Positional accuracy improvement programme

Limited Real World Change

Background

Although the PAI Programme completed in March 2006, there was one element of the programme that Ordnance Survey could not control, the weather. Wherever and whenever possible, the rural revision programme (including positional accuracy improvements to existing data) was undertaken using recently captured imagery. The aim was always to capture imagery for an area as close to PAI production work taking place so as to ensure that the maximum amount of Real World Change (RWC) ccould be included. This was one of the reasons for the apparent ‘pepper pot’ approach to the programme scheduling.

By delaying capture, there was always a risk that insufficient imagery would be available to allow completion of the programme as intended. 2004 proved to be a poor season for flying and consequently only 30% of the planned programme for that year was captured. 2005 started off slightly better but in the end it was impossible to capture all that was required and therefore a change to the process had to be made.

Please note these developments do not affect the Rural Towns programme which is already complete.

Options

Two main options were identified as potential solutions to this issue

Delay the rural revision programme (including the PAI components) until new imagery can be captured.
Use imagery previously captured in support of Ordnance Survey activities to complete the PAI component of the rural revision programme and to meet the planned completion date of end March 2006 for the PAI elements of the programme. All imagery would meet the expected high standards necessary for photogrammetric survey, however the downside of this option is that some RWC may not be captured at this stage.

Decision

In order to complete the rural revision programme with minimal delay, Option 1 was only viable if we could be certain that any remaining areas would be captured early in 2006. Unfortunately, as it is dictated almost soley by the weather, no such guarantee could be made.

After discussion with a number of customers, Ordnance Survey chose to implement Option 2. Thus the PAI components of the rural revision programme were completed as planned by the end March 2006 and were surveyed from existing imagery. A strong emphasis has been placed on the capture of imagery in this year’s flying season so as to complete those elements of RWC that remain outstanding. Every effort will be made to complete this work during 2006 if at all possible. [Nov 06 update - So far so good. Out of the 10,717 kms to be flown all bar 519 have been captured]

Implications

The main implication of using older imagery to complete the rural revision programme is that less RWC will be included. The amount of ‘missing’ RWC will be dependent upon the age of photography used. Typically imagery will be less than 4 years old however in some instances it may be that imagery is up to a maximum of 8 years old. It is important to stress that the quality of positional improvements to the data (i.e. those aspects of the PAI programmed) will be delivered to the same exacting specification as is used at present.. Points to note:

All revised ‘PAI’ data released will incorporate all of the positional accuracy improvement components expected under the wider rural revision programme
All significant change that has been captured by ground survey methods (Category A change) since the date of the imagery used for PAI survey (and that currently exists in the large scale data) will be preserved in the data.
Where topographic features have been removed from current large scale data (e.g. demolitions) these will not be re-captured even where the original feature appears on the imagery being used. Hence, these features will not be present in the revised ‘PAI’ data
Real word change that has taken place since the date of imagery and that has not been captured by Ordnance Survey using ground survey techniques will not be present in the data. It is these RWC components that are planned for capture and survey during the 2006 season.
Where a 10km*10km survey block has been partially flown as part of the rural revision programme, new imagery will be used for all those areas for which it is available. Therefore some supply blocks will contain a mixture of survey from recent and older photography.

Ordnance Survey Commitment

Ordnance Survey remains committed to capturing the imagery for the remaining areas and the outstanding RWC will be captured at the earliest possible opportunity even where the PAI components of the rural revision programme have been captured using older imagery. The outstanding RWC will be supplied as part of normal product update cycles.

Release Programme

The link provides a list of blocks (xls 46 KB) where older imagery has either been released or are in the process of being worked on and their month of release. As soon as any of these are flown and the remaining RWC becomes available we will add this information. Last Updated November 2006