Real-time synchronization is increasingly available in web-based environments for editing textual data, and this has changed how groups of people collaborate. We present a novel approach for real-time collaborative editing of geo-data. We introduce Ethermap, an open source web-application that implements this approach and enables multiple users to map data concurrently. It supports synchronous and collaborative mapping in several ways: it visually highlights mapping activities, it allows for fine-grained reviewing of all changes, and it enhances text-based communication with cross-modal references to geo-objects. We report on key results from a multi-tiered evaluation of Ethermap based on a user-study and on expert interviews. The concept of real-time collaborative editing was received favorably by users and experts. Participants of the study learned to use Ethermap quickly, and successfully completed a collaborative mapping task. Experts and users agreed that, given the right scenario (e.g., disaster mapping, teaching, planning), the approach could benefit the process of working on geo-data collaboratively.