7 things Ecologists get wrong carrying out a Biodiversity Net Gain assessment in QGIS
Introduction
You’re an ecologist, I don’t need to tell you BNG is complex.
I’ve been helping clients with the mapping for BNG for some time, integrating pre and post development data, exporting to the metric. Then last week I went on a CIEEM BNG training course, and I now see the full picture. Wow, I’m in awe of the range of skills ecologists need.
So, if you are an Ecologist trying to get to grips with all the mappy-data bits of BNG, here is the Maplango listicle of 7 things Ecologists often get wrong carrying out a BNG assessment using QGIS.
1. Starting from scratch
Yes, you need a QGIS project. But, no, you don’t need to do it all yourself. Natural England (NE) have helpfully released a BNG-QGIS template. There are some clever elements to the project, and you can adapt to your needs.
2. Not configuring QGIS for use in the UK
A map projection enables the 3D spheroid-shaped world to be displayed on a 2D surface, like a piece of paper, PDF, or computer screen.
Out of the box, QGIS defaults to a globally applicable projection (called WGS84) but BNG projects in the UK almost always use the British National Grid (the original BNG!). This uses Eastings/Northings coordinates and, importantly for BNG calculations, metres for distance & area measurements.
To set QGIS to default to BNG go to Settings > Options > CRS > Use a default CRS > Search & set to 27700 (the EPSG projection code).
3. Not using good quality background data
Background data is not included in the NE template so you need to source it yourself. QGIS makes it easy to connect to Web Mapping Services, which allow you to add background data to your map without downloading anything, in the same way you use Google maps on your phone.
For the best available aerial images you’ll need to purchase something like OS Mastermap Imagery from an Ordnance reseller. As a fall back you could consider Google satellite. You can add this to your project quickly and easily using the QuickMapServices plug-in.
In addition to aerial images, you will also need accurate boundaries of your site and surrounding land. Ordnance Survey now also make their high-quality background mapping data available as web services via osdatahub. Sign up for the premium plan and you even get some Mastermap data free each month, giving you unrivalled context.
4. Not using an app to collect data in the field
If you are still doing your field survey using paper and pen, you are likely adding time to your project by having to digitise the data in QGIS back in the office. You may also inadvertently be introducing errors like positional change during geo-referencing, mis-interpretation of field notes, and typos in the transcription of data.
If you aren't already, it's really time to start using an app to collect data in the field. It's cheaper and easier than you might think. Mergin maps allows you to use your QGIS projects on an Android or IOs device, meaning you can take the NE template project with you in the field.
5. Not knowing how to work with CAD files in QGIS
It is very common for (landscape) architects to provide post-development plans in an AutoCAD format. You’ll need to add to your QGIS project and interpret to create a dataset of post-development habitats.
Often the contents, format and projection of the DXF files you receive is likely to vary quite a bit. You will need to learn to scale and reposition them (see v.transform function) and extract relevant information (e.g. select by attributes).
To get started, though, you can simply drag and drop a DXF file to load it into QGIS, though I would recommend using ‘Another DXF converter’ plug-in. This will convert it to a GIS format and maintain the styles used by the (landscape) architect, making the DXF a lot easier to interpret.
6. Not digitising accurately
BNG is driving a higher level of data standards. Previously you might’ve gotten away with poor quality digitising (gaps, overlaps and slither polygons) because you just needed to display a map in a report. Now, there’s nowhere to hide. Clients expect data outputs, and BNG requires 100% coverage of the site. Plus, spatial queries in QGIS won’t run if data is invalid.
To capture GIS data quickly and efficiently, and not introduce digitising errors, you need to get to grips with the snapping options. I recommend: 20px, avoid overlap, and enable tracing.
7. Integrating parcels one-by-one
The NE QGIS template is set up to integrate baseline and post-development data, which is a big help. And it’ll help you track change in a parcel, an essential part of BNG. But if you have a lot of small features on your site (quite normal, say, on a housing development), then you will probably want to speed things up and combine baseline and post-development layers in a more automated way.
For more complex sites, I recommend using the Union geoprocessing tool. This creates separate features for overlapping and non-overlapping parts and maintains attribute values from both layers for overlapping features.
Conclusion
Used correctly, QGIS is a big help in delivering BNG assessments. But I do sympathise with ecologists who feel they are now having to be part ecologist, part data-scientist. A lot is being asked of you. If you need to upskill with QGIS there are a few options.
If budget is an issue, and time less so, then it is possible to learn the skills you need via free resources. There are umpteen QGIS YouTube tutorials online. For advice, the QGIS for Ecologists facebook group and technical fora like Stack exchange will come in handy.
Personally, I don’t think it’s too much to ask for employers to support and invest in staff as they develop BNG skills and set up new workflows. Professional training will pay off massively in terms of time saved.
If you would like to find out more about how Maplango supports ecologists, building their capacity and confidence, give us a call for a free 30-minute consultation. And if you have a specific query, we’ll try to solve it there and then.
Psst: we do run a limited number of courses each year with CIEEM (you’ll have to be quick though, they sell-out fast).