Problem with CRS metadata for UK Environment Agency LiDAR DSM tiles?

Decoding the Mystery: When UK LiDAR Data Gets Lost in Translation (and How to Fix It)

So, you’re diving into some sweet UK Environment Agency LiDAR data, ready to map floods, monitor habitats, or just geek out on terrain models. Awesome! The EA has been capturing this stuff since way back in 1998, building a massive archive covering most of England. Seriously valuable stuff. You grab a Digital Surface Model (DSM) tile, drag it into your GIS software… and BAM! It’s floating somewhere off the coast of Africa. What gives?

Yep, you’ve stumbled upon a classic head-scratcher: missing Coordinate Reference System (CRS) metadata. It’s like forgetting to put a return address on a letter – the software just doesn’t know where the data’s supposed to be.

Here’s the deal: some of these EA DEMs (Digital Elevation Models) are missing that crucial CRS info. Your GIS, being the helpful but sometimes clueless sort, defaults to WGS 84 (latitude/longitude). Now, the UK uses the British National Grid, a totally different system based on meters. Hence, the unexpected trip to West Africa. I remember the first time this happened to me; I spent a good hour convinced I’d downloaded the wrong file!

Why does this happen, anyway? Good question. The truth is, it’s not always clear. Maybe it’s a quirk of the data format. The EA often provides data as zipped archives of text files, which, while super compatible, might lose CRS info during conversion. Or perhaps it’s a legacy thing. Data capture standards evolve, right? Older tiles might predate the consistent use of embedded CRS data. It could also be due to how the EA creates those big composite datasets, merging data from different surveys. If the source data is inconsistent, or the merging process isn’t perfect, metadata can get lost in the shuffle. The newest and highest resolution data takes precedence in the merging process.

Okay, enough with the problem. Let’s talk solutions! Luckily, this is a super easy fix. You just need to tell your GIS what the CRS should be. The magic code is EPSG:27700, also known as OSGB 1936 / British National Grid.

In QGIS, it’s as simple as right-clicking the layer, going to “Set CRS,” and punching in that code. Boom! Your data snaps back to the right spot. While you’re at it, set your project CRS to EPSG:27700 too, to keep everything playing nicely together.

But hold on, there’s more! Just fixing the location isn’t the whole story. Here are a few extra tips to keep in mind when wrangling EA LiDAR data:

• Metadata is your friend: The EA provides metadata catalogues that tell you exactly which survey was used to create the composite. Dig into these – they’re goldmines of info.
• Accuracy matters: The EA aims for a vertical accuracy of around ±15cm. That’s pretty darn good, but keep it in mind when you’re doing your analysis. Newer surveys can be even better, down to ±5cm!
• Watch out for coastal weirdness: Those composite datasets can sometimes have glitches near the coast, like odd bumps or dips. This is often due to merging surveys taken at different tides.
• Transform and conquer: The EA uses a specific transformation (OSTN15/OSGM15) to convert between different coordinate systems. Knowing this helps ensure everything lines up perfectly.
• Stay updated: The Environment Agency is constantly updating its LiDAR data. Always check the Defra Data Services Platform to make sure you’re using the latest and greatest.

So, there you have it. Missing CRS metadata in EA LiDAR data can be a pain, but it’s a totally solvable one. By understanding the issue, knowing the magic EPSG code, and following these best practices, you’ll be well on your way to unlocking the full potential of this incredible resource. The Environment Agency deserves kudos for making this data available, and with a little know-how, we can all make the most of it. Now go forth and map!