## PostGIS: An Introduction to Spatial Data Management

PostGIS (Geographic Information System) is a computer system designed for storing, analyzing, and manipulating geographical data. One of the most popular applications in GIS technology is PostGIS.

It is an open-source spatial database manager that stores, manages, and manipulates spatial data. If you want to manage location-based data in your applications, PostGIS might be the solution for you.

PostGIS is developed under the PostgreSQL license and can support a wide range of GIS operations. It provides additional functions, indexes, and data types necessary for running geospatial queries and calculations.

### Data Types Added by PostGIS

PostGIS extends the PostgreSQL database with two data types, geography and geometry.

The geometry data type represents the location of objects in a two-dimensional coordinate system. This data type can be used to represent points, lines, polygons, and other geometric shapes.

The geography data type, on the other hand, represents the location of objects on the Earth’s surface. It uses a spherical coordinate system to store information about the latitude and longitude of a point, line, or polygon.

Both data types support the same set of functions and operators. However, geography supports additional operations that are specific to geographic computations like distance and area calculations.

## Spatial Functions Offered by PostGIS

PostGIS provides hundreds of spatial functions that allow you to analyze and manipulate spatial data. Some of the primary functions include:

- ST_Intersects: Tests whether two geometry or geography objects intersect.
- ST_Contains: Determines whether one geometry or geography object contains another.
- ST_DWithin: Determines whether the distance between two geometry or geography objects is within a specified range.
- ST_Area: Computes the area of a geometry or geography polygon.
- ST_Length: Computes the length of a geometry or geography linestring.
- ST_Distance: Computes the distance between two geometry or geography objects.

### Using Geography Data Type

Geography data type can be used to store spatial data as points, lines, polygons, or multi-point, multi-line, or multi-polygon geometries.

When working with geography data type, we use the Well-Known Text (WKT) format to express geographic elements. To create a table with geography data type, we have to define a column of type geography.

For example, let us create a table named “locations.”

```
CREATE TABLE locations (
id SERIAL PRIMARY KEY,
name VARCHAR,
location GEOGRAPHY(Point)
);
```

Once we have created a table with geography data type, we can use the ST_POINT function to populate it with spatial data. For example, let’s insert a point into our “locations” table.

```
INSERT INTO locations (name, location) VALUES ('New York City', ST_POINT(-73.935242, 40.730610));
```

Here, we have created a row with a point location that represents the geographic coordinates of New York City.

## Calculating Distance and Area

PostGIS offers several functions that allow us to calculate distances and areas between geometric elements.

However, the calculation of distance and area differs between geometry and geography data types. Geometry data type represents objects in Euclidean space, while the geography data type considers objects as points on the surface of the Earth.

So, the calculation of distances and areas for geometry data is relatively straightforward, whereas the calculation of distances and areas for geography data needs specialized algorithms to account for the curvature of the Earth. The ST_DISTANCE function returns the distance between two geography data points.

The function takes two geography points as input and returns the distance between them in meters.

```
SELECT ST_DISTANCE(
ST_GeographyFromText('POINT(-73.935242 40.730610)'), -- New York City
ST_GeographyFromText('POINT(77.1025 28.7041)') -- Delhi
) AS distance_in_meters;
```

Here, we have used the ST_DISTANCE function to calculate the distance between New York City and Delhi in meters. The function returns the distance of about 11,053.07 km.

Storing and calculating area for complex shapes using polygons are a common task when working with PostGIS. The area calculation for polygon data types is relatively simple, and you can use the ST_AREA function to calculate the area of a polygon.

```
SELECT ST_AREA(
ST_GeographyFromText(
'POLYGON((
-74.005941 40.712784,
-73.968734 40.720225,
-73.937540 40.738680,
-73.941860 40.807580,
-73.980057 40.781173,
-74.016574 40.752021,
-74.009261 40.704921,
-74.005941 40.712784
))'
)
) as area_in_square_meters;
```

Here, we have calculated the area of a complex polygon that represents the New York City boundary. The ST_GeographyFromText function allows us to create a polygon with lat-long coordinates that the ST_AREA function takes as input and returns the area in square meters.

Joining tables based on spatial conditions can be achieved using the ST_INTERSECTS function. The function returns true if two geometric objects intersect with each other.

For example, suppose we have a table named “cities” with columns “name,” “population,” and “location,” and another table named “states” with columns “name” and “boundary.” To get a list of cities that fall within states, we can use the following query.

```
SELECT cities.name, cities.population, states.name
FROM cities
INNER JOIN states
ON ST_Intersects(cities.location, states.boundary);
```

Here, we have used the ST_INTERSECTS function to join the tables based on the spatial condition that cities location intersects with the state boundaries. This query returns the name, population of cities, and the name of their respective states.

### Interactive PostGIS Course

For those who want to learn PostGIS in a structured and interactive way, the interactive PostGIS course is an excellent resource. This course offers over 140 exercises that allow you to learn PostGIS and geospatial data manipulation practically.

The course has eight chapters, each with several exercises that start with basic spatial queries and gradually increases in complexity. The course covers topics such as spatial functions, geometries and projections, geography data types, and spatial joins.

The course also provides an interactive code editor that allows you to practice writing queries and spatial functions. So, if you are interested in learning PostGIS practically, this interactive course is an excellent resource.

## Conclusion

In summary, PostGIS is an open-source spatial database manager that allows us to store, analyze, and manipulate geographical data. PostGIS offers two data types, geography, and geometry, and hundreds of spatial functions to help us manage spatial data.

Calculating distances and areas in PostGIS differs between geometry and geography data types. PostGIS also offers an interactive course with over 140 exercises for hands-on learning.

With PostGIS, you can streamline your location-based data processing and analysis and make informed decisions. Whether you are building GIS-related web applications, working with online maps, or doing geospatial analysis, PostGIS provides a robust foundation to work with.