Initial Data Capture and Update Routes

The data has been captured by four routes:

• Digitizing existing field documents;
• Scanning and vectorising existing field documents;
• Field survey;
• Air survey.

These have different approaches to data capture and produce different outputs.

Digitizing.

The vast majority of the data has been captured by blind digitizing of the field documents. This involves tracing out the graphics and adding feature coding on a digitizing table, and then checking against the original record. Centre lines are added for roads, double water features, railways, named paths and paths in parks and forests. Squaring is also applied to buildings (unless they are classified as non-square), so that they appear square on printed maps.

Scanning & Vectorising.

This was introduced to speed up production but was found to be ineffective. It created separate sets of solid lines, pecked lines and text, which were manually identified, and feature coded.

Graphical Field survey

Field survey uses many data capture methods, Electronic Distance Measurement, Global Positioning System, Mobile pen-computers and traditional surveying methods. Together these make up the main route for updating OSNI data. The surveyor takes the current map (in either hardcopy or digital form) into the field and either records changes or resurveys an area. The data is either fed back for digitizing or updating.

Files are updated either as part of the Periodic Revision (PR) or Continuous Revision (CR) programmes. PR operates three cycles: -

· 5 year cycle - Built up urban areas

· 7 year cycle - High density rural areas

· 10 year cycle - Low density rural areas

The CR programme takes in major change in urban areas (roads, building development) and can react to customer requests (Govt. Departments, Public Bodies and Private customers)

The surveyor takes a copy of the map into the field either as a Master Survey Document (MSD), using traditional methods to record changes to the MSD, or as a digital file (DST) installed on a GEMS hand held computer. On completion in the field, MSD's and DST's are returned to HQ for digitising or post-field processing.

Large surveys need control points, fixed by Global Positioning System (GPS) equipment, from which, surveys are carried out using Electronic Distance Measuring equipment (EDM).

OSNI is in the process of introducing new technology into field survey, which will ultimately give the surveyor the ability to record, structure and add associated data, to changes in the field. This will eliminate post-field processing of DSTs.

Air Survey

A large part of the survey work is undertaken by aerial photogrammetry, for resurvey and map revision purposes. The photography is scanned and the digital data is used in ATLAS photogrammetry systems. Once controlled and rectified, the images are overlaid on the existing Large Scale digital map and compared for change and error. Changes to the topography are identified, and three files are produced, one each for:

• Additions (new features)
• Supressions (deletions)
• Shifts (correction of position due to merging of survey methods)

These files are passed to digital update. Textual information, such as identification of objects, has still to be collected by field survey.

Digital Update

Updating procedures are carried out on database files, incorporating the changes identified by Field and Air Survey. These changes are date stamped for archival purposes, and any associated data is amended.