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# transformations

CORE
This module contains functions that compute geometric constructions, or alter geometry size or shape.

### ST_BUFFER

ST_BUFFER(geog, distance [, segments])
Description
Calculates a buffer for the input features for a given distance.
• `geog`: `GEOGRAPHY` input to be buffered.
• `distance`: `DOUBLE` distance of the buffer around the input geography. The value is in meters. Negative values are allowed.
• `segments` (optional): `INTEGER` number of segments used to approximate a quarter circle. The default value is `8`.
Return type
`GEOGRAPHY`
Example
SELECT CARTO.CARTO.ST_BUFFER(ST_POINT(-74.00, 40.7128), 1000);
-- { "coordinates": [ [ [ -73.98813543746913, 40.712799392649444 ], ...
SELECT CARTO.CARTO.ST_BUFFER(ST_POINT(-74.00, 40.7128), 1000, 10);
-- { "coordinates": [ [ [ -73.98813543746913, 40.712799392649444 ], ...

### ST_CENTERMEAN

ST_CENTERMEAN(geog)
Description
Takes a Feature or FeatureCollection and returns the mean center (average of its vertices).
• `geom`: `GEOGRAPHY` for which to compute the mean center.
Return type
`GEOGRAPHY`
Example
SELECT CARTO.CARTO.ST_CENTERMEAN(TO_GEOGRAPHY('POLYGON ((30 10, 40 40, 20 40, 10 20, 30 10))'));
-- { "coordinates": [ 26, 24 ], "type": "Point" }

### ST_CENTERMEDIAN

ST_CENTERMEDIAN(geog)
Description
Takes a FeatureCollection of points and computes the median center. The median center is understood as the point that requires the least total travel from all other points.
• `geog`: `GEOGRAPHY` for which to compute the center.
Return type
`GEOGRAPHY`
Example
SELECT CARTO.CARTO.ST_CENTERMEDIAN(TO_GEOGRAPHY('POLYGON ((30 10, 40 40, 20 40, 10 20, 30 10))'));
-- { "coordinates": [ 25, 27.5 ], "type": "Point" }

### ST_CENTEROFMASS

ST_CENTEROFMASS(geog)
Description
Takes any Feature or a FeatureCollection and returns its center of mass (also known as centroid).
• `geog`: `GEOGRAPHY` feature to be centered.
Return type
`GEOGRAPHY`
Example
SELECT CARTO.CARTO.ST_CENTEROFMASS(TO_GEOGRAPHY('POLYGON ((30 10, 40 40, 20 40, 10 20, 30 10))'));
-- { "coordinates": [ 25.454545454545453, 26.96969696969697 ], "type": "Point" }

### ST_CONCAVEHULL

ST_CONCAVEHULL(geojsons [, maxEdge] [, units])
Description
Takes a set of points and returns a concave hull Polygon or MultiPolygon. In case that a single or a couple of points are passed as input, the function will return that point or a segment respectively.
• `geojsons`: `ARRAY` array of features in GeoJSON format casted to STRING.
• `maxEdge` (optional): `DOUBLE` the length (in 'units') of an edge necessary for part of the hull to become concave. By default `maxEdge` is `infinity`.
• `units` (optional): `STRING` units of length, the supported options are: miles, kilometers, degrees or radians. By default `units` is `kilometers`.
Return type
`GEOGRAPHY`
Examples
SELECT CARTO.CARTO.ST_CONCAVEHULL(
ARRAY_CONSTRUCT(
ST_ASGEOJSON(ST_POINT(-75.833, 39.284))::STRING,
ST_ASGEOJSON(ST_POINT(-75.6, 39.984))::STRING,
ST_ASGEOJSON(ST_POINT(-75.221, 39.125))::STRING,
ST_ASGEOJSON(ST_POINT(-75.521, 39.325))::STRING
)
);
-- { "coordinates": [ [ [ -75.221, 39.125 ], [ -75.833, 39.284 ], [ -75.6, 39.984 ], [ -75.221, 39.125 ] ] ], "type": "Polygon" }
SELECT CARTO.CARTO.ST_CONCAVEHULL(
ARRAY_CONSTRUCT(
ST_ASGEOJSON(ST_POINT(-75.833, 39.284))::STRING,
ST_ASGEOJSON(ST_POINT(-75.6, 39.984))::STRING,
ST_ASGEOJSON(ST_POINT(-75.221, 39.125))::STRING,
ST_ASGEOJSON(ST_POINT(-75.521, 39.325))::STRING
),
100
);
-- { "coordinates": [ [ [ -75.833, 39.284 ], [ -75.6, 39.984 ], ...
SELECT CARTO.CARTO.ST_CONCAVEHULL(
ARRAY_CONSTRUCT(
ST_ASGEOJSON(ST_POINT(-75.833, 39.284))::STRING,
ST_ASGEOJSON(ST_POINT(-75.6, 39.984))::STRING,
ST_ASGEOJSON(ST_POINT(-75.221, 39.125))::STRING,
ST_ASGEOJSON(ST_POINT(-75.521, 39.325))::STRING
),
100,
'kilometers'
);
-- { "coordinates": [ [ [ -75.833, 39.284 ], [ -75.6, 39.984 ], ...
SELECT CARTO.CARTO.ST_CONCAVEHULL(
ARRAY_CONSTRUCT(
ST_ASGEOJSON(
ST_POINT(-75.833, 39.284))::STRING,
ST_ASGEOJSON(ST_POINT(-75.6, 39.984))::STRING
)
);
-- { "coordinates": [ -75.833, 39.284 ], "type": "Point" }
If points are stored in a table, a query like the one below can be used (multiple polygons are generated in this case, one for each `cluster_id` value):
WITH _array AS (
SELECT
cluster_id,
ARRAY_AGG(ST_ASGEOJSON(geom)::STRING) as geomarray
from mytable
group by cluster_id
)
SELECT
CARTO.CARTO.ST_CONCAVEHULL(geomarray) as geom,
cluster_id
from _array

### ST_CONVEXHULL

ST_CONVEXHULL(geog)
Description
Computes the convex hull of the input geography. The convex hull is the smallest convex geography that covers the input. It returns NULL if there is no convex hull.
This is not an aggregate function. To compute the convex hull of a set of geography, use ST_COLLECT to aggregate them into a collection.
• `geog`: `GEOGRAPHY` input to compute the convex hull.
Return type
`GEOGRAPHY`
Examples
SELECT CARTO.CARTO.ST_CONVEXHULL(
TO_GEOGRAPHY('LINESTRING (-3.5938 41.0403, -4.4006 40.3266, -3.14655 40.1193, -3.7205 40.4743)')
);
-- { "coordinates": [ [ [ -3.14655, 40.1193 ], [ -4.4006, 40.3266 ], [ -3.5938, 41.0403 ], [ -3.14655, 40.1193 ] ] ], "type": "Polygon" }
SELECT CARTO.CARTO.ST_CONVEXHULL(ST_COLLECT(geog))
FROM <database>.<schema>.<table>;
warning
The aggregate function ST_COLLECT has an output limit of 16 MB. This is equivalent, approximately, to 300K points.

### ST_DESTINATION

ST_DESTINATION(startPoint, distance, bearing [, units])
Description
Takes a Point and calculates the location of a destination point given a distance in degrees, radians, miles, or kilometers; and a bearing in degrees. This uses the Haversine formula to account for global curvature.
• `origin`: `GEOGRAPHY` starting point.
• `distance`: `DOUBLE` distance from the origin point in the units specified.
• `bearing`: `DOUBLE` counter-clockwise angle from East, ranging from -180 to 180 (e.g. 0 is East, 90 is North, 180 is West, -90 is South).
• `units` (optional): `STRING` units of length, the supported options are: `miles`, `kilometers`, `degrees` or `radians`. If `NULL`the default value `kilometers` is used.
Return type
`GEOGRAPHY`
Examples
SELECT CARTO.CARTO.ST_DESTINATION(ST_POINT(-3.70325,40.4167), 10, 45);
-- { "coordinates": [ -3.6196461743569053, 40.48026145975517 ], "type": "Point" }
SELECT CARTO.CARTO.ST_DESTINATION(ST_POINT(-3.70325,40.4167), 10, 45, 'miles');
-- { "coordinates": [ -3.56862505487045, 40.518962677753585 ], "type": "Point" }

### ST_GREATCIRCLE

ST_GREATCIRCLE(startPoint, endPoint [, npoints])
Description
Calculate great circle routes as LineString or MultiLineString. If the start and end points span the antimeridian, the resulting feature will be split into a MultiLineString.
• `startPoint`: `GEOGRAPHY` source point feature.
• `endPoint`: `GEOGRAPHY` destination point feature.
• `npoints` (optional): `INT` number of points. By default `npoints` is `100`.
Return type
`GEOGRAPHY`
Examples
SELECT CARTO.CARTO.ST_GREATCIRCLE(ST_POINT(-3.70325,40.4167), ST_POINT(-73.9385,40.6643));
-- { "coordinates": [ [ -3.7032499999999993, 40.4167 ], ...
SELECT CARTO.CARTO.ST_GREATCIRCLE(ST_POINT(-3.70325,40.4167), ST_POINT(-73.9385,40.6643), 20);
-- { "coordinates": [ [ -3.7032499999999993, 40.4167 ], ...

### ST_LINE_INTERPOLATE_POINT

ST_LINE_INTERPOLATE_POINT(geog, distance [, units])
Description
Takes a LineString and returns a Point at a specified distance along the line.
• `geog`: `GEOGRAPHY` input line.
• `distance`: `DOUBLE` distance along the line.
• `units` (optional): `STRING` units of length, the supported options are: `miles`, `kilometers`, `degrees` and `radians`. By default `units` is `kilometers`.
Return type
`GEOGRAPHY`
Examples
SELECT CARTO.CARTO.ST_LINE_INTERPOLATE_POINT(TO_GEOGRAPHY('LINESTRING (-76.091308 18.427501,-76.695556 18.729501,-76.552734 19.40443,-74.61914 19.134789,-73.652343 20.07657,-73.157958 20.210656)'), 250);
-- { "coordinates": [ -75.5956489839589, 19.273615818183988 ], "type": "Point" }
SELECT CARTO.CARTO.ST_LINE_INTERPOLATE_POINT(TO_GEOGRAPHY('LINESTRING (-76.091308 18.427501,-76.695556 18.729501,-76.552734 19.40443,-74.61914 19.134789,-73.652343 20.07657,-73.157958 20.210656)'), 250, 'miles');
-- { "coordinates": [ -74.297592068938, 19.449810710315635 ], "type": "Point" }