Managing GeoData with PostGIS

Volodymyr Gamula

KhmelnytskyiPy #1

Agenda:• Current situation with GeoData

• What is PostGIS?

• Spatial Data types in PostGIS?

• Trivial problem

• SRID

• Indexing

• Another ways to use PostGIS for

• How to start working with PostGIS now?

• Program interfaces

• Conclusions

GeoData is an information about geographic location,

stored in a specific format that can be used in geographic information systems (GIS)

What is GeoData?

Current situation with GeoData

GeoData is everywhere!

Suggested timeslots

What is PostGIS?

– Paul Ramsey, Chair of PostGIS Steering Committee

“It is a Relational Database System that in addition to text and numbers and dates, it can also index spatial objects - points, lines, and

polygons.”

History

• PostgreSQL: May 1, 1995

• PostGIS: April 19, 2005

• Very well tested

• Massively used in production

Spatial Data types in PostGIS

Point• (x, y, z) coordinates

• Represents locations such as buildings, train stations, etc.

• Also represents GPS latitute and longitude

• POINT(-51.1231325 12.213155)

LineString

• Multiple points connected by straight lines

• Represents things such as roads, routes, cables, etc.

• LINESTRING(30 10, 10 30, 30 40)

Polygon

• Multiple points which encloses an area

• Represents boundaries such as properties, towns, countries.

• POLYGON(30 10, 10 20, 20 40, 40 40, 30 10)

Trivial problem

Distance between two points

2 dimensions:

Geographical distance:

What if we need to recalculate distance for

every request?

Standard formula brings a lot of complexity :(

So, what should we do?

Geohashing

PostGIS has a function ST_Distance

ST_Distance can work with the next types, which are basic PostGIS types:

• Geometry • Geography

We should represent (lat, lng) to PostGIS

Geography field

There are a lot of ways to do it:• 'SRID=4326;POINT(83.106560 54.838971)'::geography

• ST_GeographyFromText('SRID=4326;POINT(-110 30)’)

• ST_GeomFromEWKT('SRID=4269;POINT(-110 30)’);

• ST_GeomFromGeoJSON(‚{"type":"Point","coordinates":[-48.23456,20.12345]}')

• ST_GeomFromGML(' <gml:Point srsName="http://www.opengis.net/def/crs/EPSG/0/4326"> <gml:coordinates>45.67, 88.56</gml:coordinates></gml:Point>’);

• etc.

SRID

Query:

Why will this query be so fast?

Indexes!

Indexes:

• B-Tree

• R-Tree

• GiST (Generalized Search Trees)

GiST Indexing for GeoData with calculations

Another PosGIS calculations:

• Area

• Distance

• Length

• Perimeter

• Union

• Difference

• Symmetric difference

How to start work with PostGIS now?

• Install PostgreSQL

• Install PostGIS

• CREATE EXTENSION postgis;

• CREATE EXTENSION btree_gist;

• ALTER TABLE geodata_test ADD COLUMN location geography(Point, 4326);

• CREATE INDEX ix_geodata_test_geohash ON geodata_test USING GIST(location);

Program interfaces:GeoAlchemy

GeoDjango

Conclusions

Thanks!

@vgamula