Can I let the color of the GPX track be determined by values associated with each track point, e.g. elevation or speed? - colors

Question

My gpx file already contains elevation information for each trkpt and I can augment this with a speed for each trkpt. I would like to represent the elevation or the speed at each trkpt by varying the color of the track. For instance: slow is blue, fast is red.
How can I do this?
And this probably means: Which files and functions in Openlayers do I have to change to do this?

Answer 1

You can try the ol/style/FlowLine of ol-ext to achieve this.
Using this style, you can change the with/color of the feature along the line using a function. This example show how to: http://viglino.github.io/ol-ext/examples/style/map.style.flowline2.html.
You just have to calculate the width (or color) along the feature geometry varying according the speed or altitude:
const flowStyle = new ol.style.FlowLine({
width: function(f, step) {
// calculate the with of the feature f at the given step
// step is the curvilinear abscissa between 0,1
// (0: first coordinate, 1: last one)
const width = ...
return width;
}
});
#+

Answer 2

You should go with a stylefunction for the vector layer:
https://openlayers.org/en/v4.6.5/apidoc/ol.html#.StyleFunction
This function is checked for each feature to be displayed on the vector layer and the related style can be set/returned programmatically. For example:
function gpxStyle(feature) {
var style = null;
if (feature.get("speed")>="100") {
style = new ol.style.Style({
image: new ol.style.Circle({
radius: 6,
stroke: new ol.style.Stroke({
color: 'red',
width: 2
}),
fill: new ol.style.Fill({
color: 'red'
})
})
});
}
else {
style = new ol.style.Style({
image: new ol.style.Circle({
radius: 6,
stroke: new ol.style.Stroke({
color: 'blue',
width: 2
}),
fill: new ol.style.Fill({
color: 'blue'
})
})
});
}
return [style];
}
var gpxLayer = new ol.layer.Vector({
source: new ol.source.Vector(),
style: gpxStyle
});