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https://github.com/mapbox/postgis-vt-util

postgres helper functions for making vector tiles
https://github.com/mapbox/postgis-vt-util

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postgres helper functions for making vector tiles

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README 

        
PostGIS Vector Tile Utils [![Build Status](https://travis-ci.org/mapbox/postgis-vt-util.svg?branch=master)](https://travis-ci.org/mapbox/postgis-vt-util)

=========================



A set of PostgreSQL functions that are useful when creating vector tile sources,

either at the query stage in [Mapbox Studio][1] or in the earlier data

preparation stages.



[1]: http://mapbox.com/mapbox-studio



Installation

------------



Everything you need is in `postgis-vt-util.sql`. This project is also

available [as an NPM module][2] if that's useful to your workflow.



[2]: https://www.npmjs.com/package/@mapbox/postgis-vt-util



Load the file into your database with `psql` or whatever your usual method is.

Eg:



    psql -U  -d  -f postgis-vt-util.sql



Function Reference

------------------



### Bounds ###



Returns an array of the bounding coordinates of the input geometry -

`{xmin, ymin, xmax, ymax}`. Useful for interfacing with software outside of

PostGIS, among other things.



If an SRID is specified the output will be the bounds of the reprojected

geometry, not a reprojected bounding box.



__Parameters:__



- `geometry` g - Any geometry

- `integer` srid (optional) - The desired output SRID of the bounds, if

  different from the input.



__Returns:__ `float[]` - an array of 4 floats, `{xmin, ymin, xmax, ymax}`



### CleanInt ###



Returns the input text as an integer if possible, otherwise null.



__Parameters:__



- `text` i - Text that you would like as an integer.



__Returns:__ `integer`



### CleanNumeric ###



Returns the input text as an numeric if possible, otherwise null.



__Parameters:__



- `text` i - Text that you would like as an numeric.



__Returns:__ `numeric`



### LabelGrid ###



Returns a "hash" of a geometry's position on a specified grid to use in a GROUP

BY clause. Useful for limiting the density of points or calculating a localized

importance ranking.



This function is most useful on point geometries intended for label placement

(eg points of interest) but will accept any geometry type. It is usually used

as part of either a `DISTINCT ON` expression or a `rank()` window function.



__Parameters:__



- `geometry` g - A geometry.

- `numeric` grid_size - The cell size of the desired grouping grid.



__Returns:__ `text` - A text representation of the labelgrid cell



__Example Mapbox Studio query:__



```sql

(   SELECT * FROM (

        SELECT DISTINCT ON (LabelGrid(geom, 64*!pixel_width!)) * FROM (

            SELECT id, name, class, population, geom FROM city_points

            WHERE geom && !bbox!

        ) AS raw

        ORDER BY LabelGrid(geom, 64*!pixel_width!), population DESC, id

    ) AS filtered

    ORDER BY population DESC, id

) AS final

```



### LargestPart ###



Returns the largest single part of a multigeometry.



- Given a multipolygon or a geometrycollection containing at least one polygon,

  this function will return the single polygon with the largest area.

- Given a multilinestring or a geometrycollection containing at least one

  linestring and no polygons, this function will return the single linestring

  with the longest length.

- Given a single point, line, or polygon, the original geometry will be

  returned.

- Given any other geometry type the result of `ST_GeometryN(, 1)` will be

  returned. (See the documentation for that function.)



__Parameters:__



- `geometry` g - A geometry.



__Returns:__ `geometry` - The largest single part of the input geometry.



### LineLabel ###



This function tries to estimate whether a line geometry would be long enough to

have the given text placed along it at the specified scale.



It is useful in vector tile queries to filter short lines from zoom levels

where they would be unlikely to have text places on them anyway.



__Parameters:__



- `numeric` zoom - The Web Mercator zoom level you are considering.

- `text` label - The label text that you will be placing along the line.

- `geometry(linestring)` g - A line geometry.



__Returns:__ `boolean`



### MakeArc ###



Creates a CircularString arc based on 3 input points.



__Parameters:__



- `geometry(point)` p0 - The starting point of the arc.

- `geometry(point)` p1 - A point along the path of the arc.

- `geometry(point)` p2 - The end point of the arc.

- `integer` srid (optional) - Sets the SRID of the output geometry. Useful

  when input points have no SRID. If not specified the SRID of the first

  input geometry will be assigned to the output.



__Returns:__ `geometry(linestring)`



__Examples:__



```sql

SELECT MakeArc(

    ST_MakePoint(-100, 0),

    ST_MakePoint(0, 100),

    ST_MakePoint(100, 0),

    3857

);

```



### MercBuffer ###



Wraps ST_Buffer to adjust the buffer distance by latitude in order to

approximate real-world measurements. Assumes input geometries are Web Mercator

and input distances are real-world meters. Accuracy decreases for larger buffer

distances and at extreme latitudes.



__Parameters:__



- `geometry` g - A geometry to buffer.

- `numeric` distance - The distance you would like to buffer, in real-world

  meters.



__Returns:__ `geometry`



### MercDWithin ###



Wrapper for ST_DWithin that adjusts distance by latitude to approximate real-

world measurements. Assumes input geometries are Web Mercator and input

distances are real-world meters. Accuracy decreases for larger distances and at

extreme latitudes.



__Parameters:__



- `geometry` g1 - The first geometry.

- `geometry` g2 - The second geometry.

- `numeric` distance - The maximum distance to check against



__Returns:__ `boolean`



### MercLength ###



Wrapper for ST_Length that adjusts distance by latitude to approximate real-

world measurements. Assumes input geometries are Web Mercator.  Accuracy

decreases for larger y-axis ranges of the input.



__Parameters:__



- `geometry` g - A (multi)linestring geometry.



__Returns:__ `numeric`



### OrientedEnvelope ###



Returns an oriented minimum-bounding rectangle for a geometry.



__Parameters:__



- `geometry` g - A geometry.



__Returns:__ `geometry(polygon)`



### Sieve ###



Filters small rings (both inner and outer) from a multipolygon based on area.



__Parameters:__



- `geometry` g - A multipolygon

- `float` area_threshold - the minimum ring area to keep.



__Returns:__ `geometry` - a polygon or multipolygon



### SmartShrink ###



Buffers a polygon progressively (on an exponential scale) until the

area of the result hits a certain threshold ratio to the original area.

The result is also simplified with a tolerance matching the inset

distance.



__Parameters:__



- `geometry` g - A (multi)polygon.

- `float` ratio - The threshold for how much smaller (by area) you want

  the shrunk polygon to be compared to the original. Eg a value of 0.6

  would result in a polygon that is at least 60% as large as the input.

- `boolean` simplify - Defaults to false. Whether or not you would

  like the shrunk geometry simplified.



__Returns:__ `geometry`



### TileBBox ###



Given a Web Mercator tile ID as (z, x, y), returns a bounding-box

geometry of the area covered by that tile.



__Parameters:__



- `integer` z - A tile zoom level.

- `integer` x - A tile x-position.

- `integer` y - A tile y-position.

- `integer` srid - SRID of the desired target projection of the bounding

  box. Defaults to 3857 (Web Mercator).



__Returns:__ `geometry(polygon)`



### ToPoint ###



Helper to wrap ST_PointOnSurface, ST_MakeValid. This is needed because

of a ST_PointOnSurface bug in geos < 3.3.8 where POLYGON EMPTY can pass

through as a polygon geometry.

If the input geometry is a polygon with less than 5 points the ST_Centroid

of the polygon will be used instead of ST_PointOnSurface to speed up calculation.



__Parameters:__



- `geometry` g - A geometry.



__Returns:__ `geometry(point)`



__Example:__



```sql

-- Create an additional point geometry colums for labeling

ALTER TABLE city_park ADD COLUMN geom_label geometry(point);

UPDATE city_park SET geom_label = ToPoint(geom);

```



### ZRES ###



Takes a web mercator zoom level and returns the pixel resolution for that

scale, assuming 256x256 pixel tiles. Non-integer zoom levels are accepted.



__Parameters:__



- `integer` or `float` z - A Web Mercator zoom level.



__Returns:__ `float`



__Examples:__



```sql

-- Delete all polygons smaller than 1px square at zoom level 10

DELETE FROM water_polygons WHERE sqrt(ST_Area(geom)) < ZRes(10);



-- Simplify geometries to a resolution appropriate for zoom level 10

UPDATE water_polygons SET geom = ST_Simplify(geom, ZRes(10));

```



### Z ###



Returns a Web Mercator integer zoom level given a scale denominator.



Useful with Mapnik's !scale_denominator! token in vector tile source

queries.



__Parameters:__



- `numeric` scale_denominator - The denominator of the scale, eg `250000`

  for a 1:250,000 scale.



__Returns:__ `integer`



__Example Mapbox Studio query:__



```sql

( SELECT * FROM roads

  WHERE Z(!scale_denominator!) >= 12

) AS data

```