This document summarizes work to define citizen observatories and map existing initiatives across Europe. It reviewed literature for a consolidated definition of citizen observatories and frameworks for classifying initiatives based on characteristics like geographic scope, participants, data accessibility, and more. Existing citizen observatory projects from the EU's Horizon 2020 and FP7 programs are mapped according to these classifications to benchmark features and functions. Practitioners are invited to share additional project details to compile a more comprehensive landscape of citizen observatory initiatives in Europe.