About PRONI Historical Maps viewer

PRONI and Land and Property Services have worked together to digitise and make available a range of historical Ordnance Survey of Northern Ireland (OSNI) maps held within the PRONI archive.

Historical Ordnance Survey Maps

The historical OSNI maps available on this application are from the 6 inch County Series mapping; and latterly the Irish Grid.  Maps available cover the six counties of present-day Northern Ireland: counties Antrim, Armagh, Down, Fermanagh, Londonderry and Tyrone.  The following map editions are available on the application:

  • Edition 1 (1832 – 1846)
  • Edition 2 (1846 – 1862)
  • Edition 3 (1900 - 1907)
  • Edition 4 (1905 - 1957)
  • Edition 5 (1919 – 1963)
  • 6” Irish Grid (1952-1969)
  • 1:10,000 metric Irish Grid (1957-1986)

Contemporary mapping

The application also features a number of more modern OSNI maps (Data owner: Ordnance Survey of Northern Ireland | Land & Property Services), including:

OSNI Basemap

The basemap is generated using a range of Ordnance Survey of Northern Ireland map products such as 1:50,000 Vector and Largescale data.

Aerial imagery

Orthophotographs are photographs of the earth’s surface from which accurate measurements can be taken.  OSNI orthophotographs are created from high quality aerial images with distortions removed, resulting in a scale-accurate bird's eye view of the landscape.  

This secured orthophotography service is updated on a quarterly basis using the latest ortho-rectified imagery.  The flown date indicates the date the imagery was collected.  Some tiles may have anomalies, for example cloud cover, and in such cases a previous ortho image may be stitched to the most recent.

OSNI 10k Raster

The 1 : 10 000 scale raster mapping has been derived directly from OSNI branded large-scale database and provides clear, detailed and textual information on roads, buildings, fields, administrative boundaries, water features and vegetation, overlaid by contours at 10m intervals. 

OSNI 50k Raster

The 1 : 50 000 scale raster mapping has been derived directly from our OSNI branded large-scale database and provides small scale information on roads, buildings, administrative boundaries, water features and vegetation, under laid by contours at 10m intervals.


A range of datasets have been added to the PRONI Historical Maps viewer to enhance its functionality and provide additional information for users, including:

Geographical boundaries

County boundaries

The area a county covers can be compared to an English shire.  It was created by the English administration in Ireland as the major subdivision of an Irish province in the years following the Anglo-Norman invasion of Ireland. 

Counties were formed over a period of time from the 13th to the 17th centuries.  The present day counties were planned in the early 16th century, although some existed long before this time.  This application reflects current county boundaries.

Civil parish boundaries

The parish was once an ecclesiastical unit of territory, at first based on early Christian and monastic settlements.  Later used by the Established Church of Ireland after the Reformation, it was then also adopted as a civil administrative area.  Over time, some civil and ecclesiastical boundaries came to vary.  This application reflects current civil parish boundaries.

Townland boundaries

The townland is the smallest administrative division throughout the island of Ireland that is still in use.  These small divisions of land were used as the basis for plantation grants in the 16th and 17th centuries and for the leasing of land on the great landed estates from the early 17th century.  Up until the early 19th century, townland boundaries altered considerably following subdivisions.

While townlands are almost all compact units, it is possible to find parts of a townland in different civil parishes.  Townlands can vary enormously in size from a few acres to over 7,000 acres.  In Northern Ireland there are over 9,000 townlands.  This application reflects current townland boundaries.

Other land divisions referenced in the viewer

The application also references barony.  The barony is a now obsolete administrative unit that is mid-way in size between a county and a parish.  Many historical PRONI records are organised by barony and further subdivided by Parish and Townland. 

Exceptions and peculiarities

In the majority of cases, smaller divisions fit neatly into larger - a number of townlands are grouped together to form a civil parish; a number of parishes are grouped together to form barony and so on.  However, this is not always the case:

  • Detached townlands:
    Several parishes have a single or several townlands located away from the main group and that lie surrounded by another parish, for example the parish of Downpatrick in county Down which has six detached townlands.
  • Split townlands:
    In some cases, a townland is split between two or even three parishes, with each 'part' still retaining the same townland name.  For example, the townland of Ballyalton in county Down is split between the parishes of Comber and Newtownards.
  • Townlands with the same name:
    Normally the parish is the easiest way to differentiate between townlands which have the same name, however, occasionally a townland name repeats within one parish.
  • Baronies and counties:
    There are many parishes in more than one barony, and several in more than one county for example, Ballywillin in Antrim and Derry/Londonderry, Lissan in Derry/Londonderry and Tyrone.  There are also parishes which cross the border between Northern Ireland and the Republic of Ireland, for example Creggan in counties Armagh and Louth, Clones in counties Fermanagh and Monaghan, Kinawley in counties Fermanagh and Cavan, and Urney in counties Donegal and Tyrone.

Points of interest

Significant buildings and landmarks

Apart from purely geographical data, there's also a wealth of additional details that have been annotated on Ordnance Survey maps.  These ‘points of interest’ include a range of landmarks and significant buildings – for example, churches, graveyards, schools, court houses, workhouses and hospitals. 

As part of the original exercise to digitise a range of historical PRONI OS maps, a Points of Interest dataset was developed to capture some of the information of historical interest which related to the themes of Religion, Health, Education and Law.  It should be noted that the Points of Interest dataset has been compiled using information gathered from a range of historical maps - including other historical OS maps which do not appear on this application.

Historic sites and monuments

The Northern Ireland Sites & Monuments Record dataset (provided courtesy of the Historic Environment Division, Department for Communities) holds information on almost 16,000 historic and archaeological sites. These range from Mesolithic camp sites, Bronze Age landscapes preserved under bog, through the Early Christian monasteries, the castles of the Norman conquest, the defended houses of the Plantation settlers, up to the pill boxes, airfields and observation posts of World War II.

PRONI Ordnance Survey archive

The PRONI Ordnance Survey archive has a range of historic Ordnance Survey maps which are over fifty years old, including the 6” County Series mapping available in this application. 

Referencing and arrangement

The Ordnance Survey archive at PRONI is prefixed with the letters ‘OS’.  Each of the c.40,000 maps in this archive has a unique reference number which begins ‘OS/’ and each series at a certain scale has been allocated a number.  For example, the county maps at 6" scale are classified under OS/6, and the county maps at 25" scale are under OS/10. 

Records are then arranged by county (alphabetically) - so for the 6" county maps OS/6/1 = Co. Antrim, OS/6/2 = Co. Armagh, OS6/3 = Co. Down and in a similar way in OS 10 (the 25" county series) OS/10/1 = Co. Antrim, OS/10/2 = Co. Armagh.

The next number is the sheet number, so OS/6/1/1 is sheet 1 of the 6" maps for Co. Antrim.

The final number relates to the edition/revision (with /1 being the earliest edition, /2 the next available revision).  This reference in its entirety relates to a single map.  For example, OS/6/1/1/1 is the item reference required to order the first edition of Co. Antrim Sheet 1 (Surveyed 1832) from the 6” county series.

When it comes to the Irish grid series references, the same principle applies, except there is no county reference and the class and item numbers instead reflect the plan numbers. 

For further information, refer to the main description of the ‘OS’ collection in PRONI’s eCatalogue.

Identifying archive references on the Historical Maps viewer

The viewer includes a function whereby users can display associated PRONI archive references for various scaled maps in PRONI’s Ordnance Survey collection (PRONI Reference prefix OS/...). 

To apply this function, select the Survey Date widget and select Get Survey, then click on a point on the map to return available details.  Note, only a selection of OS maps from PRONI’s collection have been captured on the database.  For details of all available historic maps for an area, you can browse the OS collection via PRONI’s eCatalogue .

How historical maps are displayed electronically

Modern maps reference geographical locations on the earth's surface through a system of coordinates (look at an Ordnance Survey map and you will notice a series of numbers along the sides, associated with a grid covering the whole map area) so all locations and shapes can be defined in terms of x and y coordinates from a given grid system (in this case, the Irish Grid system).  These numerical values are used to translate map information into digital form. This applies in both vector and raster formats.

To include the 6 inch county series, high quality images of the historic maps were captured, then  Georeferenced and Georectified.   Georeferencing means assigning a spatial reference and coordinates to the image.  Georectification means translating, transforming, and warping it into position, relative to some other spatial data such as survey locations, street intersections and more. 

The maps have been georeferenced to the Irish Grid, having been adjusted to transform the original Cassini-Soldner projection to the current Transverse Mercator projection, allowing a reasonable degree of fit.  As each county was surveyed separately, there is not a perfect fit between the transformed sheets resulting in some overlap along the borders between counties, however, the discrepancies are minimal (when purchasing the data from the OSNI mapping website, the full data is provided with no overlap).

Search PRONI Historical Maps viewer

Search and browse a range of historical Ordnance Survey of Northern Ireland (OSNI) maps and find information on sites, buildings and landmarks of historical interest:

Find out more about the Public Record Office of Northern Ireland (PRONI) - the official archive for Northern Ireland.


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