Static Vector Tiles II: Openlayers with Custom Projection

Building off the work from my previous post on vector tiles, I wanted to develop a second process for use in Openlayers. There are many tutorials out there demonstrating the use of vector tiles with predefined styles and tutorials explaining how to build a fully vector tile map online. My aim instead is to demonstrate the processes to build:

  1. A vector tile set up overlaying a raster tile cache
  2. How to integrate a custom style.json
  3. How to do all of this in a custom projection using Openlayers

As always, there is more than one method to do any of this. What is shown below can definitely be improved upon. Please take this and make it better.

Github Respository is here.

Website example is here.

This example uses Openlayers6. The map is served in the NZTM projection (EPSG:2193). No effort has been made in ordering the labels. The labels display as the Openlayers decluttering orders them. Included in the JS code is a basic pop-up window demonstrating how to get information from the vector tile.

The project is built as a static set up. The vector tile cache is built directly into the website. THIS IS NOT OPTIMAL, but does demonstrate the principle. Ideally, you would have a location like AWS S3, to serve your tile cache from.

In order to use a custom projection, you will need to build an XYZ tile cache. MBTiles do not handle projections other than Web Mercator (EPSG:3857).

Basic steps

  1. Download or reproject the shapefile in NZTM
  2. Upload shapefile to PostgreSQL database with PostGIS extensions
  3. Tile PostgreSQL table into NZTM (EPSG:2193) XYZ tile cache using TRex
  4. Construct Openlayers6 JS for tile consuption

Sample Data

https://data.linz.govt.nz/layer/50280-nz-geographic-names-topo-150k/

Note:

The Geographic Names layer is clipped and filtered for this example. I clipped only to the Wellington Region and filtered the data only to use:

desc_code = BAY, METR, LOC, POP, TOWN, SBRB

Upload to PostgreSQL

shp2pgsql -s 2193 /data/wellyRegion_townBay.shp public.wellyRegion_townBay_nztm | psql -h localhost -d <yourDatabaseHere> -U <youUserNameHere>

TRex Tiling

TRex will create an XYZ tile cache in the projection of your choosing. You will need to know the resolutions and bounding box of your projection in order to make this work. I was fortunate to have this information at hand thanks to a great tutorial from LINZ.

TRex uses a config file for tiling. The config used in this example is here

The command used to run TREX:

t_rex generate --progress true --maxzoom=14 --minzoom=0 --extent=174.627603,-41.613839,176.259896,-40.737190  --config /configpsql_points.toml

TRex will generate gzip pfb’s. If you prefer to unzip them:

find . -type f | xargs -n1 -P 1 -t -I % gzip -d -r -S .pbf %
find . -type f | xargs -n1 -P 1 -t -I % % %.pbf

Openlayers JS

The Openlayers for this is version 6.  <script> tags needed are:

<link rel="stylesheet" href="https://cdn.jsdelivr.net/gh/openlayers/openlayers.github.io@master/en/v6.5.0/css/ol.css" type="text/css">

<script src="https://cdn.jsdelivr.net/gh/openlayers/openlayers.github.io@master/en/v6.5.0/build/ol.js"></script>

<script src="//cdnjs.cloudflare.com/ajax/libs/proj4js/2.3.15/proj4.js"></script>

<script src="https://unpkg.com/ol-mapbox-style@6.3.2/dist/olms.js" type="text/javascript"></script>

For the full JS example

NZTM Construct in Openlayers

Building the projection for Openlayers

// set NZTM projection extent so OL can determine zoom level 0 extents.
// Define NZTM projection
proj4.defs("EPSG:2193","+proj=tmerc +lat_0=0 +lon_0=173 +k=0.9996 +x_0=1600000 +y_0=10000000 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs");

// Register projection with OpenLayers
ol.proj.proj4.register(proj4);

// Create new OpenLayers projection
var proj2193 = new ol.proj.Projection({
	code: 'EPSG:2193',
	units: 'm',
	extent: [827933.23, 3729820.29, 3195373.59, 7039943.58]
});

// NZTM tile matrix origin, resolution and matrixId definitions.
var origin = [-1000000, 10000000];
var resolutions = [ 8960, 4480, 2240, 1120, 560, 280, 140, 70, 28, 14, 7, 2.8, 1.4, 0.7, 0.28, 0.14, 0.07 ];
var matrixIds = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16];

Applying to Raster and Vector Tiles

Raster Tiles

Another great tutorial from LINZ regarding the set up of an XYZ for raster tiles.

// Tile Services Map
var urlTemplate =
  "https://tiles.maps.linz.io/nz_colour_basemap/NZTM/{z}/{x}/{y}.png";

// Set raster layer
var layer = new ol.layer.Tile({
  source: new ol.source.XYZ({
    url: urlTemplate,
    projection: proj2193,
    attributions: ['<a href="http://data.linz.govt.nz">Data from LINZ. CC BY 4.0</a>'],
    tileGrid: new ol.tilegrid.TileGrid({
      origin: origin,
      resolutions: resolutions,
      matrixIds: matrixIds,
      extent: [827933.23, 3729820.29, 3195373.59, 7039943.58]
    })
  })
});
Vector Tiles

Set up the vector layer to use custom projection:

// Set vector layer
var placeSource = new ol.source.VectorTile({
  cacheSize: 0,
  overlaps: true,
  tilePixelRatio: 1, // oversampling when > 1
  tileGrid: new ol.tilegrid.TileGrid({ 
    origin: [-1000000, 10000000],
    maxZoom: 16,
    tileSize: 4096,
    extent: [827933.23, 3729820.29, 3195373.59, 7039943.58],
    resolutions: resolutions,
  }),
  extent: [827933.23, 3729820.29, 3195373.59, 7039943.58],
  format: new ol.format.MVT(),
  projection: ol.proj.get('EPSG:2193'),
  url: 'https://xycarto.github.io/static.vector.tiles.openlayers.nztm/tiles/wellyRegion_townBay_nztm/{z}/{x}/{y}.pbf'
});

var vectorMap = new ol.layer.VectorTile({
  declutter: true,
  source: placeSource,
  renderMode: 'vector',
  zIndex: 10
  })

Styling

For the style file example

  1. The method in this example is loading the vector tile and overaying it on a raster tile cache. In order to accomplish this, a vector tile cache must be loaded first to the map, THEN the rules from the style JOSN are applied using:
fetch('./styleText.json').then(function(response) {
  response.json().then(function(glStyle) {
    olms.applyStyle(vectorMap, glStyle, 'wellyRegion_townBay_wgs');
  });
});
  1. The above uses olms.applyStyle. To access this function you will need to add the scipt tag to your HTML:
<script src="https://unpkg.com/ol-mapbox-style@6.3.2/dist/olms.js" type="text/javascript"></script>

Notes

  1. Not fully complete. Working example only
  2. Runs slow in Safari
  3. This is in no way the only way to do this. My hope is someone takes this and make it better.
  4. Still needs label work for use on mobile devices.

One thought on “Static Vector Tiles II: Openlayers with Custom Projection

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