Trees Outside of Woodlands

Welsh Government

The Trees Outside Woodland (TOW) for Wales dataset comprises polygon features representing tree canopy cover located outside the National Forest Inventory (NFI) woodland extent. Included features have a minimum tree canopy top above 3m in height and more than 5m2 in area. Canopy extent is then measured down to 1.3m above the ground. There may be areas of contiguous tree canopy greater than 0.5 hectares within this TOW dataset if they do not meet all parameters for inclusion to the NFI. However, for areas of woodland more than 0.5 hectares the NFI should be used.

Canopy polygons were generated from continuous patches extracted from Canopy Height Models (CHM) derived from the Wales LiDAR Survey. Each polygon includes calculated area attributes, a classified woodland type, and associated height metrics. Canopy polygons are further subdivided into defined height-class polygons. This data was created by the Forest Research ‘Remote Sensing Team’, in February 2026.

To note: To maximise drawing performance, the data is only visible when zoomed to 1:100,000 or closer. 

Woodland type classifications are derived using a rule-based spatial algorithm that evaluates:

  • The number of individual treetops detected within each canopy polygon;
  • The polygon area; and
  • Polygon geometry (e.g., roundness).

These parameters are used to assign each feature to one of four woodland type classes:

  • Lone Trees – Single trees, shown as small, separate canopy areas with usually one treetop within them.
  • Groups of Trees – Small clusters of trees growing close together within the same canopy patch.
  • Small Woodlands – Larger, continuous tree areas that meet a minimum size or number of trees.
  • Overhanging Canopy (NFI OHC) – Tree areas that hang over or are next to National Forest Inventory woodland.

Height Classes

Canopy polygons are stratified into height classes based on derived canopy height metrics. The defined height bins are:

  1. Class 3: 1.3 m – <10 m
  2. Class 10: 10 m – <15 m
  3. Class 15: 15 m – <20 m
  4. Class 20: 20 m – <30 m
  5. Class 30: ≥30 m

These classes enable structured analysis of canopy structure and vertical distribution.

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Attributes (16)

Attribute name Label Description
tow_id Trees Outside of Woodlands ID
tow_area Trees Outside of Woodlands Area (m2)
woodland_type Woodland type
height_class Height class
bin_area Bin area
min_height Minimum height
max_height Maximum height
mean_height Mean height
stdv_height Standard deviation of height
sub_id Substitute ID
lidar_year LiDAR year
lidar_date LiDAR date
british_gr British Grid
type_and_height
math_o_goedwig Math o goedwig
geom

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Type:
Spatial data
Category:
Environment

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Publication date:
Licence:
Open Government Licence for Public Sector Information (OGL)

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None

Keywords:
TOW
Point of contact:
Edition
1
Language
English
Data Quality
<p>A Canopy Height Model (CHM) was created using a Digital Surface Model (DSM) and Digital Terrain Model (DTM).</p> <p>The CHM was then masked, to create a Vegetation Object Model (VOM), using several datasets available to FR. This step removes non-vegetation features from the raster.</p> <p>Existing NFI Woodland is also removed during this step.</p> <p>Canopy delineation was then carried out, to create the Trees outside of Woodlands polygons. This canopy was then split into height bins and attributes assigned.</p> <p><u>Data Processing and Additional Notes</u></p> <ul> <li style="list-style-type: none;"> <ul> <li>Canopy features with an area of less than 5 m&sup2; were excluded from the dataset.</li> <li>A treetop detection algorithm was applied to the CHM, with a minimum treetop height threshold of 3 m. Consequently, any polygons with a maximum canopy height below 3 m were excluded from the dataset.</li> <li>The 2024 National Forest Inventory map was used to remove NFI woodland areas from the TOW dataset.</li> <li>Small woodland polygons were further subdivided into groups of trees when identified as thin, linear arrangements, providing a more accurate representation of actual canopy structure.</li> <li>The overhanging canopy areas (NFI OHC) should not be included in routine forest statistics, but may be included when considering the total canopy area of trees across Wales.</li> </ul> </li> </ul>
Supplemental Information

<p><em><u>Attribute Name Description</u></em></p>
<p>TOW_ID (unit: N/A): Unique identifier for each TOW canopy polygon. The ID suffix corresponds to the Briti…

Spatial Representation Type
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