Averaging methods
Two averaging methods are used for calculating a mean diameter and a mean mass if more than one value is given: the conventional weighted average method (w.avg) and the "Expected Value Method (EVM)".
The w.avg use weights w = 1/err^2, where err is the (symmetric) 1-σ error of the value. Weighted averages are easily biased by values which have underestimated (low) errors because they
got strong weights. The EVM is more robust concerning outliers, can handle (in some way) asymmetric errors and usually gives more conservative (i.e. larger and maybe also more realistic) errors than the weighted average method.
For an overview and comparison of these and more averaging algorithms see Singh & Birch (2016) and
Birch & Singh (2014).
Asymmetric errors While the majority of reported error estimates are symmetric (value ± error), sometimes asymmetric uncertainties (err_upper, err_lower or err_plus, err_minus) for the diameter and / or mass estimate are provided, usually this comes from using parameter estimating methods which do not a-priori assume a symmetric (or even Gaussian) distribution, like for example Monte-Carlo ML estimation. SiMDA stores and displays asymmetric errors as well. Nevertheless currently for any (diameter / mass) estimate with asymmetric errors, which is used for averaging, a symmetric 'mean' error = (err_upper + error_lower) / 2 is taken instead. The mathematical correct handling of asymmetric errors (in averaging and error propagation) will be adressed in a future version of the system. The Catalog The catalog of diameter, mass and density 'best values' is computed on the fly (thus it takes a moment). For each object an average diameter and mass is computed from all archived estimates. Which estimate is used and which not, is manually set (and maintained) in the DB (Note 2 in the diameter / mass estimate table of the object). Data from Ref = M123 (Goffin, 2014) Four asteroids in Table5.dat (stored in the CDS) gives a mass error of 0.000 (x 10E-10 Solar Masses), probably due to the truncation to 3 digits after the decimal. This values are changed abitraryly to 0.001E-10. The objects are: (11) Parthenope, (17) Thetis, (50) Virginia, (110) Lydia. |
Updates
DB version: 2023/06/28, 23:28:34 01.03.2020: First initial working version, containing data from Carry (2012) plus some more. 31.08.2020: Storage and handling of asymmetric errors added, but yet not used in calculation. 15.09.2020: First (v1.0) SiMDA catalog release on Zenodo (DOI). 26.09.2020: Indication if object is a Gaia mass determination target (new column 'G' in catalog) Issues TBD ... Acknowledgements Python + Packages, Django, Plotly, Matplotlib, Numpy, Pandas, SQLite, Astropy, NASA/JPL PDS, CDS Strasbourg, MPC, Arxiv, NASA/SAO ADS, Sci-Hub, Google + Scholar, Zenodo, ... Links (Home) of my personal Astro-Website |