Positional accuracy improvement programme

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Land-Line FAQs

I have loaded a 1:2500 scale map tile where part of one edge does not match the data on the edge of the adjacent tile. Why is this?

During the production of accuracy improvement of rural towns and other rural areas blocks any data position discrepancies or real-world change at the external edges of these areas are resolved as soon as practicable in the map tiles adjacent to them.

These adjacent map tiles are not reissued as part of the PAI block as their complete map tile areas have not been improved, but depending on customers' supply parameters, they could be received before the issue of PAI map tiles. This is a temporary situation during the initial delivery stages of the PAI programme blocks.

A similar situation will occur where a block of PAI map tiles are reissued and the map tiles adjacent to the block are not received by customers until their normal resupply occurs. This will occur throughout the PAI programme. Our improvement programme has two main threads:

• the resurvey of selected="selected" rural towns; and
• the accuracy improvement of the remaining rural areas.

The resupply of these areas will either be by all the map tiles covering a rural town or by 10km by 10km block (sometimes a 20km by 20km block). This will reduce any edgematch discrepancies, although inevitably there will still be external edges to those blocks of work (unless the adjacent tile is a 1:1250 scale map tile, 1:10, 000 scale map tile or a 1:2500 scale map tile already at an acceptable accuracy). At the external edge, any improved data (where the position has changed and the feature extends beyond the edge) will be improved into the adjacent map tile if it is currently unimproved.

However, the link file will only be produced for use up to the tile edge. To prevent problems with customers' data at the tile edge, the link file creation software ignores positional accuracy improved data on the before map if it is within 100 metres of the tile edge. Thus a control point that would give a false reading to the normal shift of data within any area will not be created.

Overall, the edgematch issue is comparatively small; it is estimated that, nationally, less than 8,500 of the 620,000, 1:2500 scale map tile edges will be affected at any one time. That said, you will need to recognise that discrepancies between detail across tile edges will occur during the course of the PAI programme. Data presentation may as a result be affected during this period, which could impact on business services (internal and external) that you provide.

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How will I identify the improved maps and features in Land-Line?

The improved maps will be reissued separately from your normal regime.

The rural towns will be surveyed to ±0.4m absolute accuracy. The CONT_IND (content indicator) field will be updated from B to A where the extent of the rural town covers the whole map and from B to AR where the extent of the town does not cover the whole map tile. In most cases they will also be stored at 1:2500 scale, so the SCALE field will be set to 2500. These two fields together will normally identify most improved maps. However, as there are quadrantised plans in existence (that is, where the SCALE value will be 1250, though maintained as 1:2500 scale overhaul maps), there could be a few instances where we resurvey these to ±0.4m absolute accuracy. In these cases, improved maps will not be separately identifiable from existing 1:1250 scale map data.

The other rural areas will be improved to ±1.1m accuracy and the CONT_ID field will be R.

Feature attributes

To satisfy customer requirements, the improved Land-Line map data tiles now contain an additional attribute (CHG_TYPE) at feature level that identifies the reason for change. The attribute values are:

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