  Heatmap is a common geovisualization method that interpolates and visualizes
a set of point observations on a map surface. Most of online web mapping
libraries implement a one-pass heatmap algorithm using HTML5 canvas or WebGL
for efficient heatmap generation. However, such implementation applies additive
operations that accumulate the rendering of point weights on the map surface
grid, making it inappropriate for visualizations that require the highlighting
of both low and high weights. We introduce \textit{hilomap}, an online heatmap
algorithm that highlights surface areas where points with both low and high
trends are located. An HTML5 Canvas-based reference implementation on
OpenLayers is presented and evaluated.
