Introduction to PostGIS

What is PostGIS?

PostGIS is an open-source extension for the PostgreSQL object-relational database that allows the storing, managing, and analysing geographic data.

Allows GIS (Geographic Information Systems) objects to be stored in the database.

It provides support for a wide range of spatial data types and formats, spatial indexing, and spatial functions for querying and analysing spatial data.

With PostGIS, users can build location-based services, perform geospatial analysis, and create maps and visualisations. It is widely used in both commercial and open-source software applications and has a large and active user community.

Spatial Objects for PostgreSQL

1. Adds support for geographic objects and functions to the PostgreSQL object-relational database –

With PostGIS, users can store, query, and manipulate spatial data, such as points, lines, and polygons directly within their PostgreSQL databases. It enables users to perform spatial analysis, generate maps, and develop location-based services by providing a range of spatial functions and operators for querying and analysing spatial data.

PostGIS also includes spatial indexing, which helps to speed up spatial queries, and support for a range of spatial data formats, including shape files and GeoJSON.

2. PostgreSQL already has geometric types but native geometries are too limited for GIS data and analysis-

PostgreSQL has native geometric types such as point, line, and polygon, which can be used to store spatial data. However, these native geometries are limited in their capabilities for Geographic Information System (GIS) data and analysis. For example, native geometries do not support spherical coordinate systems, which are necessary for representing data on the surface of the earth, such as latitude and longitude. Additionally, native geometries do not provide a range of spatial functions and operators required for spatial analysis.

This is where Spatial Objects for PostgreSQL, or PostGIS, comes in. PostGIS extends the native geometries of PostgreSQL by providing support for a wider range of spatial data types, including support for geographic coordinates. It also adds a rich set of spatial functions and operators that allow for complex spatial analysis and queries. With PostGIS, users can perform spatial analysis on complex GIS data, generate maps and visualisations, and develop location-based services.

Why PostGIS?

1. Because databases are better than files-

Firstly, databases offer better data integrity than files. With PostGIS, geospatial data is stored in a database, which has built-in data validation and error checking mechanisms to ensure data integrity. This reduces the likelihood of data corruption or accidental modification.

Secondly, databases are more scalable than files, making them better suited to handle large amounts of geospatial data. PostGIS is designed to manage complex spatial queries and efficiently handle large spatial datasets, providing optimal performance for handling geospatial data.

Thirdly, databases offer more advanced querying and analysis capabilities than file-based systems. PostGIS provides a wide range of spatial functions and operators, which enables users to perform complex spatial queries and analyses that would be difficult or impossible with file-based systems.

Finally, databases are more integrated with other applications and systems, making it easier to share and use geospatial data. PostGIS can be easily integrated with GIS applications, web mapping frameworks, and programming languages like Python. This makes it easier to collaborate with others, share data, and create powerful data-driven applications.

 

2. Unified Storage, Management, Access-

PostGIS enables the storage, management and access of geospatial data in a unified manner. This means that users can store and access all their geospatial data in a single database, making it easier to manage and analyse data.

With PostGIS, users can store different types of geospatial data, including points, lines, and polygons, as well as raster data, in a single database. This allows for better organisation of data, eliminating the need for multiple files or databases.

PostGIS also provides advanced management features, such as spatial indexes and projections, which enable efficient querying and analysis of geospatial data. Users can easily perform spatial queries, such as finding the nearest point or calculating the distance between two objects, and generate advanced spatial analyses.

Another benefit of using PostGIS is that it provides a unified access point for geospatial data. This means that users can access geospatial data using standard SQL queries, making it easier to integrate geospatial data with other data sources and tools. It also enables easy sharing of data between teams and across different applications.

3. Transactional Integrity-

PostGIS provides transactional integrity for geospatial data, ensuring that data is consistent and accurate even when multiple users are accessing and modifying it simultaneously. This is important in applications where the accuracy and consistency of data are critical, such as in real-time mapping and navigation systems.

Transactional integrity is achieved through the use of database transactions, which provide a way to group multiple database operations into a single atomic unit. With PostGIS, users can perform multiple geospatial operations within a single transaction, ensuring that all operations are either completed successfully or rolled back in the event of a failure.

Furthermore, PostGIS provides support for multi-version concurrency control (MVCC), which allows multiple users to access and modify data simultaneously without interfering with each other. With MVCC, each transaction sees a snapshot of the database as it existed at the start of the transaction, ensuring that all operations are performed on the same consistent data.

PostGIS also provides support for transaction isolation levels, which determine how transactions interact with each other and the level of data consistency that is required. This allows users to choose the appropriate level of isolation for their specific application needs.

Simple spatial geometries in SQL

• POINT and MULTIPOINT-

In SQL, POINT and MULTIPOINT are two simple spatial geometries that can be used to represent a single point or a collection of points, respectively.

A POINT is a simple geometry that represents a single point in space, defined by its X, Y, and optional Z coordinates.

A MULTIPOINT is a collection of points, represented as a single geometry. It is useful for storing and manipulating multiple points as a single entity.

To perform operations on POINT and MULTIPOINT geometries in SQL, there are several functions available, such as ST_AsText to convert a geometry to its Well-Known Text (WKT) representation, and ST_Equals to check if two geometries are equal.

          Insert into points values (‘Point(0 0)’, ‘Origin’);

• LINESTRING-

In SQL, LINESTRING is a simple spatial geometry that represents a line segment, defined by a sequence of two or more points connected by straight lines. It is often used to represent roads, rivers or other linear features.

Once a LINESTRING is created, various operations can be performed on it using SQL functions. For example, ST_Length can be used to calculate the length of a LINESTRING, while ST_Distance can be used to calculate the distance between two LINESTRINGs.

LINESTRINGs can also be combined with other spatial geometries, such as POINTs or POLYGONs, to create more complex geometries. For example, a LINESTRING can be used as the boundary of a POLYGON to create a shape that represents a region.

          Insert into points values (‘Point(0 0)’, ‘Origin’);

• MULTILINESTRING–

In SQL, MULTILINESTRING is a simple spatial geometry that represents a collection of LINESTRINGs, which are themselves a sequence of two or more points connected by straight lines. MULTILINESTRING is useful for storing and manipulating multiple LINESTRINGs as a single entity.

Once a MULTILINESTRING is created, various operations can be performed on it using SQL functions. For example, ST_Length can be used to calculate the length of a MULTILINESTRING, while ST_Distance can be used to calculate the distance between two MULTILINESTRINGs.

MULTILINESTRINGs can also be combined with other spatial geometries, such as POINTs or POLYGONs, to create more complex geometries. For example, a MULTILINESTRING can be used to represent the boundaries of a POLYGON with multiple edges.

• POLYGON and MULTIPOLYGON-

In SQL, POLYGON is a simple spatial geometry that represents a closed shape with straight edges, defined by a sequence of points that are connected to form a closed loop. It is often used to represent land parcels, buildings, and other bounded areas.A MULTI-POLYGON is a collection of POLYGONs. It is often used to represent complex shapes with multiple non-contiguous parts.

Once a POLYGON or MULTIPOLYGON is created, various operations can be performed on it using SQL functions. For example, ST_Area can be used to calculate the area of a POLYGON or MULTIPOLYGON, while ST_Intersection can be used to find the overlapping region between two POLYGONs.

 

• GEOMETRY COLLECTION

In SQL, a GEOMETRY COLLECTION is a simple spatial geometry that contains a collection of other spatial geometries such as POINTs, LINESTRINGs, POLYGONs, and other GEOMETRY COLLECTIONs. It can be used to represent complex geometries that consist of multiple types of spatial features.Once a GEOMETRY COLLECTION is created, various operations can be performed on it using SQL functions. For example, ST_Union can be used to combine the spatial features within a GEOMETRY COLLECTION into a single geometry, while ST_GeometryN can be used to retrieve a specific geometry within the collection.

GEOMETRY COLLECTIONs can also be combined with other spatial geometries to create more complex shapes. For example, a GEOMETRY COLLECTION can be used to represent a building with multiple floors, where each floor is represented as a separate POLYGON.

 

a heterogeneous collection of any geometry (including other collections)

 (POINT, POLYGON)

 

 

Conclusion

In conclusion, PostGIS is a powerful open-source spatial database extension for PostgreSQL that enables users to store and manage geospatial data efficiently. With PostGIS, users can perform advanced spatial operations such as spatial indexing, querying, and analysis, making it an excellent tool for GIS professionals, developers, and researchers. In this document, we have introduced the basic concepts and features of PostGIS, including its spatial geometries and capabilities. We have also discussed why we prefer PostGIS because of its flexibility and versatile solution for geospatial data management. By using PostGIS, users can unlock the full potential of their geospatial data and take their spatial analysis to the next level.