Although the Planetary DWG has created this list of IAU CRSs, they need to be registered with the OGC’s Naming Authority (OGC-NA). The OGC has recently proxied an online planetary CRS registry hosted by the National Centre for Space Studies (CNES) in January 2023. Scripts to create this set are also hosted on GitHub (github.com/pdssp/csvforwkt). The OGC-NA is used by libraries like OSGeo’s PROJ to validate and retrieve CRS definitions (github.com/OSGeo/PROJ). Now the IAU CRSs are in equal standing to those from other authorities like EPSG.
An issue identified early was the lack of standardized names for coordinate reference systems (CRSs) for planetary bodies. For Earth, CRSs such as EPSG:4326 and EPSG:3785 exist, are well known, and natively available in most GIS software. While the International Astronomical Union (IAU) defines spheroids and ellipsoids for planetary bodies, well known identifiers have not been adopted by the community. Fortunately, a prior effort by the planetary community and now the OGC Planetary domain working group (DWG) have created a set of over 4000 CRSs available at voparis-vespa-crs.obspm.fr:8080/web for all planetary bodies with a consistent naming convention. The creation of this list of CRSs enables the adoption of their names as a standard against which other GIS software can be tested.
The past two decades have seen the proliferation of Earth-centric GIS standards facilitated by the OGC all with the aim of making data easier to access. Planetary science as a field has been slow to adopt these standards due to complexities inherent to updating these standards to work for extraterrestrial bodies. Fortunately, recent standards have been modified to now allow for a broader planetary use. But, work remains to make sure that popular software applications fully support Planetary use-cases enabled by these newer standards.
In late 2022, an organic discussion was held between members of the OpenPlanetary organization to identify problems in existing software and standards that could be solved efficiently.
Filling in the Gaps in Maps:
Embracing Open GIS Standards
In Planetary Science.
A. M. Annex1, T. M. Hare2, J. R. Laura2, N. Manaud3, J.-C. Malapert4, B. Seignovert5 1Division of Geological and Planetary Sciences, California Institute of Technology, MC 150-21, 1200 E. California Blvd., Pasadena, CA 91125, USA (annex@caltech.edu), 2USGS Astrogeology Science Center, Flagstaff, AZ, 3SpaceFrog Design, Toulouse, FR, 4CNES, Toulouse, FR, 5OSUNA/CNRS-INSU, Nantes, FR.
Intro
IAU CRSs
Naming Authority Updates
TileMatrixSets
RGB Terrain Quantizers
Cloud Optimized Geotiffs (COGs)
Spatio-Temporal Asset Catalog (STAC)
Next steps
Conclusions
We are contributing to open-source GIS software to better support planetary science data.
The OGC Tile Matrix Set (TMS) standard defines a way to specify regular grids for various CRSs and map projections that clients and servers can use. The morecantile project (github.com/developmentseed/morecantile) was used and patched twice as part of this effort in order to produce a preliminary set of 551 TMS specs from the IAU CRSs which includes Mercury, the Moon, Venus, Mars and numerous moons with equatorial and polar map projections. Work remains to resolve issues with generating TMS specs for a subset of bodies including Pluto. The TMS specifications that were made are freely available at github.com/AndrewAnnex/planetcantile.
Elevation data intended for use in WebGIS clients are typically provided as PNG formatted image tiles with the data quantized lossily from 32 bits to 24 bits to make efficient use of bandwidth. We identified that a quantizer for terrain data is defined only by two parameters: the minimum elevation to encode and the precision desired. We defined a set of quantizers for all inner planets and the Moon based on their elevation ranges with respect to their geoids with precisions finer than 2 mm are available at github.com/AndrewAnnex/topography-quantizer-spec.
A universal quantizer for rocky bodies can represent all topography from -12000 meters in the inner solar system with 2.04682 mm vertical precision.
A cloud optimized GeoTiff (COG) is an extended GeoTiff that allows for HTTP GetRange requests on some or all of the underlying data (cogeo.org). This is accomplished by placing additional information into the GeoTIFF header (gdal.org/drivers/raster/cog.html) and by making use of pyramid overviews whereby GET requests access and stream lower resolution representations of the data when appropriate (e.g., when the client is zoomed out). COGs are beneficial because of their ability to be efficiently tiled, compressed, and streamed over the web.
The Spatio-Temporal Asset Catalog (STAC) is an open standard for defining and distributing collections of geospatial data (typically COGs) and metadata so that these data can be indexed and searched easily (stacspec.org/) enabling data discovery. Extensions to STAC can be written by the community to add new functionality, and a Solar System Extension Specification (SSYS) has already been created to add planetary bodies as a target field to STAC catalogs (github.com/thareUSGS/ssys).
Titiler
TiTiler is an open source python framework for creating dynamic tile servers (developmentseed.org/titiler/). TiTiler acts as a backend server for STAC catalogs, Cloud Optimized GeoTiffs (COGs), Web GIS frameworks and clients, and can serve data to clients using the TMS specifications from Planetcantile. Dynamic tile servers are beneficial as they help minimize maintenance costs by adopting the serverless computing paradigm with minimal configuration. Screenshots on the poster were generated in part by a TiTiler server as well.
A number of front ends have already been built around Titiler including the mars-tiler project (github.com/davenquinn/mars
-tiler).
To support the planetary community, the USGS ASC now hosts a publicly accessible and free-to-use Titiler endpoint available at astrogeology.usgs.gov/titiler. This endpoint includes support for the IAU TMSs from Planetcantile to enable users of planetary data.
Adding support for triaxial ellipsoids within PROJ would enable more accurate distance and area calculations, and would allow additional TMS specification for oblate icy moons and other small bodies. The creation of a community curated list of STAC catalogs and collections would enable scientists to quickly find and contribute data that is potentially analysis-ready across a suite of GIS tools.
Vector data is the next area of consideration for this effort. Tiled vector data formats exist but conventionally only support Web Mercator projections, although in principle, could be adapted to use TileMatrixSet indexes and other projections.
3D-tiles, terrain data encoded in quantized mesh format is also an area for further development.
These improvements also make it possible to consider additional future changes to the ecosystem of popular web-based visualization software to better support planetary data. These include a number of open source javascript libraries like Deck.gl, Maplibre, Leaflet, Openlayers, Proj4js, CesiumJS, and geotiff.js.
This work describes the current state of the cloud-based planetary data ecosystem, identifies key components of the ecosystem defining a software stack for cloud native PSDI, and describes initial work to ensure that planetary data are well supported. One can track a path through mutually supporting standards to find a place where cloud enabled planetary spatial data simply work in off-the-shelf geospatial software.
Foundations
Front Ends
User �Interfaces
Clients & APIs
TMS
OGC Features
Serverless
Backend
PSDI Software Stack
github.com/AndrewAnnex/planetcantile
Mars MOLA color shaded relief overlying MOLA topography encoded using RGB terrain quantizers
Fergason et al 2020 HiRISE ortho map overlying Mangold et al 2021. CTX map (semi transparent) overlying MOLA Global topography colorshaded and hillshaded using 49900 TMS and Titiler.
Mangold et al 2021. CTX DEM quantized to RGB overlying MOLA Global topography colorshaded using 49900 TMS and Titiler.
github.com/AndrewAnnex/topography-quantizer-spec
Basemap of Europa Voyager - Galileo SSI Global Mosaic 500m v2 (astrogeology.usgs.gov/search/map/Europa/Voyager-Galileo/Europa_Voyager_GalileoSSI_global_mosaic_500m.cub) overlain by Galileo SSI image G7ESTYRMAC02 obtained from the USGS STAC catalog https://stac.astrogeology.usgs.gov/browser-dev/#/collections/galileo_usgs_photogrammetrically_controlled_mosaics/items/G7ESTYRMAC02_GalileoSSI_Equi using the 50200 TMS spec and Titiler
astrogeology.usgs.gov/titiler
Mangold et al 2021. CTX map underlying False Color CRISM MTRDR (Seelos et al. 2016) HRL000040FF generated from summary parameters R2529, R1506, and R1080 dynamically from float32 data (Viviano-Beck et al. 2014).