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Most of the time, when you need to host vector tiles, you’ll use one of the two main (only?) commercial vector tile hosting services: Mapbox or Maptiler Cloud. But what if you don’t want to? Maybe you want to host tiles which don’t meet Mapbox’s 500KB-per-tile limit. Maybe you have some very specific requirement which can’t be met by those services.
We’ll use Glitch: your free NodeJS server edited through a web interface.
And Tessera: a NodeJS vector tile server. You tell it where your .mbtiles files are, and it serves them through an HTTP interface.
We actually won’t use any of the provided template at all. You can start with the hello-express template.
The easiest way to add dependencies is to select “package.json” then click “Add Package”.
Actually, because of a weird disagreement between Tessera and Glitch about the interpretation of a non-standard obscure HTTP header, you’ll have to use my patched version instead. Add this dependency directly into the package.json:
“tessera”: “stevage/tessera.git#x-forwarded-proto”
Now, we need to tell Glitch to run Tessera instead of the templated code that was set up for us. Change the “scripts” section to look like this:
Under “New File”, select “Upload a file”. Pick an .mbtiles file you have, upload and wait.
Now, here’s the one tricky, rather clunky step. The asset you uploaded will actually live on Glitch’s assets CDN. It needs to be inside your server, so that Tessera can access it.
Inside the Glitch console (under “Tools”), first find out where the mbtiles file is, by running “less .glitch-assets”.
Look for a line like:
{“name”:”lgas.mbtiles”,”date”:”2019-12-17T22:56:15.660Z”,”url”:”https://cdn.glitch.com/982234c74-247f-4e51-9416-944436678291%2Flgas.mbtiles”
Extract out that url, and download it, choosing a sensible name:
$ wget -O lgas.mbtiles “https://cdn.glitch.com/982234c74-247f-4e51-9416-944436678291%2Flgas.mbtiles”
Now we’re going to make the config.json file promised in our package.json. In the main editor, create a new file, config.json.
My tiles contain local government areas (lgas), so this is what my config.json looks like:
Make sure the URL there points to the location of your specific file. It must start with “mbtiles://./”.
Under “Show”, choose “Next to the Code” so we can whether Tessera is running correctly.
Click “Change URL” to access the TileJSON for your tile layer. In my case that looks like:
Check the JSON carefully. If you see ”
"filesize":0
or
"format":"png"
it means that Tessera couldn’t find your .mbtiles file, and created a blank one, assuming the file format was .png. (Yes, these are some interesting choices.)
You can now use your tiles in any Mapbox-GL-JS project. Instead of an identifier like “mapbox://stevage.nt2h43nh”, you’ll have a URL like “https://demo-tessera.glitch.me/lgas/index.json”. (You can use either HTTP or HTTPS – Glitch and Tessera support both.)
For instance, using mapbox-gl-utils:
I recently re-engineered the data processing behind OpenTrees.org. It’s a website that lets you explore the combined open tree databases of 21 local councils around Australia (over 800,000!), with some pretty data visualisations. Working on this site has taught me a lot about processing data into vector tiles. Today’s lesson: “You might not need PostGIS”.
Trees from Melbourne, Hobson’s Bay and Brimbank.
The architecture of v1 looked like this: (See “OpenTrees.org: how to aggregate 373,000 trees from 9 open data sources“).
It worked fairly well, but with the huge disadvantage of having to host a web-accessible server, complete with database.
When I lost access to my free hosting, I re-architected it using Mapbox-GL-JS: v2.
Now we don’t need a server (Github Pages and Mapbox are serving everything we need, and are free). But we still have the heavy dependency of PostGIS.
What is PostGIS actually doing in this scenario? Mostly it’s doing very simple row-oriented, non-relational operations like:
or:
(Yes, I should have used SPLIT_PART())
And then finally we just dump the whole table out to disk.
I began trying to replace it with Spatialite, but that didn’t seem to play very nicely with NodeJS for me. As soon as it got fiddly, the benefits of using it over Postgres began to disappear.
And why did I even need it? Mostly because I already had scripts in SQL and just didn’t want to rewrite them.
So, the disadvantages of PostGIS here:
So, I rewrote it as v3:
The workflow now looks like:
The processing scripts now look like:
For now, each GeoJSON file is loaded entirely in one synchronous load operation.
(Processing all the GeoJSONs this way takes about 55 seconds on my machine. Loading them asynchronously reduces that to about 45. Most of the time is probably in the regular expressions.)
The only slight hurdle is generating the species count table. With PostGIS, this is just one more query run after all the others:
In NodeJS, our “process each tree once” workflow can’t support this. After processing them once (counting species as we go), we process them all again to attach the species count attribute.
If we were doing a lot of statistics, possibly PostGIS would start to look attractive again.
The next dependency I would like to remove is OGR2OGR. It is there because datasets arrive in formats I can’t control (primarily CSV, Shapefile, GeoJSON). I love using Mike Bostock’s shapefile library, but it doesn’t currently support projections other than EPSG:4326. That’s not a showstopper, just more work.
It would also be great not to have to maintain VRT files (in XML!) to describe the CSV formats in which data arrives.
For my next web mapping project, we’ll use vector tiles. Great. And the data will come from OpenStreetMap. Excellent. Now you only have five more questions to answer.
For the front-end web application developer who wants to stick a map in their site, vector tiles open up lots of options and flexibility, but also lots of choices.
We’re very lucky that in a couple of these areas, a single standard dominates:
Which leaves three decisions to make.
There are several viable options for displaying your vector tiles, depending on whether you also want to display raster tiles or need creative styling, if WebGL (IE11+ only) is ok, and what else you need to integrate with.
Mapbox Terrain, a style rendered with Mapbox-GL-JS.
There are other combinations as well, such as Leaflet.VectorGrid.
The style mechanism tends to be closely tied to the display engine. (That was also true of CartoCSS, which was a pre-processor for Mapnik. RIP).
Finally, there are three distinct, well-defined schemas for packaging OpenStreetMap data into vector tiles. There doesn’t seem to be a formal specification for how you define a schema, so each is presented as documentation: a list of layers, each with a list of attributes (and their possible values), and at which zoom levels they appear.
Dark Matter, a Mapbox Style for OpenMapTiles.
class=major_rail”).
Now, the style needs to be designed for the schema: if the schema contains a layer called “roads”, your style can’t be expecting a layer called “transportation”. But it also needs to be expressed in the right format supported by the engine: don’t go feeding no YAML to Mapbox-GL-JS.
For instance:
Tron, a highly stylised style from Mapzen for Tangram.
Style Specification. And hence can be rendered by Mapbox-GL-JS, or OpenLayers. (Other standard Mapbox styles include Mapbox Streets, Mapbox Terrain and Mapbox Dark)
These styles kind of live within their company affiliations, however. How about styles rendered by one company’s engine, using data from a different schema:
Which brings us to the point of this post. How do you mix schemas and styles? That is, how do you take a style you designed for Mapbox Streets, and make it work on OpenMapTiles? Or port one of Mapzen’s kooky open-licensed styles so it works with Mapbox Streets? Well, you can’t – yet.
(Adapting a style from one engine to another, like what ol-mapbox-style does, is a tough ask, because engines’ capabilities differ.)
But adapting a Mapbox Style file from one OpenStreetMap schema to another? That seems totally doable – even if there isn’t yet a tool to make that happen.
My quick little proof of concept in NodeJS converted OpenMapTiles’ “OSM Bright” style (left) to versions for Mapbox Streets (centre) and Mapzen (right).
Want to give me a hand? Get in touch!
Steve Bennett blogs 
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