How accurate is the map ?

Land Registry title plans are based on Ordnance Survey maps, mostly on National Grid Series maps published between 1945 and 1990 or on the digital (computer-readable) maps derived from these National Grid Series maps. Some older title plans, and many conveyance plans, are based on the older Ordnance Survey County Series maps published between the 1840's and 1950's.

Whilst Ordnance Survey maps enjoy a worldwide reputation for accuracy, it is very unwise to attempt to measure distances from them in order to scale those distances up and to set out on the ground the theoretical position of the boundary. One reason for this is that any error you make in measuring distances on the map is magnified by the scale of the map when you set it out on the ground. So if you make an error of half a millimetre (one-fiftieth of an inch) in measuring on a 1:1250 scale map, you will produce an error of 625 mm (about 2 feet) on the ground.

Another effect of map scale is that objects can appear to be much wider on a map than they are on the ground. For example, consider a thin line representing a fence:
- Most Land Registry title plans are produced at 1:1250 scale. The thickness of the line on the map actually represents 10 inches (250 mm) on the ground.
- Other title plans are either produced at 1:2500 scale (rural areas) or are enlarged to 1:1250 from 1:2500 scale. On both of these the thickness of the line on the map actually represents 20 inches (500 mm) on the ground.
Compare those line thicknesses with the average larch-lap panel fence, supported on fence posts only 75 mms (3 inches) thick!

More important than this, you have to consider the accuracy of the map you are using. Accuracy can mean a number of different things:
- absolute accuracy: is a feature shown in its true position on the surface of the earth?
- relative accuracy: is the position of any feature on the map shown in its true relationship to other nearby features?
- currency: does the map show all of the things that have been built, and exclude all of the things that have been demolished, at the date the map claims to portray (the publication date)?
- selection: does the scale of the map require certain features to be omitted for the sake of clarity?
- generalisation: does the map scale require certain features to be shown out of their correct position for the sake of clarity?

The absolute accuracy of a map is not an issue in boundary disputes. This is because a high level of absolute accuracy assures us that the map shows a Hertfordshire mansion in its correct position relative to the owner's holiday cottage in Cornwall. With boundary disputes we are concerned only with the Hertfordshire mansion and the immediately adjacent land (or perhaps with the Cornish cottage and its immediate neighbour).

Relative accuracy is a major concern in boundary disputes. Is the fence shown in its correct position relative to the houses either side of it? Ordnance Survey has over many years tested both the absolute and the relative accuracy of a large sample of their published National Grid Series of maps. As a result they have published an accuracy statement for their large scale maps. The following (in green) is an extract of what Ordnance Survey says about the relative accuracy of its large scales maps.

Achieved relative accuracy of Ordnance Survey map data.

The following table shows the expected relative accuracy values for well defined points within each accuracy category. The values apply up to the stated maximum measured distances quoted in the table.

Table 3 - Relative Accuracy

* Some generalisation of detail does occur for cartographic reasons on 1:10000 mapping therefore some points of detail may appear to be less accurate than these standards.

The accuracy statement will tell you that even in the best of circumstances (a 1:1250 scale map), whilst some of the lines will be spot on, you can expect the position of any line on the map to be shown up to 400 mm (16 inches) out of position relative to other lines within 60 metres (185 feet) of it. For a "1:2500 scale Overhaul" map the comparable relative accuracy of the lines on the map is 1.2 metres (4 feet) relative to other lines within 200 metres (656 feet). I have come across a few lines on some maps that are much further out of position than this.

To be fair to Ordnance Survey, they have a Positional Accuracy Improvement programme running that will improve the relative accuracy of the 1:2500 scale Overhaul maps. But if you are working from a title plan based upon a 1970 edition 1:2500 scale plan then you are not helped by the availability of a later, more accurate map.

Remember also that the relative accuracy of County Series maps will be worse than that of the National Grid maps quoted above.

Currency is often misunderstood. If you have, say, the 1911 edition of a County Series sheet then you can be reasonably sure that everything that was on the ground and should have been mapped is shown on that map, and that any demolished buildings have been removed from it. The same holds true if you have a fully revised 'new edition' National Grid paper map, which can be identified by having an edition letter such as A, B, C, D printed in the bottom margin of the sheet. If the edition letter takes the form A* then selective revision may be present and the map cannot be considered complete and up-to-date at the date of publication.

Since the cessation of printed 'new editions' in about 1990 the situation regarding currency has, in one sense, worsened. Generally, Ordnance Survey has made strenuous efforts to ensure that significant ground detail finds its way onto its maps within a specified number of months. However, their revision programme is driven by an intelligence system, and new ground detail gets added to a map only if the local surveyor becomes aware that it has appeared on the ground. It is perfectly possible for a surveyor to map a new house built as infill development in one street, and miss another house in an adjacent street. There is therefore no longer any guarantee that the version of the map you are working from is current over the whole area it covers. This applies to any map derived from the National Topographic Database, ie. Superplan and Siteplan plots, Land-Line, Superplan Data, Promap, MasterMap.

Currency is also an issue with Land Registry title plans. You may for example find a title plan that consists of an Ordnance Survey map extract bearing a copyright date (eg. 1968), which we can expect to have been current at that date, to which Land Registry have added the red edging to a title registered in, say 1975. We can usually be confident that no revision was added to the map extract at the date the red edging was applied, so we can assume that the map extract was not necessarily current at the date of registration of that particular title.

If you are trying to use the presence or absence of a feature on a particular version of a map to prove the age of a ground feature (such as a fence) then you have to be absolutely certain that the map was current (ie. complete and up to date) at its publication date.

Selection: there is not sufficient space, even at the relatively large scales of 1:1250 and 1:2500, to show on the map everything that occurs on the ground. Examples of selection include:
- where a hedge and ditch run alongside each other it is the hedge that is shown and the ditch that is omitted;
- where a hedge separating two fields has a fence on either side (presumably to isolate livestock in one field from livestock in the other field) then the hedge will be shown and the fences omitted;
- buildings smaller than 8 square meters in extent are usually omitted;
- bay windows and porches are omitted.

Generalisation: Sometimes features that are too important to omit from the map stand too close together to be shown in their correct positions on the map. The rule is that parallel features either have to be shown at a minimum distance from each other (1 metre on 1:1250 scale maps and 2 metres on 1:2500 scale maps) or they have to be merged into a single feature. For example:
- a hedge and a drain (rather than a hedge and ditch - a ditch may be used to mark a boundary or to drain water whilst a drain is never used to mark a boundary) less than 2 metres apart will be shown 2 metres apart on a 1;2500 scale map;
- two detached houses with a gap between them of between 1 and 2 metres will be drawn at 2 metres apart on a 1:2500 scale map, whilst two detached houses with a gap of less than 1 metre between them will drawn as if they were semi-detached houses on a 1:2500 scale map.
- on a flight of steps, individual steps will be drawn at 1 m horizontal spacing on a 1:1250 scale map, even though the step treads will be narrower than this.

Map interpretation: Further complications can arise because published (as opposed to digital) Ordnance Survey maps use the same line symbol for a wall, fence, hedge, bank, ditch and stream, making it sometimes difficult to know just what the line represents.

Correct interpretation of the map can only be achieved by taking the map onto the site and comparing it with the features on the ground to decide what has been shown, what has been omitted, what has been deliberately moved out of its correct position for the sake of clarity, and what has been shown in the wrong position because of inaccuracies in the survey that produced the map.

A correct interpretation of the map usually concludes that the line on the map identifies the physical feature to which the boundary is related, and it is the position of the actual feature on the ground - not the position of the line on the map - that is important in attempting to trace the position of the boundary. Even then, the general boundaries rule means that the line on the map may not be the legal boundary.
 

Example of map generalisation and interpretation

On the basis of Fig. 1, an Ordnance Survey County Series 1:2500 scale map, the owner of 'A' told the owner of 'B' that their common boundary ran from the north-east corner of B's house perpendicular to the front hedge. A claimed that some of his land had been taken into B's garden, and A wanted it back.

Fig. 2 results from an accurate survey. It shows that more than 0.5 metre (nearly 2 ft) separates the two houses that Ordnance Survey has shown joined together. The survey also reveals that the long-established hedge between the two front gardens (right hand side of Fig. 2) had been drawn by Ordnance Survey at about 1.3 m (4 ft 6 in) to the south of its true position. Both the closing of the gap between the two buildings and the mis-positioning of the hedge, whilst misleading to the naïve map reader, are within specification for a County Series 1:2500 scale map.

Fig. 3 overlays the map onto the accurate survey to illustrate the relationship between the map and the reality.

Incidentally, it did not occur to the owner of 'A' that if he followed through with his own logic then he would have to voluntarily give up part of his rear garden to the owner of 'B'.

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.This page was last updated on 22 Feb 2005
by Jon Maynard FRICS

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