Meet some popular open geospatial standards!

Some inspiration for this post came from the beautiful books of Lovelace et al. (2020), Pebesma & Bivand (2019) and Hijmans (2019), and from various websites.

Why use open standards?

• Open file standards ease collaboration, portability and compatibility between users, machines and applications.
• Their (file) structure is fully documented.
  • Consequently, scientists and programmers can build new software / packages and make innovations that use these standards, while maintaining interoperability with existing applications.
  • And, there is a much higher chance that your data will still be readable in a hundred years from now. The standard’s open documentation makes it relatively easy to build tools that can read an ancient open-standard file!

Luckily, quite a list of open standards is available! Below, some powerful and widely-used single-file formats are introduced. Single-file data sources are readily amenable to exchange and publication.

I see you can’t wait to start practicing, so you can also head straight over to the tutorial on vector formats and the tutorial on raster formats! In these tutorials, a comparison table of vector/raster file formats is also presented.

A few words on the GDAL library

GDAL (Geospatial Data Abstraction Library) is by far the most used collection of open-source drivers for:

• a lot of raster formats;
• a lot of vector formats.

In other words, it is the preferred workhorse for reading and writing many geospatial file formats, used in the background by a lot of geospatial applications . Using GDAL is the easiest way to conform to open standards.

So, in R we use packages that use GDAL in the background, such as rgdal, sp, sf, raster and stars.

The GeoPackage file format

• Its website is https://www.geopackage.org.
• It is a standardized implementation of an SQLite database for geospatial data. Hence, a GeoPackage is a binary file (filename.gpkg). It shares this property with shapefiles, which however pose multiple limitations,¹ so the GeoPackage is a more than suitable replacement.
• The GeoPackage can store one or multiple vector layers (points, lines, polygons and related feature types). Besides vector data, it can also store raster data or extra standalone tables. These properties make it somehow comparable to the ‘personal geodatabase’ of ArcGIS – ESRI’s closed, Windows-only format.²
• The GeoPackage standard is maintained by the Open Geospatial Consortium (OGC), which stands out as a reference when it comes to open geospatial standards.

The GeoJSON file format

• One GeoJSON file (filename.geojson) contains one vector layer. Note that one vector layer can combine different feature geometry types, e.g. points and linestrings. JSON itself is a common and straightforward open data format. It is a text file readable both by humans and machines (see the tutorial for an example). GeoJSON adds the necessary specification to JSON for standardized storage of geographic feature data, but it is still a plain JSON text file.
• The GeoJSON standard is maintained by the Internet Engineering Task Force (IETF), a large open standards organization that develops Internet standards under the auspices of the Internet Society.
• Although the previous version of the GeoJSON standard – GeoJSON 2008 – is still a lot in use, it is obsoleted and a new version RFC7946 is establishing.
  • This version is strict about the coordinate reference system (CRS) – it is always WGS 84 – and it also differs on a few other aspects (such as the recommendation for applications not to inflate decimal coordinate precision).
  • RFC7946 solves the problem that quite a few libraries – including GDAL – simply assumed WGS 84 in GeoJSON 2008 (without checking or transforming), even though WGS 84 was not a requirement of GeoJSON 2008 (it did support an explicit crs declaration). This resulted in inconveniences (cf. this post in the sf-repository).
  • A specific section in the documentation of GDAL’s GeoJSON driver gives a summary of the differences between both GeoJSON versions.
• While GDAL by default still follows the GeoJSON 2008 format,³ RFC7946 is supported by the option RFC7946=YES. Here, reprojection to WGS 84 will happen automatically. It applies 7 decimal places for coordinates, i.e. approximately 1 cm. Given the advantages, we advise to explicitly use RFC7946. Several functions in R allow the user to provide options that are passed to GDAL, so we can ask to deliver RFC7946 (see the tutorial).
• In order to keep it manageable (text file size, usage in versioning systems⁴ ) it can be wise to use GeoJSON for more simple cases (points and rather simple lines and polygons), and use the binary GeoPackage format for larger (more complex) cases.

The GeoTIFF file format

• GeoTIFF is the preferred single-file open standard for raster data. It adheres to the open TIFF specification; hence it is a TIFF image file (filename.tif). It uses a small set of reserved TIFF tags to store information about CRS, extent and resolution of the raster.
• A GeoTIFF file can contain one or multiple rasters with the same CRS, extent and resolution.
• The GeoTIFF standard is maintained by the Open Geospatial Consortium (OGC), which stands out as a reference when it comes to open geospatial standards.

Literature