🆕Known limitations

OS carries out a flying programme each year, capturing each area of Great Britain at least once every 3 years. The aerial imagery derived from this capture programme can be impacted by seasonal variances, that is, the time of year when the area was flown over, or even the time of day. Earlier in the year vegetated features may be captured with leaf-off, and long shadows are created which may impact the quality of the feature’s classification and width measurements.

Additionally, the automated process uses aerial imagery as a top-down view to classify Field Boundary features so some features may have their true nature obscured by overhanging trees. For example, fences running through woodland can be obscured by trees and therefore classified as Tree Canopy.

Limitations exist with existing OS NGD Structure lines, which is output in Field Boundary data:

1. Where the OS NGD Structure Line data is an ‘Edge or Limit’ of vegetation change rather than ‘Built Obstruction’ the Field Boundary is not classified.

2. Where features are close together and parallel e.g. a Ditch and a Hedge, the capture specification for NGD features will generalise and select one of the features, in this example the Ditch rather than the Built Obstruction Line is captured. This generalisation results in no Field Boundary features being classified and can commonly occur in low-lying areas. Additionally where Built Obstruction Lines are closely aligned, only one of these is picked up by the automated process and classified as a Field Boundary feature.

3. Moorland areas have a positional accuracy (RMSE) of 4.09m. The automated process has a search buffer of 2m to find potential vegetated Field Boundary features and 3m buffer for ‘Wall’ features. A search buffer any higher than this will result in many false positive classifications with the surrounding area, therefore some features in Moorland areas maybe inaccurately positioned.

4. Field Boundary features are classified through areas of woodland. This can occur when Built Obstruction Lines exist across areas of woodland and were classified when still visible from aerial imagery. Due to progressive changes in the natural environment, the trees have grown over time and obscured the underlying Built Obstruction Line. Classification from imagery will always classify the Field Boundary feature as 'Tree Canopy’ even if in the real world this is a fence.

Data Quality

There are known areas of Great Britain where the data quality maybe lower. See Known Data Issues for additional information.