Scott, That spherical value of 470 km, as defined in ESRI's projection, was intended. We are still working with the IAU to update these rules, but when that projection was defined, we used this rule:
If the IAU defined a "mean" radius for a body (see Table 6.0 page 33<https://astropedia.astrogeology.usgs.gov/download/Docs/WGCCRE/WGCCRE2015reprint.pdf#page=33> of the report), we default to that value as a sphere. This value was provided to the IAU by Thomas Roatsch (DLR) from the Dawn Team. The Dawn team, which produced the control mosaic for Ceres, also used 470 km as a sphere for the final mosaic and digital terrain model ( DTM<https://astrogeology.usgs.gov/search/map/ceres_dawn_fc2_hamo_global_dtm_137m>): reference: https://doi.org/10.1016/j.pss.2016.05.011 quote, "Ceres' shape model was then used for the calculation of the ray intersection points, while the map projection itself was placed onto a reference sphere of Ceres with a mean radius of 470 km." image mosaic: https://astrogeology.usgs.gov/search/map/ceres_dawn_fc_global_mosaic_140m While it might seem strange to use a sphere, this is common for irregular shaped bodies. This is mostly true for bodies defined as a triaxial since mapping applications do not commonly support triaxial map projections. The IAU defines these mean radii to help support interoperability across applications. We are working with the IAU and could update some of these rules. For example, I will say it is odd to see that Ceres is NOT defined as a triaxial, but the Dawn Team and the IAU promote a "mean" radius. I'm sure we will revisit this in the future. BTW, it doesn't mean you can't define your own projection for Ceres if you need to. But I would recommend matching the current best-of image mosaic and DTM. I hope that helps, Trent ________________________________ From: Newcomb, Doug <doug_newc...@fws.gov> Sent: Sunday, December 22, 2024 8:12 PM To: Reynolds, Scott <sc...@agi.com> Cc: Hare, Trent M <th...@usgs.gov> Subject: Re: [EXTERNAL] [gdal-dev] Shape update/addition for Ceres check with Trent Hare at USGS ________________________________ From: gdal-dev <gdal-dev-boun...@lists.osgeo.org> on behalf of Reynolds, Scott via gdal-dev <gdal-dev@lists.osgeo.org> Sent: Saturday, December 21, 2024 9:17 AM To: gdal-dev@lists.osgeo.org <gdal-dev@lists.osgeo.org> Subject: [EXTERNAL] [gdal-dev] Shape update/addition for Ceres This email has been received from outside of DOI - Use caution before clicking on links, opening attachments, or responding. As a user of GDAL in some of our products I’m trying to find a contact, probably/possibly at Esri, that could add a new entry for Ceres that uses the oblate parameters defined here<https://astropedia.astrogeology.usgs.gov/download/Docs/WGCCRE/WGCCRE2015reprint.pdf> (487.3km, 446km). ESRI:104972 exists for the spherical definition (470 km). Thanks, Scott Disclaimer The information contained in this electronic message and any attachments to this message are intended only for the individual(s) addressed in the message and may contain proprietary and confidential information. If you are not the intended recipient, you should not disseminate, distribute or copy this e-mail. Please notify the sender and delete this message. WARNING: Computer viruses can be transmitted via email. The recipient should scan this email before opening it. The company accepts no liability for any damage caused by any virus transmitted by this email.
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