Undersampling fraction (the fraction of SOM cells containing fewer than five galaxies; purple) and quantization error (the average Δ in Eq. (1); orange) as a function of SOM size, given a limited data volume of 7507 galaxies. While the undersampling fraction increases monotonically with size, the quantization error first decreases and then increases, reaching an optimal value at a SOM size of 30 × 30.