How can I map temperature readings to a location?

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You could use a marker array to drop squares with a relatively low alpha value (so that they are almost transparent) and scale the RGB values based on the temperature, and the marker location where the robot is. Essentially, your robot will drop little poos as it goes.

A temperature sensor will measure the reading at a point whereas a heatmap will tell you the temperature of every point in that area. So I don't know how you would create a heatmap using just a sensor. You will need an array of sensors and stack them together or scan every point in your area with the same temperature sensor and transform/aggregate them into a heat map.

I want to make a map that is a heatmap of the readings

GMapping should give you your current pose. You can then take these temperature readings (or even better, heatmap images) and transform them to your current pose and just accumulate them. To visualize such a map maybe use rviz markers as mentioned by @joekeo

I have personally never done it, but I would start thjinking about using markers in rviz Maybe choose a slim shape and make the colour proportional to the temperature

http://wiki.ros.org/rviz/DisplayTypes...