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Mastering PostGIS

You're reading from   Mastering PostGIS Modern ways to create, analyze, and implement spatial data

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Product type Paperback
Published in May 2017
Publisher Packt
ISBN-13 9781784391645
Length 328 pages
Edition 1st Edition
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Authors (4):
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Dominik Mikiewicz Dominik Mikiewicz
Author Profile Icon Dominik Mikiewicz
Dominik Mikiewicz
Michal Mackiewicz Michal Mackiewicz
Author Profile Icon Michal Mackiewicz
Michal Mackiewicz
Tomasz Nycz Tomasz Nycz
Author Profile Icon Tomasz Nycz
Tomasz Nycz
George Silva George Silva
Author Profile Icon George Silva
George Silva
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Toc

Table of Contents (10) Chapters Close

Preface 1. Importing Spatial Data FREE CHAPTER 2. Spatial Data Analysis 3. Data Processing - Vector Ops 4. Data Processing - Raster Ops 5. Exporting Spatial Data 6. ETL Using Node.js 7. PostGIS – Creating Simple WebGIS Applications 8. PostGIS Topology 9. pgRouting

Reprojecting geometries


Reprojecting, also called transformation, is a process of converting a geometry's coordinates from one coordinate system to another. Since PostGIS spatial analysis functions can't operate on geometries with different coordinate systems, it's a very important functionality. It's provided by the ST_Transform function.

ST_Transform accepts two arguments: the input geometry and the target SRID. For example, to transform the coordinates of a point feature (city center) to latitude-longitude, we write the following:

SELECT ST_AsText(ST_Transform(wkb_geometry,4326)) FROM points WHERE osm_id = '253525668'; 

st_astext 
POINT(18.5419933 50.0955793) 

While ST_Transform can be used on the fly, it's computationally expensive and can cause a complex query to run very slowly. If it's necessary to run a spatial analysis using tables with different SRIDs, it's will be wise to create a materialized view with geometries reprojected beforehand:

CREATE MATERIALIZED VIEW boundaries_3857 AS...
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