Algorithm for generating heat maps
You have to interpolate the missing heat values. There's some mathematics that explains this pretty well (man, it's been a while) but, if I recall correctly, the correct physical way to do this (assuming no heat sources in the area being interpolated) is the following:


For each pixel for which there wasn't original data, assign the value of the pixel to the average value of all surrounding pixels.
Repeat (1) until no pixel changes its value by at least some $\epsilon$


To be safe, you should probably double-buffer this process, so that while doing the first step, changing the value of the first pixel doesn't affect the value for the second pixel. Example:

   5.0 4.0 4.0 3.0    5.0 4.0 4.0 3.0    5.0 4.0 4.0 3.0    5.0 4.0 4.0 3.0
   2.0 --- --- 1.0    2.0 3.0 2.5 1.0    2.0 2.6 2.6 1.0    2.0 2.8 2.5 1.0
   1.0 --- --- 3.0 =&gt; 1.0 2.0 2.5 3.0 =&gt; 1.0 2.4 2.4 3.0 =&gt; 1.0 2.3 2.5 3.0
   3.0 3.0 2.0 4.0    3.0 3.0 2.0 4.0    3.0 3.0 2.0 4.0    3.0 3.0 2.0 4.0

=&gt; 5.0 4.0 4.0 3.0
   2.0 2.7 2.6 1.0
   1.0 2.3 2.5 3.0 ...
   3.0 3.0 2.0 4.0


Already the values are beginning to settle down a bit. This is easy and fast to automate, though naturally, you'll only get an approximation. See here and here for more information on the physics and mathematics behind this (and whether this is even applicable to your situation!!!)