Here are some ideas on how best to debug GeoJSON. This was inspired by Adam’s use of GeoJSON.io, and my laziness with copy and paste!

Generate GeoJSON and view online

The workflow is as follows:

1. Generate a geojson output file from a unit test (for example see testVectorToGeoJsonMilngavie which generates milngavie.geojson). The filename must have the suffix .geojson.
2. Upload the file as a github gist. This is how I do it, first of the following 3 steps are done once, and then the 4th one can be repeated each time the GeoJSON is updated.
   1. Install the github client program gh
   2. Upload the gist e.g. gh gist create app/milngavie.geojson
   3. Get the ID for the new gist with gh gist list (referred to as XXX_GIST_ID_XXX in the following examples)
   4. When the file changes, upload a new version of it e.g. gh gist edit XXX_GIST_ID_XXX app/milngavie.geojson
3. Open the gist either
   1. In github (click on the user avatar and select Your Gists) which overlays the GeoJSON on Bing Maps, or
   2. In geojson.io by opening the URL `https://geojson.io/#id=gist:USERNAME/XXX_GIST_ID_XXX

The only limitation of this method is that the gist must smaller than 1MByte. Otherwise, it’s a very quick way of viewing GeoJSON without having to copy or paste any text.

Generate protomaps tiles to use in the above workflow

Getting hold of tiles from our server is straightforward e.g.

wget https://server_address/protomaps/15/17509/11948.pbf -O 17509x11948.mvt

However, an additional bit of workflow is required when making changes to the map where we want to test it prior to it being uploaded to the server. For this we have to build planetiler (see the instructions in the mapping doc). Then we need to use that to build a small map to test with. I build the whole of Scotland, though building Monaco is the default and very slightly quicker option e.g.

java -Xmx30g -jar target/planetiler-openmaptiles-3.15.1-SNAPSHOT-with-deps.jar --force --download --area=monaco --fetch-wikidata --output=monaco.pmtiles --nodemap-type=array --storage=mmap --maxzoom=15 --render_maxzoom=15 --simplify-tolerance-at-max-zoom=-1

It will take slightly longer to run the first time as it has to download the various data sources. However, it still takes around 100 seconds to run on my fairly fast linux PC. Once the mapping file has been generated there are various ways to use it:

1. View it via a server e.g.run tileserver-gl-light: tileserver-gl-light --file monaco.pmtiles. This allows viewing of the map both as a graphical UI map or as tile data where points can be queried.
2. Extract MVT tiles from the file. I use go-pmtiles which does the trick:

   git checkout git@github.com:protomaps/go-pmtiles.git
   cd go-pmtiles
   go build main.go
   go install main.go
   main tile monaco.pmtiles 15 17509 11948 > 17509x11948.mvt
   

   For larger protomaps files (e.g. Europe, the output tiles are gzipped, and so they need to be unzipped as they come out e.g. main tile monaco.pmtiles 15 17509 11948 | gunzip > 17509x11948.mvt). The resulting mvt files can then be used from within unit tests etc.