This map shows the locations of bat roosts identified during visits by Natural England volunteer bat roost visitors (VBRVs) to dwellings, churches and public buildings between January 2013 and September 2018.
The visualisation uses two Leaflet plugins: Leaflet.markercluster and Leaflet.heat. These were easy enough to implement and are similar in versatility to the marker clustering library and heatmap layer available with the Google Maps API.
However there are drawbacks to using Leaflet for this map – most of the bat roost locations are at building level, and Leaflet doesn't provide the ready access to aerial and street imagery available with Google Maps.
The map displays 7,401 geocoded records extracted from a dataset produced by the Bat Conservation Trust (BCT) and published via the NBN Atlas: Natural England bat roost visit records from 2013 onwards.
The data is ostensibly re-usable under the Open Government Licence, though the NBN also requires users to agree the NBN Atlas terms of use. I obtained my copy second-hand from GBIF, which re-licenses the dataset under CC BY. GBIF also requires data users to agree additional site terms but they are less problematic than the NBN's.
BCT's bat roosts dataset contains occurrence records from January 2013 onwards and is regularly updated, most recently to the end of September 2018. The records describe sightings or other evidence of bat roosts at dwellings, churches and public buildings where there have been issues with bats or work is proposed that may affect bats.
Bats also roost in other types of location, of course, so mapping this dataset does not provide a reliable picture of how bat populations are distributed in England. There are many other datasets about bats available from the NBN Atlas, and from other sources.
Most of the occurrence records in the BCT datasets are geocoded to individual buildings. (A few of the records in the BCT dataset seem to have incorrect coordinates, so the map shows some markers in the English Channel and other odd places.)
There are a couple of data protection considerations for users of this dataset.
The occurrence records include recorder names, which are mostly identifiable individuals and therefore personal data. This is a common practice in biological recording, though I'm not clear which lawful basis the NBN is relying on to publish the names. In any case users of the data will need to consider whether they have a lawful basis to process this personal data for their own purposes, and also bear in mind that the OGL does not cover personal data. (I have not used the recorder names to produce the Leaflet map above.)
Many of the records also identify individual residental properties. The dataset does not contain address fields, but the coordinates are mostly specific enough that any user with access to Google Maps will be able to match the record to a property. There is a wider debate about the extent to which information about a home is also information about its owner or occupier. Some homeowners may argue that the presence of a bat roost in their home is personal data and that disclosure of this information could harm their interests. Bats are not pests and don't usually cause property damage, but some people are iffy about them. As bats are a protected species, the presence of a bat roost can reduce the potential for renovation of a property and this can discourage some prospective buyers.
The BCT website has useful information about the different types of bat commonly found in the UK, with photos.