Different types of maps

Different types of maps - Surveying

Maps used for measuring land and building boundaries come in three types outlined below. The only way to ensure your property is mapped with reliable is an onsite survey from professional surveys such as PCA.

Ordnance Survey Ireland (OSI)-Limitations

OSI produce large scale (1:5000, 1:2500, 1:1000) maps that cover the whole Country.
OSI maps are Surveyed by aerial photogrammetry. This means that features and boundaries are mapped from aerial photographs. This is good for large scale mapping but poor for recording detail. If there are trees along a boundary an aerial photograph can estimate where the centre line of the trees are, but not a wall, fence or building that may be obscured by the tree canopy. If a wall beneath tree canopy changes direction this may not be recorded on an OSI map. Buildings with large overhanging eaves may also yield innaccurate measurement when compared to ground dimensions.
Roads in rural areas may not measure the same as the paved surface but rather the width from boundary to boundary on either side of road.
Many ditches and hedges measuring several metres wide on the ground may be shown as a single line on OS maps.

OSI maps are fit for purpose as national mapping. However, they are not intended be relied on for a high level of accuracy. Accoring to the OSI their maps are accurate only up to:

±0.60m for 1:1000 maps (urban areas)
±0.69m for 1:2500 maps (peri-urban areas)
±1.22m for 1:5000 maps (rural areas)
These are at the 66% confidence interval and can be taken to be 3 times greater at the 95% Confidence Interval.

Property Registration Authority (PRA)

The PRA map is based OSI mapping and may miss detail missed by the OSI.

The PRA map is supposed to represent legal proprties not necessarily physical features. The OS maps physical features.

The PRA may now moves legal boundaries onto OSI physical detail. So if the OSI records a boundary as the centre line of a row of trees, not wall hidden beneath the trees, the new PRA map can also show the boundary as the centre line of a row of trees and not the wall or fence previously registered.

For this reason, surveyors are recently and increasingly encountering problems where the new PRA mapping has changed the legal title boundary to an incorrect physical boundary feature.

The PRA map "identifies property, not boundaries" for more information see Property Registration Authority (PRA).

The following is taken from Land Registry FAQ 10 :

I have a dispute with my neighbour over where the boundary lies. Can you tell me who is right?

No. The Land Registry map is an index map and identifies property, not boundaries. Therefore, we are not in a position to advise.

The above statement on the PRA website makes the process of legal boundary definition in Ireland a very complicated process which should only be dealt with by professional surveyors with a lot of experience in this area.
The above also means that it is extremely unsafe practice to use an OS or PRA map for contract purposes without having the boundaries verified on the ground by a professional Surveyor. Property purchasers and legal conveyancers beware!

Professional Boundary surveyors

On site surveys can determine exactly what features are on the ground.
Maps can be generated to any level of accuracy required
Contract mapping can use these more accurate maps to define what is being transferred.
Application can be made to Land Registry for Conclusive registration.

Prendergast WP, Flynn M, Corrigan P, Sweeny BF, Martin Á, and Moran P 2008,

Green Paper Proposing Reform of Boundary Surveys in Ireland, Irish Institution of Surveyors, Dublin, p. 109.

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