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This paper discusses the way that the Land Information System (LIS) being used at Milton Keynes Development Corporation (MKDC) captures and maintains legal boundaries using conveyance plans as the source of the data. It deals with the way conveyance plans and other source documents are related to the map base, often using common physical features and not National Grid Co-ordinates. As a result of using conveyance plans as source documents, the boundaries input to the system often fail to obey topological rules. This characteristic is fundamental to the interpretation of legal plans transferred to a computerised mapping system. To assist in the rectification of conflicting boundaries, the necessity to hold information concerning the quality of each source document is discussed. In the decision making processes when adjoining boundaries are undergoing rectification, a pseudo-weighting system is required which is based upon the quality of the original conveyance plan.
Steve Gill is a GIS Consultant with Midsummer Computing. A graduate in Earth Sciences, he entered the mapping industry in 1966 and has been involved with the Milton Keynes project since its inception. Ken Robinson (BA) is a long serving officer with MKDC and Manages the LIS Department on behalf of the Corporation.
Milton Keynes Development Corporation is responsible for securing the development of the New Town of Milton Keynes and has already enabled the population, within the designated area, to rise from 40,000 in 1967 to over 145,000 in 1989. However, after 1992 the role presently exercised by MKDC will be performed by other public and private bodies.
As an aid to the continuing development of the city, up to and beyond 1992, MKDC has installed a Land Information System to speed up the land transactions which are essential to the development programme. The LIS is being used to capture the boundaries of the land transactions (both acquisitions and disposals) entered into by the corporation. The data set on the system will therefore record the total land area which will have passed through the Corporation's hands during the period from designation in 1967 to wind up in 1992.
This major information overlay comprises data which was previously held on many thousand disparate maps and plans. By superimposing this wealth of information on to a common map base the relationships between properties can be analysed rapidly and with increased confidence. For example, the boundaries of properties surrounding planned disposals can be rapidly reviewed and potential (or existing) problems involving property boundaries can be identified. Thus, not only does the actual disposal process move more quickly, but the process is likely to generate fewer problems due to conflicting boundaries.
Unfortunately the system was introduced after many thousand transactions had been completed. The visual and analytical benefits provided by an LIS were not then available; consequently the legally defined boundaries of adjacent properties have not always been coincidental. The digital conversion of the retrospective transactions has however enabled many of the assumed, but previously unidentified, boundary problems to be isolated. For example, overlapping disposals have been identified, highway adoptions have been noted where they impinge upon private property, small slivers of land ( which still belong to the corporation ) have been discovered and small areas of land have been found which, although sold by the corporation, had never previously been acquired! The system, having identified these problems, is also being used, via the database, to monitor the progress of any subsequent rectification procedures.
Besides enabling the analysis of property boundaries, the system also provides a sophisticated plotting resource. The plans associated with new disposals are generated on the system as are all rectification drawings. Customised Ordnance Survey map sheets are also produced to satisfy the demands of Corporation staff. These maps can be produced with either a single superimposed MKDC overlay or, if required, a combination of such themes. If several categories of polygon are required on the map then each theme can be prioritised such that the boundary definitions and the associated colour- infills meet the requirements of the user and are also easy to interpret.
From the above it can be seen that the input of legal boundaries is central to MKDC's land information system. To this end data capture commenced in September 1989 and currently (November 1990) it is expected to complete the conversion of these records by mid 1991. The total number of transactions that will be input to the system will be in excess of 35,000.
During the late spring and summer of 1989 a full analysis of the MKDC land transactions was undertaken and the various overlays or themes corresponding to each category of transaction were designed. Subsequently symbology tables were defined which describe the colours, line styles, line weights and levels associated with each of the identified features. The basic attribute set for each feature was also finalised during this important analysis phase of the project.
During this period the known existence of overlapping boundaries within a single theme reinforced the belief that the use of strict topological rules would be of limited use in the search for conflicting boundaries. The expectation was for the boundaries of at least some adjacent properties within a single theme to be non-coincidental. However strict topological rules do not allow this situation to occur because each defined area is expected to abut its neighbours. To circumvent this expected problem, contingency plans were formulated which enabled the isolation of overlapping properties within a single theme.
The discrepancies between adjacent boundaries occur for a number of reasons, not least that adjacent conveyance plans will have been prepared using differing map bases, which may be at differing scales and drawn perhaps a century apart. The various map bases used to represent adjacent properties may, for example, comprise of a 1:1250 OS map sheet based on the National Grid, a 2500 County Series map sheet, a building site plan (not as built) at say 1:200 and an engineering survey at 1:500 using perhaps co-ordinates to define a property extent. With such diverse information it is hardly surprising that the
common boundaries of adjacent properties often fail to coincide once the property records have been transferred to the common map base. The adjoining land owners may well agree that the boundaries are co-incident but if the information on the plans is transferred to a common base then overlaps and gaps can be expected. The independent transference of conveyances to the common digital base is necessary because it is essential for the resultant overlay to be a true representation of each conveyance. Each property will be represented on the map base and thus the true relationship between each conveyance and all adjoining conveyances will be illustrated.
In order that this transfer of information results in a true representation of the conveyance within the LIS some ground rules were defined. These rules were formulated following discussions involving the Corporation's Land Terrier Officer and members of the implementation team who possessed extensive knowledge of the mapping industry, and in particular the historical changes which have affected the Ordnance Survey large scale map products. The resultant draft rules were subsequently discussed with HM Land Registry staff who have been involved in their own digital pilot schemes. It was perhaps satisfying to know that, completely independently both MKDC and HMLR had devised a set of rules which were basically very similar.
The rules are stated quite simply as a hierarchy:-
In entering the boundary of the land parcel into the system the highest applicable method will be used, PROVIDED it yields acceptable results. For instance, in one case where the land parcels were based on a rectilinear grid, co-ordinates were noted on a legal plan which when entered into the system resulted in the parcel being displayed in an obviously wrong location - the plot being displaced by approximately 50 metres relative to its correct position. In this instance the co-ordinates shown on the plan were absolutely accurate but related to the wrong piece of land.
Using these rules operators enter the information from the conveyance plan into the graphics files on the LIS - the associated textual information having previously been entered into the database via an alpha numeric terminal. Information is entered using the Conveyance plan itself as the source document.
If the source document is based upon an OS National Grid map sheet, then usually there are few problems recreating a conveyance within the LIS. In such instances physical features common to both map series are used to locate the conveyance. However, even under these circumstances transfer may not necessarily be straight forward, for it is not always a simple task to identify common features. Established features may appear displaced within the map base or may appear to be realigned. The technician entering the data thus has to decide whether any such discrepancy is due simply to resurvey or whether a genuine change to the physical feature under consideration has occurred prior to a re-survey. If the latter is true then the position of the original feature would need to be recreated as the true boundary of the conveyance. The situation previously referred to may appear perhaps trivial because such positional discrepancies may be only one or two metres; but these discrepancies are typical where a property owner has performed an unauthorized repositioning of his fences. When such occurrences are suspected the appropriate officers within the Corporation are informed.
Acquisitions which are defined on National Grid map sheets are often positioned within the LIS using a combination of feature identification and National Grid intersections. Most of the Corporation's acquisitions comprise green field sites and perhaps represent complete farms. The digital map information available within the LIS has all been supplied since mid 1989 and thus displays not open farmland with occasional villages and farms, but a large expanse of planned urban development. Considerable ingenuity is thus often required when inputting acquisitions to the system. Following the development of any particular area within Milton Keynes there often remain vestiges of the old hedges - or at least apparent vestiges. Obviously it is not possible to positively identify a short linear feature on an OS map as part of an ancient hedge, so it is sometimes necessary to resort to aerial photography or perhaps a site visit for confirmation. When common features, if any, have been identified on the different map bases, then local scales, supplemented by National Grid intersections are used to input the acquisition.
Many older transactions (particularly acquisitions) are delineated on OS County Series map sheets. These sheets, of course, have no grid, and thus the transfer of these records to the LIS is performed using only the remnants of old features. If the remnants define a junction or other identifiable sector of a feature (eg a corner), then the set-up prior to input can be performed with some degree of confidence. However, in most instances the remnants of hedges are simple linear features which give no indication of the location of the remnant along the original complete feature. In such circumstances set-up can be a lengthy process. Careful searches for all remnants need to be conducted and then a best possible fit using all available information needs to be achieved prior to digitising the old hedge boundaries, stream centre-lines or parish boundaries which may jointly define the extent of a pre-existing property. Using these techniques properties of up to 700 hectares have been re-created within the LIS.
It should be noted that the re-creation of long-gone physical features may merely be the first-step in the generation of an acquisition, for in many instances the legal boundaries were off-set from the feature. For example if the "hedge and ditch" rule is applicable to a property, then the legal boundary may follow the line of an ancient ditch which may have been infilled long ago and thus not represented on the map base. The adjacent hedge, grown originally on the spoil removed from the ditch would therefore lie either within or without a property, depending upon which side of the hedge the ditch was located. The true property boundaries, in such instances, would thus be created by generating linework at the correct off-set parallel to the recreated feature.
Initially it was considered that most of the data-capture problems would be associated with transactions based upon County Series map sheets. It was not envisaged that serious difficulties, albeit of a different nature, would be encountered during the input of disposals. Where the conveyance plan is based on an OS map sheet then few problems occur. However, unfortunately most disposals, whether freehold or leasehold, are depicted on site lay-out plans or, worse still, simple sketches which use no recognisable base mapping. Where site lay-out plans are used it is almost inevitable that following construction work the as-built boundaries rarely coincide with those depicted on the plans. Worse still, in rare instances extra properties are fitted into a site or a site has undergone some fundamental form of redesign
Thus even with disposals the interpretative skills of the LIS technician can be severely tested. Basically, if the configuration of a property boundary corresponds to that shown on the conveyance and there is also positional correspondence, then the features within the OS base map will be used to generate the legal boundaries within the LIS. If however, there are major positional, rotational or alignment discrepancies between the lay-out plan and the as-built OS map base, then the boundaries will be input independently of the OS map features. Discrepancies will therefore exist between the legal boundaries and the physical boundaries of certain properties. It may therefore be necessary to rectify some legal boundaries to ensure correspondence with the accepted physical property boundary.
An increasing number of disposals are now being defined using co-ordinates obtained using land survey techniques. Where such information is available the legal boundary is entered into the LIS by keying in the co-ordinates. Although this method can be considered the most accurate means of input, it is also guaranteed to generate problems. For example, slivers of land or over-laps are likely to be generated because it is extremely unlikely for the co-ordinated boundary to be co-incidental with the boundary of an adjacent property which may perhaps be based upon an OS map feature copied through from the map base.
Co-ordinated boundaries should not be accepted at face value. False origins and off-sets have been encountered, especially where plans originate outside the Corporation. Similarly co-ordinates rounded to the nearest metre should be treated with suspicion. These are usually obtained by very inaccurate survey methods - ie reading co-ordinates from an OS map sheet!
When the operator has completed entry of a number of conveyances which can be conveniently considered as a unit (for example a building site), then a check plot is produced at the same scale as the source conveyances. This plot is then checked by land terrier staff using a light table - the check plot being superimposed over the source conveyance and then being thoroughly scrutinised with recommended edits clearly marked.This check exposes genuine errors, differences in interpretation and also highlights those transactions where the land terrier officer has knowledge of existing problems.
Differences in interpretation are, in most instances, quickly resolved by the land terrier officer, although occasionally more prolonged discussions are required. The following scenario perhaps illustrates why detail interpretation can cause conflicts:
A further example which may generate problems:
Operator errors during input are inevitable but they can be minimised by a training programme which should include field trips to help develop map interpretation skills. The monitoring of input of each technician also helps to identify and therefore eradicate interpretation problems.
Identified edits are subsequently incorporated into the LIS and changes are made both to the graphics display and to the associated database to indicate that those boundaries have been checked.
During the capture of the property boundaries, information concerning the source document is entered into the database. This information is both factual (eg scale and document type) and subjective (eg the quality of the source plan). This information may subsequently be used to assist in the interpretation of boundary conflicts. If a problem exists for example due to the incorporation of a piece of land into more than one transaction, then rectification will be required. Normally the older transaction will be deemed correct and the later transaction will thus undergo rectification to ensure coincidental boundaries. If, however, an interrogation of the database shows that the source document associated with the older transaction was of "poor" quality, then the boundary in the area of conflict would undergo further investigation prior to the initiation of any rectification.
Once a rectification process has begun, its progress through the various departments of the Corporation is tracked within the database of the LIS.
As previously implied the boundaries defining a transaction, once validated, become fixed within the system unless they undergo subsequent rectification. The graphical elements within the LIS which represent validated boundaries are thus set to "locked" so that they cannot be either deliberately or accidentally modified or deleted. In an ideal situation this would be the end of the story as far as any particular boundary element was concerned. However, in reality the supply of updates to the OS digital map base necessitates the unlocking and subsequent modification of at least some boundary elements within the LIS.
Ordnance Survey issues updated map sheets to Milton Keynes on a monthly basis. These sheets are incorporated into the digital map base and the superseded map sheets are transferred to an archive directory. Now, hopefully the new sheet will contain only additions to the map base rather than modifications. Additions should have little impact upon the LIS overlays except where a boundary has been input to a theme prior to the same physical boundary being incorporated into the digital map base. In such instances the positional accuracy of the LIS boundary should be compared with the OS boundary, and adjusted if necessary. If however large scale resurvey work or major edge matching exercises have been performed on the new issue, then boundaries within the LIS will need to be investigated and perhaps modified so that physical and legal boundaries are, where applicable, again made coincident.
The archived map sheets play an important role in the checking process. By displaying both the archived and current map sheets simultaneously, along with the LIS boundaries, it is possible to determine whether or not a legal boundary which is displaced relative to a physical boundary on the updated map sheet was displaced deliberately. If this is the case, then the LIS boundary will not be altered. If however, the apparent displacement is because the position of the boundary within the map base has been moved then the legal boundary will be modified with respect to the revised data.
Similar procedures need to be adopted due to the moving of linework during the edge-matching of previously un-matched map sheets.
The incorporation of OS updates and the implications regarding the maintenance of an LIS should not therefore be underestimated. The idea of superimposing diverse information from many sources upon a common map base is fundamental to many GIS applications. If the common map base is continually being modified then the superimposed data must also be modified to maintain the required referencing system.
The input and maintenance of the legal boundaries within an LIS is not a straightforward exercise. The boundaries defining each transaction should be input without reference to the surrounding properties. If adjacent property boundaries are used then there would never be any conflicting boundaries or gaps! Transactions which are not wholly based upon existing OS map detail require to be interpreted relative to all available information. These transactions undergo not a simple transfer to the LIS but a transformation into the LIS. This transformation is obviously not a mathematical transformation but an intuitive transformation which is based upon excellent map interpretation skills.
The input of property boundaries on the Milton Keynes system was driven by four major requirements:
Without the workflows,monitoring and validation procedures which have been implemented within the Development Corporation's LIS department, the data set would not be able to satisfy these demanding requirements.
| Copyright © 2001 Ken Robinson |
