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How to Make Maps Using Leaflet.js, PostGIS and Chicago Open Data (samc1213.github.io)
194 points by samblogs 3 months ago | hide | past | web | favorite | 20 comments

A fun tip, if you're using PostGIS you can actually generate the Geojson directly in the SQL query with http://www.postgis.org/docs/ST_AsGeoJSON.html

and if you're running a version of PostGIS with protobuffers enabled (Not AWS RDS) you can generate vector tiles on the fly with https://postgis.net/docs/ST_AsMVT.html which are great for displaying large datasets.

RDS now supports libprotobuf-c, so AsMVT will work (at least on some postgres versions)

Well thats the best news i've had today. Thanks!


v10.5 of postgres by the looks of it, don't know how I missed that.

The vector file seems really useful. I ran a query that resulted in a 1.5 GB geojson file... much too big for Leaflet. Maybe the Vector tile would be compact enough? Thanks!

Yeah I know that pain.

You'll need the MapboxVectorTiles plugin for leaflet but you can definitely improve the sizes by a fair bit.

When you're doing tiles, you need to simplify to highest resolution actually discernible in that tile. So, for each zoom level, generate bbox tile bounds, and then simplify or cluster everything in each tile for the pixel resolution you want. That way you will never have 1.5GB tile. After all, all that data is wasted space if it can't be seen.

I've done this for GeoJSON tiles for a long time, before the MVT tiles. The MVT tiles pack way more data though, but are not a solution to the O(n2) problem.

Reduce the precision, 15 decimal places are useful for picometers, not a web map. Try 5-6.

It never seems worth the effort in my testing, as you have to come up with your own loop to do the truncating, and the benefit is marginal. In this case he’d have 1.2gb file, I doubt that fixes the problem.

for what it's worth, ST_AsGeoJSON lets you specify decimal precision as an optional argument, so you at least don't have to write a truncating loop.

Another good option with many polygon geometries is Base64 encoding the output of ST_AsGeobuf -- it is many times smaller than GeoJSON and still pretty easy to handle from Javascript. (Needs libprotobuf-c on the PostGIS side and geobuf.js+pbf.js on the JS side)

Thats cool, I should try AsGeobuf as a soft upgrade to one of the older tile GeoJSON site :)

Does GCP Cloud SQL have protobuffers enabled?

Nice! I think you could have used pg COPY .. CSV[0] plus a CREATE TABLE .. AS to avoid the custom python script and do the importing with only SQL.

Also, as others have mentioned you can generate the st_asmvt format directly from pg. This plus PostgREST binary output[1] can get you a way to query your table directly through a REST API.



Kudos on writing a solid comprehensive article on mapping, the tooling available, and how to do it step by step. (GDAL and everything!) Not an easy task, and I found the format engaging. Great stuff.

It looks like the city of Chicago might be sitting on top of the Socrata API, which allows you to run a distinct - if you are visualizing counts for each tract (or averages and other types of calculations) it would save you the Digital Ocean resources to grab the API counts either on-the-fly or as static files for the tracts in Chicago and render them that way via Leaflet, but obviously this does not allow for quite as novel an analysis as PostGIS offers! (here's the API page that the City of Chicago links to from their page https://dev.socrata.com/docs/functions/#2.1,)

Thanks for your feedback! I have 0 experience with mapping so it was fun to see all the tools out there. There's much more out there than I expected. I did notice they had the API, but figured I would learn something a little more universally applicable by learning PostGIS. I should try that out though for a more lightweight and cheaper option next time

Absolutely and that's critical knowledge - PostGIS is super powerful and the experience with GDAL is definitely good to have (and to understand the transformations that are possible)

One of my favorite primers is by Robert Simmon who I had the pleasure of seeing at OpenVis: https://youtu.be/N_dmiQI1s24

RE mapping tools - I've been working a lot lately with Vector Tiles and their embedded props and from a web-mapping perspective it's nice to download the geometry once and then style many times in terms of over-the-wire efficiency.

Happy hunting and thanks for your post.

A similar article, this one is a lot of fun: https://blog.patricktriest.com/game-of-thrones-map-node-post...

Does anyone have a link to a decent primer on PostGIS please?

I would seriously throw Leaflet down the cliff and replacd with Mapbox for just the performance alone. All of the features plus documentation are there with Mapbox and the performance is ridiculously better.

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