The following pages contain specific advice on how to make the most of the OS NGD Field Boundary data:

• Field Boundary definition

• Field Boundary Feature Type

• How Field Boundary data is created

• Field Boundary attribution

• Known limitations

• Summary

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 also created which may impact the quality of the feature’s classification and its 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 can therefore be mistakenly classified as Tree Canopy.

Limitations exist with existing OS NGD Structure Lines, which are 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 (for example, a Ditch and a Hedge), the capture specification for OS NGD features will generalise and select one of the features; in this example, the Ditch rather than the Built Obstruction Line would be captured. This generalisation results in no Field Boundary features being classified and this 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 numerous false positive classifications with the surrounding area; therefore, some features in Moorland areas may be 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 aerial imagery will always classify the Field Boundary feature as 'Tree Canopy’, even if in the real world the feature is actually a fence.

Data Quality

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

Field Boundary definition

A Field Boundary is a line which identifies the nature of the physical barrier feature and its characteristics when adjacent to, or contained within, specific types of land cover. Field Boundary features are predominately classified in Rural and Moorland areas of Great Britain. Vegetated Field Boundary features are given height and width values to give an indication of size. As both man-made (for example, wall) and vegetated Field Boundary features (for example, hedge) are regarded as structures, they are included in the OS NGD Structures Theme.

Field Boundary data is created by an automated process. The inputs to the process are OS NGD Structure 'Built Obstruction' Lines, OS aerial imagery (25cm), Digital Surface Model (DSM) and Digital Terrain Model (DTM).

1. After new imagery is available and the topographic area updates in 10km-by-10km areas are complete, the automated process is run to classify features and calculate height and width values.

2. Field Boundary features are coincident with and reference the underlying parent OS NGD Structures ‘Built Obstruction’ Line. Field Boundary features follow the existing geometry of the Built Obstruction line.

3. Field Boundary features may be subdivisions of the Built Obstruction line where different classifications can be identified from OS aerial imagery. Where the line is not recognised as a vegetated or wall feature, the Field Boundary feature is classified as ‘Other’ (for example, fences, or newly planted hedges where features are too small to be identified from 25cm aerial imagery). Where the classification of Field Boundary features has not yet taken place by the automated process, these features are classified as ‘Unknown’.

4. Where more than one classification is present, the classification hierarchy is used to decide which classification should be applied, that is, Tree Canopy (highest) to Unknown (lowest). Any classified section of Field Boundary feature less than 4.0metres is ignored and subsumed into the surrounding classified sections based on the hierarchy.

Field Boundary features are coincident with and reference all, or part of, an underlying OS NGD Structure (Built Obstruction) Line. They represent physical barriers between adjoining areas of land. There are five distinct categories to describe Field Boundary features: Tree Canopy, Wooded Strip, Hedge, Wall, and Other. An 'Unknown' description is applied when a feature hasn’t yet been classified.

Field Boundary descriptions

Field Boundary features are predominately classified across Rural and Moorland areas in Great Britain where they are adjacent to, or contained within, topographic areas of Agricultural Land, Trees, Rough Grassland or Heath, or when they are adjacent to land in the Rural Payment Agency’s (RPA) Rural Land Register (RLR) where they are situated within urban areas in England. Features adjacent to land in the RPA’s RLR are buffered by 2metres.

See the Field Boundary Description Value Code List for Field Boundary feature definitions.

A Field Boundary is a line feature representing the field boundary features adjacent to, or contained within, areas of agricultural land, trees, rough grassland, or heath. It also includes features adjacent to land in the Rural Payment Agency's (RPA’s) Rural Land Register (RLR) that are situated within urban areas in England. Field Boundary features replicate all, or part of, the underlying OS NGD Structure (Built Obstruction) Line and are classified as Tree Canopy, Wooded Strip, Hedge, Wall, Other, or Unknown (in hierarchy order).

Height and width

Vegetated field boundary features are given minimum, maximum and average (mean) height and width values. Values are rounded up to the nearest 0.5metres and provide an indication of the height or width rather than an absolute value.

Consideration is given to the natural movement of these features and seasonal variances; therefore, during the calculation process, the Hedge width calculation includes a tolerance of 2.0metres, and the Tree Canopy and Wooded Strip width calculations include a tolerance of 3.0metres.

The minimum, maximum and average height and width values are calculated using the 10th and 90th percentile values, sampled along the length of the Field Boundary feature at regular intervals. The calculated average mean value represents the most appropriate average height or width for each section of vegetated Field Boundary. Wall, Unknown and Other features are not given a height or width value.

Imagery date

The is the date on which the latest imagery capture was gathered and used to identify the Field Boundary feature. Where a feature has not yet been classified, it is ‘Unknown’ and the Imagery Evidence Date will be 'null'.

Woodland Boundary flag

The is a Boolean flag to identify whether a field boundary is adjacent to a land cover area classified as woodland. ‘Woodland’ is defined as a land cover polygon with a classification of Coniferous Trees and / or Non-Coniferous Trees.

Vegetated Field Boundary features adjacent to an area of woodland in the OS NGD Land Theme, which include a classification of Coniferous Trees, Non-Coniferous Trees, Scattered Coniferous Trees or Scattered Non-Coniferous Trees, will have the Is Woodland Boundary attribute populated with ‘True’ and height and width values will be 'null'.