Family Search have an interactive map called "England and Wales Jurisdictions 1851" showing parish (and other) boundaries with optional background maps such as Ordnance Survey. There is also a Search facility, do read the guidance notes to get maximum benefit from this useful resource. See here for further background information to assist in the interpretation of this data
The Village of Merthyr Cynog by Brian Wagstaffe
You can see pictures of Merthyr Cynog which are provided by:
You can see the
administrative areas in which Merthyr Cynog has been placed at times in the past. Select one to see a link to a map of that particular area.
Victorian Powys for schools site - Map of Merthyr Cynog in 1840, and 1887
You can see maps centred on OS grid reference SN985375 (Lat/Lon: 52.026596, -3.480787), Merthyr Cynog which are provided by: