CGM has provided large scale maps for downloading showing all co-ordinates for the actual buildings and the garden sites as follows:

Estate Map 1a: Cottages East (JPEG 350kb) showing all co-ordinates

Estate Map 1b: Cottages West (JPEG 95kb) showing all co-ordinates

Estate Map 2: Lodges - North East (JPEG 356kb) showing all co-ordinates

Estate Map 3: Lodges- North West (JPEG 350kb) showing all co-ordinates

Estate Map 4 Lodges South (380kb) showing all co-ordinates

Estate Map 5: Landscape Plan (JPEG 450kb)

Estate Map 6: Full 200 acre site. Refer to the print brochure (very large PDF file 3 megs)

CGM also provides a word document showing the precise location of each co-ordinate "node". Click here to download:

Clifden Glen Site Co-ordinates (word file).

Clifden Glen House Co-ordinates (word file)

The root mean square error (RMS) in the ordnance survey maps are 0.69 metres at 1:2500 and 1.22 metres at 1:5000. Both scales are present at Clifden Glen. Everything east of house 91 approximately is based on 1:5000 scale. Effectively two parallel lines 3.44m apart on the ground could appear to be 1 metre apart or 5.88m apart and still be within tolerance at 1:5000 (although its seldom as severe as that in reality).

The Ordnance Survey (OS) detail is based on aerial photography (photogrammetry). At its best when you see a representation of a building you really are seeing the roof. Housing estate houses will therefore seem bigger than their footprint, and closer together. As a result the scheme map is a compromise between the boundaries as intended on the ground and the OS background.

In an environment at Clifden Glen where we have no physical boundaries the only way to be absolutely precise in relation to a property boundary is by using co-ordinates which can be read off a larger scale map. The official Land Registry maps are just not accurate enough to show boundaries.

The Land Registry recognises the fact that in most circumstances there will be a margin of error. Indeed a potential purchaser is always advised to check the boundary position on the ground as Land Registry maps are not definitive and do not purport to be so. The Registry in recognising that variances exist provides a tolerance level in order to deal with mapping inadequacies.

Extract from an OS document on accuracy tolerances:

3.5 Quality Assessment

Another key element of the last two years has been the development of a set of measures on the
quality of OSi large-scales mapping. The quality of OSi mapping with regard to these statistics has
been developed by sampling 3% of the large-scales database, and producing statistics for the five
quality themes as defined by ISO (ISO, 2001):

• Positional accuracy;
• Thematic accuracy;
• Logical consistency;
• Completeness; and
• Temporal accuracy.

The statistics are still being refined; the following paragraphs therefore give a flavour of the findings to date, with more definitive statements on each of the quality themes to be produced during 2006. Elements of the quality of the mapping, particularly the thematic accuracy and logical consistency, cannot effectively be quantified at this stage, as the mapping specification has developed over the course of the new mapping programme. The current quality improvement programme (described in section 3.2) is addressing all issues under these two headings and will result in all maps conforming to the current specification. The quality assessment programme of these two elements will therefore concentrate on testing the mapping on which upspeccing has been completed.

Temporal accuracy has been measured by checking whether all OSi mapping has been revised to the specified frequency and by confirming in detail (map by map) that this is the case.

Completeness has been measured by comparing the large-scales vector mapping database with orthophotos of a similar date to the revision date. This has to date been completed for a sample of 1:2,500 and 1:5,000 plans and has confirmed that in excess of 75% of plans are fully complete with regard to content. Further refinement of the statistics, and of how to deal with differences in date between photography and revision, is continuing.

Positional accuracy has been tested by surveying a random selection of points by GPS methods.
Given that this method is better than the expected accuracy of the mapping by an order of magnitude, the GPS coordinates have been assumed to provide ground truth, and the map coordinates have been compared to them to give a measure of absolute positional accuracy. The initial work has provided the following preliminary results:

• The average RMSE (root mean square error) of detail on the 1:1,000 plans tested is 0.60 metres;
• The average RMSE (root mean square error) of detail on the 1:2,500 plans tested is 0.69 metres;
• The average RMSE (root mean square error) of detail on the 1:5,000 plans tested is 1.22 metres.

These results are in line with what might be expected in light of the age of resurvey of the plans, the methodologies used, and the number of revisions to which the mapping has been subject (and the methods used for those revisions). They highlight the very good quality, when compared to international norms, of the 1:2,500 and 1:5,000 mapping, which in the sample both consistently
exceed the target levels set internally by OSi. With regard to the 1:1,000 mapping, the sample is not yet large enough definitively to determine the effects of the various factors, but initial indications are that the age of the mapping (and therefore the technology employed for the initial survey) is of considerable importance.

Further work to refine the measures and statistics is continuing, referencing various sources
including the developing ISO work in this area.

Thanks and Summary

The mapping project is now complete and the Clifden Glen "Scheme Map" is approved and in place at the Land Registry. The project started in late 2004 and has taken nearly 20 months to bring to a satisfactory conclusion.

Our thanks to Shane Joyce, consulting engineer and Eileen O'Gorman, solicitor, Gleeson McGrath Baldwin for their considerable contribution to site mapping at Clifden Glen. Our thanks also to OSL Consulting Engineers, Cork who carried out the original survey identifying the position of each building.

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Issued by Clifden Glen Management (2005) Plc.,
Market House, Churchtown, Mallow, Co Cork, Ireland