Generic selectors
Exact matches only
Search in title
Search in content
Post Type Selectors
podcast
Filter by Categories
ArcGIS Pro
GDAL
GeoJson
Map
Python
QGIS
Uncategorized

# The difference between Geography and Geometry

Before we dive too deep, you might want to check out these resources:

## Geometry Data Type

In SQL, the `GEOMETRY` data type represents data in a two-dimensional Cartesian plane. It can store points, lines, and polygons, as well as more complex shapes like circles and rectangles.

The `GEOGRAPHY` data type is similar to `GEOMETRY`, but it represents data in a round-earth coordinate system. This makes it better suited for storing data that covers large areas or tracks movement across the surface of the earth.

One key difference between the two data types is the way they store and manipulate spatial data. `GEOMETRY` uses a flat Cartesian plane, which can be useful for tasks like measuring distance and performing spatial analysis.

However, the earth is not flat, so `GEOGRAPHY` uses a more complex coordinate system to more accurately represent data on the earth’s surface. This can be helpful when working with data that needs to account for the earth’s curvature.

## The Earth is a sphere

A great circle is a circle on the surface of a sphere that has the same diameter as the sphere and passes through its center. Great circles represent the shortest distance between two points on the surface of the globe.

One example of a great circle is the equator, which is the circle formed by the intersection of the earth’s surface with a plane perpendicular to its axis of rotation. Other examples include the lines of longitude (or meridians), which run from the north pole to the south pole

Using the `GEOGRAPHY` data type in SQL can be an advantage when dealing with distances across a sphere because it uses a round-earth coordinate system that takes into account the curvature of the earth. This can be more accurate than using the `GEOMETRY` data type, which uses a flat Cartesian plane to represent spatial data.

For example, if you want to calculate the distance between two points on the earth’s surface, using the `GEOGRAPHY` data type can give you a more accurate result than using the `GEOMETRY` data type. You can use the `ST_Distance` method to calculate the distance between two `GEOGRAPHY` objects, and it will return the shortest distance between the objects along the surface of the earth.

## Converting Between Geometry and Geography data types

You can convert between `GEOMETRY` and `GEOGRAPHY` data types in SQL. There are a few different ways to do this, depending on the specific requirements of your data and the tools you are using.

One common way to convert between `GEOMETRY` and `GEOGRAPHY` is to use the `ST_GeomFromText` and `ST_GeographyFromText` functions in SQL. These functions allow you to convert a string representation of a geometry or geography object into the corresponding data type.

For example, you could convert a `GEOMETRY` point object to a `GEOGRAPHY` point object like this:

``````SELECT ST_GeographyFromText(ST_AsText(geometry_point_column))
FROM your_table;
``````

You can also use the `ST_GeomFromWKB` and `ST_GeographyFromWKB` functions to convert between `GEOMETRY` and `GEOGRAPHY` data types using well-known binary (WKB) representations of the objects.

It’s also possible to use other functions and methods to convert between `GEOMETRY` and `GEOGRAPHY` data types, depending on the specific tools and libraries you are using. For example, you might be able to use spatial functions provided by your database management system, or you could use a third-party library like `GDAL` (Geospatial Data Abstraction Library) to perform the conversion.

Once `GDAL` is installed, you can use the `ogr2ogr` utility to convert between `GEOMETRY` and `GEOGRAPHY` data types. `ogr2ogr` is a command-line utility that allows you to perform various operations on vector data, including converting between different data formats and spatial reference systems.

To use `ogr2ogr` to convert between `GEOMETRY` and `GEOGRAPHY` data types, you will need to specify the input and output data sources, as well as the desired data type for the output. For example, to convert a `GEOMETRY` column in a PostgreSQL table to a `GEOGRAPHY` column, you could use a command like this:

``````ogr2ogr -f "PostgreSQL" PG:"host=myhost user=myuser password=mypassword dbname=mydb" \
-sql "SELECT ST_GeographyFromText(ST_AsText(geometry_column)) AS geography_column FROM mytable" \
-nln mytable_geography
``````

This command uses `ogr2ogr` to connect to a PostgreSQL database, execute a SQL query that converts the `geometry_column` to `GEOGRAPHY`, and store the result in a new table called `mytable_geography`.