# Coordinate reference systems¶

Coordinate reference systems describe how the numerical coordinates of a geometry or grid relate to space in the real world. Difference coordinate systems take different approaches to reducing position on the curved surface of the earth to two-dimensional coordinates, making different assumptions about Earth’s geometry and choosing to preserve different invariants.

In Karta, coordinate systems objects handle both projection from geographical coordinates and geodetic operations such as calculating distances and azimuths. Basic coordinate systems (Cartesian and SphericalEarth) are provided, while more specialized systems employ pyproj to handle calculations.

The following predefined coordinate systems instances are available:

• SphericalEarth
• LonLatWGS84
• UPSNorth
• UPSSouth
• NSIDCNorth
• NSIDCSouth
• LambertEqualArea
• GallPetersEqualArea
• WebMercator

Additionally, the Cartesian class can be used without instantiation.

To use a pre-defined coordinate system,

from karta.crs import LonLatWGS84


The object LonLatWGS84 can be passed to Geometry and Grid instances in Karta. To define their coordinate system.

To create a specialized coordinate system, provide a proj.4 string and a spheroid definition.

from karta.crs import ProjectedCRS
my_crs = ProjectedCRS("+proj=aea +lat_1=50 +lat_2=58.5 +lat_0=45 "
"+lon_0=-126 +x_0=1000000 +y_0=0 "