A javascript library that display Solr heatmaps on a web map
Converting the Solr response to a map layer is straigntforward. The heatmap works better if the count data is binned into classifications. At least two independent groups have used Jenks classifications and used computed them with the geostats JavaScript library. From Wikipedia: The Jenks optimization method, also called the Jenks natural breaks classification method, is a data clustering method designed to determine the best arrangement of values into different classes. This is done by seeking to minimize each class’s average deviation from the class mean, while maximizing each class’s deviation from the means of the other groups. In other words, the method seeks to reduce the variance within classes and maximize the variance between classes.
The number of clusters is may be based on what looks best with your data or, more simply, the number of colors in the color map you want to use.
Once clustered, the counts can be rendered on a map. This library uses OpenLayers 2.x as well as the HeatmapLayer library from Bjoern Hoehrmann. Essentially, each non-zero count is used to create a HeatmapSource and added to the HeatmapLayer. The size of the HeatmapSource object in pixels is based on the grid size of the Solr data and the number of pixels on the map.
This code includes a quick and dirty area based filter query. Layers that cover a large area tend to create a lot of background noise for the heatmap. I expected this would all come out in the wash during classification, but it doesn't. A reasonable area filter might eliminate layers that are larger than 25% of the area of the map.
This library borrowed ideas from Jack Reed's very nice library for Solr heatmaps with Leaflet.
You can see a version of this code running at http://www.WorldWideGeoWeb.com. While that version works, this version hasn't even been tested. It will within a week or two.
This library will evolve to support both Leaflet and OpenLayers, perhaps in a Backbone framework. Suggestions and questions are most welcome, I'm [email protected].
This can be run on localhost using node server.js and going to http://localhost:8088/example.html.