653769, 5.630273 51.653846, 5.630364 51.653974, 5.630787 51.653858, 5.630852 51.653728, 5.630653 51.653531))

...etc

Is this a bug or am I doing something wrong?

Update Did some further digging using Michael Entin's answer as a hint. It turns out that bq load with WKT does NOT use the smallest polygon by default. And there is no option with bq load to change this behaviour. The imported json is very large (openstreetmap data) so there is no easy option to change this to geoJson.

To dig deeper into the actual value stored in the column, I did a

select st_asgeojson(polygon) from ...

which resulted in

{ "type": "Polygon", "coordinates": [ [ [5.598659, 51.65927], [5.598651, 51.659295], [5.598638, 51.659293], [5.598626, 51.65933], [5.598788, 51.659353], [5.598799, 51.659319], [5.598855, 51.659139], [5.598692, 51.65912], [5.598643, 51.659268], [5.598659, 51.65927] ], [ [180, 90], [180, 0], [180, -90], [-180, -90], [-180, 0], [-180, 90], [180, 90] ] ] }

So here the wrong orientation can be seen.

回答1:

Looks like some or all of these polygons might have gotten inverted, and this produces antipodal centroids: POINT(-174.333247842246 -51.6549479435566) is antipodal to POINT(5.66675215775447 51.6549479435566) etc.

See BigQuery doc for details of what this means: https://cloud.google.com/bigquery/docs/gis-data#polygon_orientation

There are two possible reasons and ways to resolve this (my bet is case 1):

The polygons should be small, but were loaded with incorrect orientation, and thus became inverted - they are now complimentary to what was the intended shape, and are larger than hemisphere. Since you don't pass oriented parameter to ST_GEOGFROMTEXT, this function fixes them by ignoring the orientation.

The correct solution is usually to load them as GeoJson (this also avoids another issue with loading WKT strings - geodesic vs planar edges). Or if all the edges are small and geodesic vs planar does not matter - replace the table geography with ST_GEOGFROMTEXT(st_astext(polygon)).

The polygons should really be large, and were loaded with correct orientation. Then when you don't pass oriented parameter to ST_GEOGFROMTEXT, this function breaks them by ignoring the orientation.

If this is the case, you should pass TRUE as second parameter to ST_GEOGFROMTEXT.

来源：https://stackoverflow.com/questions/59620818/how-to-get-the-correct-centroid-of-a-bigquery-polygon-with-st-centroid

标签

google-bigquery

gis

polygon

centroid