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So, how many OGC layers are there?

Posted on 2018-07-18

Updated: 2018-09-27 with numbers for September 2018 which also reflects improvements to how we group things together.

One of the questions we come across quite often is the deceptively simple "How many layers are there"? At the time of writing our front page says "over 1 million distinct... layers", so that's the answer right? Well, not quite, and why is that "distinct" in there anyway? There are actually quite a few potential answers so lets go through them.
Note: All numbers in this post are correct at the time of writing but will certainly change within a few weeks as we continue to index more services.

That's a lot of layers
Lets start with the largest number: 1,773,337 layers. This is also the simplest number - it's the total number of layers that we find in all of the unique capabilities documents that we download ("capabilities documents" are what map servers use to tell the world what layers they have and what features they support). This is the easiest number to give, and the one most commonly given. It is "correct" in that there really are over 1.7 million layers out there across various service endpoints, but as you'll see from the other numbers, there are a few problems with using it.
Meaningless layers
We do a lot of work to try and weed out "meaningless" layers from our index. This isn't a reflection on the data inside the layers, but on the metadata in the capabilities document. For instance there's no point us indexing a layer that has a name of "1" and no other information; for all we know these layers may have great data behind them, but if there isn't even a meaningful name our users will never be able to find those layers, so we simply remove them to stop them cluttering up the results.
It's at this stage that we also remove layers that are pre-installed defaults, like the TOPP/Tasmania data that comes with GeoServer.
In total all this filtering gets rid of over 47,000 layers, leaving us with around 1,720,000 layers.
Many endpoints and the same layer
It turns out a lot of those layers are duplicates; there are many services out there which have lots of different endpoints (the URL you use to access it) that all serve the same layer(s). In fact, there is one single layer that is served by over 2400 endpoints on the same host-domain (we group services by host-domains as part of the de-duplication process). That's an exception but there are over 580 layers that are duplicated over a hundred of times on the same host-domain, and in total we identify over 717,000 duplicate layers. We don't get rid of them entirely - you may have noticed in the results that we list multiple capabilities URLs for some layers - but we don't count them as separate layers. Once we get rid of all of those, we're down to 1,055,836 layers. It's also quite surprising to see that about half of the layers out there are duplicates.
Different service types
The final component is - what happens if a layer is served up from the same server as both WMS and WFS? Or WMS/WCS/WMTS, etc? For our purposes we try and group them together and treat them as a single layer, but as you've likely noticed in the results, we do flag that a layer is available as multiple service types. There are surprisingly few of these: only 10,752 layers are used across service types. This is where we get our final, front-page number of 1,045,059 layers.
So which is it?

As you've probably gathered by now, there isn't a "right" number. We choose to use the lowest number because it's most honest for our purposes; when you search GeoSeer you're searching 1,045,059 distinct spatial layers. It's of no help to you if you get the same layer 127 times in the results because that's how many endpoints host it. Yet across all servers and endpoints, GeoSeer is searching what represents 1,773,337 separate publicly accessible layers.