Map o ieithoedd Cymru 1800. Allwedd: Gwyrdd: poblogaeth uniaith Gymraeg; pinc: poblogaeth ddwyieithog; gwyn: poblogaeth uniaith Saesneg.
(A map of the languages of Wales 1800. Key: Green: monolingual Welsh population; pink: bilingual population; white: monolingual English population.)

Mae'r data wedi'i seilio ar ymchwil gan W.T.R. Pryce - a gyhoeddwyd yn gyntaf yn (Data is based on the research of W.T.R. Pryce. It was first published in): 

Pryce, W.T.R. (1978). "Welsh and English in Wales. 1750-1971: A spatial analysis based on the linguistic affiliation of parochial communities". Bulletin of the Board of Celtic Studies 28: 1-36.
Pryce, W.T.R. (1978). "Wales as a culture region: Patterns of change 1750-1971". Transactions of the Honourable Society of Cymmrodorion, Session 1978: 229-261., revised and reprinted in: Pryce, W.T.R. (1986) "Wales as a culture region: Patterns of change 1750-1971" in     The Welsh and Their Country: Selected Readings in the Social Sciences, Gomer Press  ISBN:  9780863832451. 
Mae Colin H. Williams yn rhoi trosolwg o ymchwil,  dadansoddi a metholdoleg Pryce (Colin H. Williams gives an overview of Pryce's research, analysis and methodology):

Williams, Colin H. (1990) "The Anglicisation of Wales" in     English in Wales: Diversity, Conflict, and Change, Multilingual Matters, pp. 23−30.  ISBN:  9781853590313. 
Gweithiau academaidd sy'n atgynhyrchu map iaith 1800 Pryce (Academic works republishing Pryce's 1800 language map):

Pryce, W.T.R. (2000) "Language Zones, Demographic Changes, and the Welsh Culture Area 1800-1911" in Jenkins,  Geraint H. , ed.    The Welsh Language and its Social Domains, 1801-1911, University of Wales Press, p. 40.  ISBN:  9780708316047. Jones, David Brian (2013). The influence of religion on the survival of the Welsh language 1801-2011: A tale of two centuries. (Masters). University of Wales, Trinity St David.  p. 13.Nodyn: Mae data ar y Gymraeg cyn gyfnod y cyfrifiad yn bodoli mewn archifdai megis yr eglwys a ffynonellau tebyg. Dyma ffynhonnell W.T.R. Pryce: yr hyn a nodwyd gan glerigwyr y gwahanol gyfnodau am iaith aelodau eu heglwysi. O'r wybodaeth honno, creodd Pryce bedwar map ar gyfer y blynyddoedd: 1750, 1800, 1850 a 1900, gan ddangos dosbarthiad y siaradwyr Cymraeg fel iaith bob dydd, ardaloedd a oedd yn ddwyieithog ac ardaloedd uniaith Saesneg.
Defnyddiodd Pryce symbolau pwynt i ddangos ei ddata parthed yr iaith o fewn yr eglwysi plwy. Rhag gor-gymhlethu'r mapiau, dydw i heb wneud hynny. Yn hytrach dw i wedi canolbwyntio'n unig ar sut roedd y sefyllfa ieithyddol yn lledaenu.
Mae fy mapiau i, felly, wedi eu sefydlu ar fapiau Pryce, ac ailgrgraffiadau o'i fapiau. Drwy gymharu'r rhain, roedd yn bosib gwiro fy mapiau i. Blociau o lwyd a du oedd gan Pryce, a'r symbolau pwynt - sy'n gwneud y weithred o'u dadansoddi nhw yn eitha anodd, yn enwedig gan bobl nad ydynt wedi arfer gyda'r pwnc, data neu fapiau! Oherwydd hyn, penderfynais ddefnyddio lliw. I sicrhau fod y lleoliadau'n gywir defnyddiais feddalwedd gartograffig megis Google Maps, Open Street Map a Bing Maps.
Note: Pre-census data on the Welsh language exists in archives such as the church and similar sources. This is the source of W.T.R. Pryce: what the clergy of the different periods noted about the language of their church members. From that information, Pryce created four maps for the years: 1750, 1800, 1850 and 1900, showing the distribution of Welsh speakers as a daily language, bilingual areas and English only areas.
Pryce also used point symbols to show his data regarding the language within the parish churches. I haven’t, as to not over complicate the maps. Instead I have focused only on how the language situation was spreading.

My maps are, therefore, based on Pryce's maps, and reprints of his maps. By comparing these, it was possible to check my maps. Pryce had blocks of gray and black, and the point symbols - which makes the task of analyzing them quite difficult, especially for people who are not used to the topic, data or maps! Because of this, I decided to use colour. To make sure the locations were correct I used cartographic software such as Google Maps, Open Street Map and Bing Maps.
Own work
Resnjari