NFI methods for NCEA surveys
Source: environment.data.gov.uk (2026-01-05)

National Woodland Bat Survey - Example Dataset. Unvalidated automated bat species classifications derived from acoustic recordings made at a sample of three National Forest Inventory squares, surveyed as part of the National Woodland Bat Survey. Data from additional squares are available on request. 
The National Woodland Bat Survey is an England-wide passive acoustic bat survey conducted across National Forest Inventory squares. Bat echolocation calls within audio recordings are detected and classified to species using automated machine-learning classifiers. These sample data have not been validated, so will include incorrect species classifications. They are of use in applications where it is preferable for classification uncertainty to be modelled and incorporated into analyses, and inference is made at regional or national scale, thereby reducing the impact of individual misclassifications. An alternative dataset of validated and accepted species records will be made available at a future date.
Data are georeferenced using a delocalised grid. The bounding coordinates for each grid square are included in the dataset. Further information about the delocalisation process is provided in the dataset evidence pack.
The 'National Woodland Bat Survey - Example Dataset’ is a project part-funded by the UK Government through Defra’s Natural Capital and Ecosystem Assessment programme (NCEA).  Users should ensure they have read the supporting documentation before viewing or making use of the data. This includes the NCEA Analysis Ready Data Limitations Statement and dataset specific limitations/caveats outlined in the supporting documentation.
 Attribution statement: © Forestry Commission & Bat Conservation Trust copyright and/or database right 2025. All rights reserved.

1 
 
 
 
NFI methods for NCEA surveys 
 
Version: 
1.0 
Version status: 
Approved by Sheila Ward [Head of Information and Statistics]  
Version date: 
18/11/2025  
Author(s): 
Information provided by Vera Correia [NFI Team Manager], Chloe Metcalf 
[Spatial Scientist / Statistical Analyst], Matt Wilkinson [NCEA Programme 
Manager], Simon Dunn [GIS Specialist] and Mark Oram [Field Station 
Manager] 
Compiled by Chloe Bellamy [Senior Spatial Scientist]  
Purpose: 
To provide an understanding of how the NFI survey is set up, including 
delocation requirements and methods, and how this might impact NCEA 
datasets from work packages which use the survey locations.  
 
Description: 
Contains: 
• 
NFI overview 
• 
NCEA reporting strata 
• 
NFI square selection 
• 
Landowner contact and access permissions 
• 
Square allocation 
• 
NFI square delocation 
• 
References 
 
NFI overview 
The National Forest Inventory (NFI) provides a comprehensive record of the size and 
distribution of forests and woodlands in Great Britain, along with information on key forest 
attributes. National-scale woodland surveys and forest inventories have been conducted in 
Great Britain since 1924. The NFI, launched in 2009, is the latest in this series of surveys. It is 
set up to continue on consecutive 5-year cycles, gathering data across Great Britain in line 
with responsibilities under the Forestry Act (1967), related legislation and international 
commitments. 
A core principle of the NFI is, and always has been, that all information collected is treated 
with the strictest confidentiality. It is never used to police, regulate, or directly influence the 
management of individual woodlands. Data is published only in summary formats that do not 
disclose details about individual woodland holdings. This principle underpins the entire NFI 
survey, ensuring that potentially sensitive information is never made publicly available. 

 
 
2 
 
Since 2023, the Defra-funded NCEA programme has integrated additional environmental and 
ecological monitoring surveys within the existing NFI estate and infrastructure. To ensure that 
NCEA data complies with the NFI confidentiality policy described above, it is necessary to 
‘delocate’ NCEA NFI squares and survey locations, preventing the publication of precisely 
georeferenced survey results. 
The NCEA programme was established by Defra with the overarching aim of producing a 
baseline of England’s natural assets by 2028, describing their status, extent, and condition. 
Future surveys beyond the current programme will be required to detect changes or trends 
against this baseline. Consequently, trend analysis is outside the scope of the current 
programme. 
 
NCEA reporting strata 
Metrics are calculated for the following areas/strata: 
• 
Each NFI square surveyed. 
• 
The eight English NFI regions: East England, East Midlands, North East England, North West 
England, South East England and London, South West England, West Midlands, Yorkshire and 
the Humber. 
• 
England. 
• 
Three NFI woodland types. These are assigned to NFI squares using their Interpreted Forest 
Type category (Forest Research, 2020), which is derived from a remote-sensing derived map 
(excluding tree nurseries and orchards) and administrative records: 
 
Conifer: Some broadleaved trees may also be present, but greater than 50% of the 
area will consist of conifers. This includes the Conifer and Mixed mainly conifer IFT 
categories. 
 
Broadleaved: Some conifer trees may also be present, but greater than 50% of the 
area will consist of broadleaves. This includes the Broadleaved, Mixed mainly 
broadleaved, Coppice with standards, and Coppice IFT categories. 
 
Other, which includes the Transition (Felled, Ground Prep, Young trees, Shrub, Failed, 
Windblow), Cloud/shadow, Low Density, and Uncertain IFT categories. 
• 
[Optional] Ownership (Forestry Commission or Private Ownership or other Public Sector 
Ownership (e.g. MoD, Highway Agency, local authorities)) 
 
NFI square selection 
To ensure the sample is representative of woodland types across England, a stratified random 
sampling approach is employed. Each year, 270 NFI sample squares are randomly selected by FR 
statisticians (including 60 reserved as backup squares). These squares are stratified by NFI region, 
woodland type (described above) and ownership (Forestry Commission or Private Ownership). 

 
 
3 
 
For each region, the area occupied by each forest–ownership pairing (e.g., Broadleaved+Private 
Ownership, Conifer+Forestry Commission) is calculated and expressed as a percentage of England’s 
total woodland area. A total of 210 sample squares are then randomly selected to match these 
regional proportions as closely as possible, by iteratively selecting squares containing the required 
forest–ownership pairing with a weighting towards those containing a higher proportional coverage 
of this pairing.    
 
An additional 60–70 back-up squares are selected to accommodate a potential 30% rejection rate, 
which may occur due to: 
• 
Inaccessibility 
• 
Unsuitability, identified during desk-based assessment or first field visit 
• 
Lack of landowner permission 
This back-up allocation is currently being refined based on the observed proportion of inaccessible 
squares during previous years’ surveys, and may be expanded to include a back-up for every selected 
square in future years.  
The NFI distinguishes between permanent and temporary sample squares: 
• 
Permanent squares are re-measured in each inventory cycle, allowing for long-term 
monitoring. 
• 
Temporary squares are replaced between cycles and used for one-time assessments. 
For this survey, only permanent squares are selected, where possible, to support consistent 
longitudinal analysis. Squares that have been surveyed or marked as inaccessible in previous years are 
also excluded from the selection pool. 
Prior to commencing the surveys, a desk-based assessment is conducted using recent aerial imagery 
to verify forest type classifications and assess suitability, and a backup square substituted where 
necessary. 
Landowner contact and access permissions 
Landowners are contacted in advance of each NCEA survey season via letter to request access 
permission and to identify any potential access constraints. If no response is received, local survey 
teams attempt direct contact using available contact details, public records, and local knowledge. 
If access is explicitly refused, the square is removed from the survey and replaced with a designated 
backup square. In cases where no response is received, surveyors may proceed with access under 
assumed permission. This is permitted under Section 48, Subsection I4 of the Forestry Act (1967), 
which grants Forest Research staff and contractors the legal right to access all forested land in England 
for survey purposes. 
Square Allocation 
Survey squares are assigned to surveyors based on proximity to reduce travel times and balance 
workloads, with the aim of completing surveys within the ideal survey window (1st July– 15th August). 

 
 
4 
 
Before fieldwork begins, surveyors receive the survey protocol, landowner correspondence, GIS site 
data (including contact details), and relevant health and safety information. 
Surveyors use a dedicated ESRI Fieldmaps survey to access data on square boundaries and record 
survey information including sensor deployment locations. This information is used by the project 
team to monitor progress. 
Surveyors attend online briefings prior to fieldwork, and less experienced staff may shadow 
experienced colleagues to become familiar with the protocol. Throughout the survey period, 
surveyors are encouraged to contact the FR or BCT project teams with any queries. 
Dynamic risk assessments are conducted on-site, and surveyors are trained to make safety decisions, 
including relocating audio sensors if necessary. If site hazards cannot be mitigated to a medium risk 
level—especially where access is based on assumed permission—surveyors may halt work and request 
an alternative survey square. Replacement squares are first selected from designated backup squares. 
If a suitable square is not available, a new square is chosen based on geographic proximity to the 
original site. Replacement squares are matched to the woodland type, region and ownership category 
of the original square wherever possible. 
NFI square delocation  
NFI square locations are confidential, precluding the publication of precisely georeferenced survey 
results. For the purposes of data sharing, visualisation and to allow pairing with other data sources, 
an approximate area within which the survey took place is instead reported. NFI squares are assigned 
to an irregular grid of delocation tiles, which was created by overlaying a 5 x 5 km grid over a map of 
woodland, and grouping grid squares until they contained 200 – 2500 ha of woodland. Due to the 
uneven spread of woodland across GB, the size of each aggregated tile is uneven. 
This analysis was performed independently within each region; however, to maintain rectangularity, 
groupings were permitted to extend beyond regional boundaries. Consequently, some groupings will 
overlap region borders or extend into marine areas. Where this is the case, only woodland located 
within the focal region contributed to the ≥ 200ha woodland threshold, and woodland located outside 
the region was excluded from woodland area calculations. 

 
 
5 
 
 
Figure 1 Example of delocated sample site plot locations – NFI Soil Fungi & Invertebrate Communities & NFI Soil 
Chemistry 
References 
Forest Research, 2020. NFI Survey Manual – Third Cycle, Chapter 4: Section Stratification. Forest 
Research, Forestry Commission. Available from https://www.forestresearch.gov.uk/tools-and-
resources/national-forest-inventory/nfi-survey-manuals-for-field-samples/