OpenLayers 3 and Vector Data

As part of a project to move most of my OpenLayers 2-powered websites to OpenLayers 3, I have recently converted two more – DataShine: Travel to Work Flows and the North/South Interactive Map. Unlike the main DataShine: Census website, both of these newer conversions include vector geospatial data, so there was additional learning involved during the migration process, mainly relating to vector styling.

North/South Interactive Map

For the North/South Interactive Map, I made use of the loading in of remote GeoJSON files.

Vector Layers

Here’s a vector layer:

layerPoints = new ol.layer.Vector({
    source: pointSource,
    style: function(feature, res) { return pointStyle(feature, res); }
});


The pointSource is a ol.source.GeoJSON, which requires the projection of the files to be defined, as well as that to be displayed, when defining the source for the Vector layer:
pointSource = new ol.source.GeoJSON({
    url: '...',
    defaultProjection: 'EPSG:4326',
    projection: 'EPSG:3857',
    attributions: [ new ol.Attribution({ 'html': "..." }) ]
});


If you wish to do further operations on your data once it is loaded in, you need to add a listener to a remotely loaded (e.g. GeoJSON file) source included within a Vector layer:

pointSource.once('change', function()
{
    if (pointSource.getState() == 'ready')
    { var features = pointSource.getFeatures(); ... }
};

Here’s a typical style function. I’m using a property “highlight” on my feature to style such features differently:

function pointStyle(feature, resolution)
{
    return [
        new ol.style.Style({
            image: new ol.style.Circle({
                radius: (feature.highlight ? 7 : feature.radius ),
                fill: new ol.style.Fill({ color: feature.fillColor }),
                stroke: new ol.style.Stroke({ width: feature.strokeWidth, color: '#fff' })
            }),
            text: new ol.style.Text({
                text: (feature.highlight ? feature.label : ""),
                font: '9px Ubuntu, Gill Sans, Helvetica, Arial, sans-serif',
                fill: new ol.style.Fill({ color: '#fff' })
            })
        })
    ]
};


Interactions

To detect clicks, I used an ol.interaction.Select – N.B. if you don’t specify which layers it applies to, it tries to apply them to all Vector layers!

var selectClick = new ol.interaction.Select({
    condition: ol.events.condition.click,
    style: function(feature, res) { return pointStyle(feature, res); },
    layers: [layerPoints]
});

selectClick.getFeatures().on('change:length', function(e)
{ ... }

olMap.addInteraction(selectClick);


In my function here I remove the flag from any already highlighted features and call features[i].changed(); to get the non-highlighed style. You don’t need to call this on what you’ve actually clicked on, as this is done implicitly. here’s likely better ways of showing selected/highlighted features, using ol.FeatureOverlay, but i couldn’t get this to work.

MousePosition

There’s quite a nice new utility function which means it was little effort to get an “old style” location indicator in, at the bottom of the North/South interactive:
new ol.control.MousePosition({ projection: "EPSG:4326", coordinateFormat: ol.coordinate.toStringHDMS, className: 'olControlMousePosition' })

DataShine: Travel to Work Flows

This loads vector data in as generic JSON through a regular (non-OL) AJAX call rather than GeoJSON so the processing is a bit more manual. This time, my source for the Vector layer is a simple ol.source.Vector which can be emptied with source.clear(); and reused.

I’m creating lines directly from the JSON, converting from OSGB grid and specifying colour (for the style) as I go – note my use of rgba format, allowing me to specify a partial transparency (of 60%) for the lines:

var startLL = ol.proj.transform([data[start][2], data[start][3]], "EPSG:27700", "EPSG:3857");
var endLL = ol.proj.transform([data[end][2], data[end][3]], "EPSG:27700", "EPSG:3857");
var journeyLine = new ol.geom.LineString([startLL, endLL]);
var lineItem = new ol.Feature({ geometry: journeyLine });
lineItem.strokeColor = 'rgba(255, 0, 0, 0.4)'; lineSource.addFeature(lineItem);


…

As previously blogged, I’m also using hand-crafted permalinks in both websites, and drag-and-drop KML display and UTF grid mouseovers in the latter, and both have also had their stylesheets tweaked to allow for easy printing – again made possible with OL3.

I’m about ready now to tackle my most complicated OpenLayers project by far, the Bike Share Map.