The nineteenth century witnessed an exponential growth in the amount of statistical data collected to define populations, necessitating new ways to process and manage information. Ethnographic cartography offered a visual method to synthesise unwieldy ethnolinguistic data and communicate it in a clear and accessible way. However, in doing so, maps profoundly impacted the very meanings of concepts like language, ethnicity, and nationhood. This dissertation examines how nineteenth-century map-makers in the Russian Empire experimented with geographical methods and graphical techniques to map the inhabitants of the Baltic provinces, constructing ethnic groups based on contemporary notions of similarity and difference. Drawing on primary source materials from archives across East Central Europe, I trace both the political and scientific debates among map-makers about how to translate statistics into cartographical form. I depart from the existing literature by deliberately emphasising the technological, socio-economic, and commercial aspects that shaped the processes of collecting data, printing, publishing, and selling maps. By drawing attention to the wide range of actors who engaged in ethnographic map-making, such as women and members of the lower classes, I challenge the prevailing historiographical tendency to view maps solely as instruments of state governance and part of the material and visual culture of intellectual elites. I reveal how ethnographic maps had a strong subversive tendency and the spread of cartographical literacy through school textbooks and popular print culture in the second half of the nineteenth century enabled local populations to use maps to assert agency and challenge the imperial state. Situating the Baltic provinces within the wider transnational information space of East Central Europe, the project enriches our understanding of how ethnographic mapping permeated multiple social and political spheres and came to hold such a powerful sway over popular imagined geographies of nationality.