Key concepts
=============

.. admonition:: **Check your understanding**

    Before diving into this week's Python lesson, you should already be familiar with some basic
    spatial data file formats and projection definitions, such as these:

    - Shapefile
    - GeoPackage
    - CRS
    - Datum
    - EPSG

.. admonition:: **Definitions**

    **Shapefile:** a vector data format for storing location information and related attributes.
    A shapefile consist of several files with a common prefix that need to be stored in the same directory.
    `.shp`, `shx` and `.dbf` are required file extensions in a shapefile. Other file extensions are not required,
    but for example the file extension `.prj` is often essential. More information about Shapefile file extensions
    in `here <http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/Shapefile_file_extensions/005600000003000000/)>`__.
    The shapefile format is developed by ESRI.

    **GeoPackage:** an open source format for storing and transferring geospatial information.
    GeoPackages are able to store both vector data and raster data. In more detail, GeoPackage is a container for
    an SQLite database with a `.gpkg` extension (all in one file!). The GeoPackage format is governed by the Open GeoSpatial Consortium.
    More information at: https://www.geopackage.org/

    **CRS:** Coordinate reference systems define how coordinates relate to real locations on the Earth.
    *Geographic coordinate reference systems* commonly use latitude and longitude degrees.
    *Projected coordinate reference systems* use  x and y coordinates to represent locations on a flat surface.
    You will learn more about coordinate reference systems during this lesson!

    **Datum:** defines the center point, orientation, and scale of the reference surface related to a coordinate reference system.
    Same coordinates can relate to different locations depending on the Datum! For example, WGS84 is a widely used global datum.
    ETRS89 is a datum used in Europe. Coordinate reference systems are often named based on the datum used.

    **EPSG:** EPSG codes refer to specific reference systems.
    EPSG stands for "European Petroleum Survey Group" that originally published a database for spatial reference systems.
    For example, `EPSG:3067 <https://spatialreference.org/ref/epsg/3067/>`__ refers to coordinate reference system ETRS-TM35FIN which is commonly used in Finland.
    `EPSG:4326 <https://spatialreference.org/ref/epsg/4326/>`__ refers to WGS84. You can search for EPSG codes at: https://spatialreference.org/