OpenLayers Legend Control

[lissyx]

Hello,

While working/prototyping with OpenLayers (I'm still quite a newbie on this library), I felt the need for a legend displaying tool, as you can find the layerswitcher component. I have no idea if it's a component that should/must be available in OpenLayers, but the fact that I saw several posts on the web about this topic makes me wonder I'm maybe not alone nor that insane.

So, using OpenLayers.Control.LayerSwitcher as a basis, I wrote a simple OpenLayers.Control.Legend component, that takes its place on the right of the screen, below LayerSwitcher. The core functionnality is quite simple: you pass an option object in the constructor that specifies:

• min value of the interval of data,
• max value --,
• function to call to calculate the color
• scale size, i.e. how many steps you want for the legend display
• unit, for formatting purpose (i.e. "value XXX (unit)")
• expected error value, i.e. a value that denotes a measure error (e.g. for a ping measure while surveying outdoor network coverage, this is network unrecheable).

Then it computes the scale to display as an array of objects:

{
  value: Value,
  intlow: Low bound of the value,
  inthigh: High bound of the value,
  color: Hex color string computed by the user's function,
  iserror: Wether it is an expected error
}

This is used as a model when drawing contents in the redraw() method.

For my usage (displaying 3G network coverage/latency measured with 'ping' over a specific path, high speed train), it works quite well.

[lissyx]

OpenLayers.Control.Legend component

[lissyx]

CSS part for description of OpenLayers.Control.Legend component

[lissyx]

An overview of the component in action