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OS MasterMap FAQs
Is the PAI programme going to improve OS MasterMap data in preference to Land-Line map tiles?
No. It can be considered that we only have one large-scale topographic database and it is this that is updated with positional-accuracy-improved data, new surveys, revisions and deletions. Land-Line and OS MasterMap are sourced from this single database.
Should I wait until the PAI is finished before migrating from Land-Line to OS MasterMap?
It doesn't matter which map product you are using as the effort will be the same. The improvement programme will only affect 1:2500 scale map tiles that have not been previously improved to a higher survey accuracy level. If you are a Land-Line customer, the improved map tiles will be issued to you as a separate issue from your normal supply and accompanied by link files. If you are an OS MasterMap customer, the improved areas will appear as part of your change-only update supply and you can choose when to accept the change.
Whether your map base is Land-Line or OS MasterMap, you will have to decide how and when to manage any resultant discrepancies in relative position between your own assets and the improved position of the map detail. As the two products are sourced from the same database, the position of the map data will be the same in both. So if you have moved your assets in Land-Line because of PAI, you will not have to do it again when moving to OS MasterMap.
Similarly, if a map area has not been accuracy improved before you have moved to OS MasterMap, you will only have to move your assets when you receive change-only update supply that contains new positional-accuracy-improved map data. The amount of effort to reposition your assets will be the same with either product and will only have to be done once.
Having said this, there a growing number of software developers that are designing transformation processes using OS MasterMap as a base for recognising shapes to transform customers’ data. These will not necessarily require link files. Where this is suitable it may be more efficient to move to OS MasterMap before dealing with any PAI alterations to your data.
How will link files apply to OS MasterMap, which is not tile based?
The PAI programme improves the absolute accuracy of map data areas in large blocks – usually in a 10km by 10km or 20km by 10km format. Link files will be issued offline initially and supplied in 1km by 1km .csv format and cover the blocks of PAI data issued to OS MasterMap through the change-only update process.
How will I identify the improved maps and features in OS MasterMap?
The accuracy-improved map data will appear as part of your change-only update and can be identified by feature-level inspection of the ChangeHistory attribute (reasonforchange value = 'Position'), together with its change in version number and date.
AccuracyofPosition shows the recorded absolute accuracy of a feature. The two used as a result of PAI are:
- 1.0m – for accuracy of features that have been resurveyed in rural towns.
- 2.5m – for accuracy of features that have been resurveyed in other rural areas.
The accuracy figures of 1.0m and 2.5m are based on 95% confidence levels. All major urban areas are also recorded at 1.0m accuracy and these have not been subject to PAI, and some features are already at 2.5m accuracy because they were historically surveyed at that accuracy. These may be inspected during the PAI programme but should not need to be improved.
ReasonforChange shows the latest reason for the existence, modification or attribute change of a feature. The full list of reasons for change are listed in the OS MasterMap user guide, but the one that records PAI movement is Position.
In most data capture or edit sessions, however, only the last event that has occurred on a feature in any one editing session is recorded at this attribute. There is a hierarchy of attributes that ensure that the most important event is recorded when an editing session is finished. For example, if a building is moved for PAI and then has an extension added to it, the last recorded change is Modified (for real world change). If it has an attribute change also during the same edit session the last recorded change is still Modified for real world change.
ChangeHistory shows a history of the reasons for change that have been recorded against the ReasonforChange attribute, including the latest ReasonforChange. This attribute is, in theory, the best place to search for whether PAI change has occurred on a feature provided that:
- PAI was the last major event of any data capture session (see explanation in ReasonforChange paragraph above);
- your GML reader can read the change history attribute (We understand that some only read the most recent change history attribute by default); and
- that all features in a PAI area have needed to be accuracy improved. If a feature is already in an accurate position and does not need to be moved for PAI then there are no changes made to its attributes since nothing has happened.
How can I keep the non-PAI areas of OS MasterMap up to date without including PAI data into my holding while I make the necessary shifts?
OS MasterMap customers with a current contract have the ability to order smaller areas of data from within their contact area. They can therefore exclude the PAI area from their order, but still receive real-world change (RWC) for the rest of their area of interest.
If you do not have a contract that enables you to order smaller areas of data, due to your contract being set up under version 1 of the OS MasterMap online service, you can set up a new contract by emailing Account Management Support (using the link below) or telephoning 023 8030 5520. When you are ready to accept PAI data, you can create a polygon that covers the PAI area or order a resupply of the whole of your area of interest. You can also specify the date from which you want to receive the update.
Email: Account Management Support
I want to obtain a version of OS MasterMap just before my area is PAI’d. When do I need to order to ensure this occurs?
PAI data is normally supplied to Land-Line customers towards the end of the month it is programmed for, though the data will have started to be archived from about the middle of the same month. Therefore when requesting change only update for an area just before it is PAI'd the latest date for on-line ordering should be at the beginning of the month in which PAI is to be supplied. If you are requesting OS MasterMap for the first time then the order should be placed by the middle of the month before the PAI supply month. The supply month for a PAI block of work is available on the Programme and Maps pages of this website.