Aerial infrared thermography: linking thermal mapping results to CAD and GIS systems

CADalyst, Sept, 2004 by Gregory R. Stockton

Surveillance, search and rescue. The military, law enforcement agencies, and border patrol officers have used ground-based IR thermography for years for surveillance purposes. In the past few years, aircraft with IR imagers have been used. Now, UAVs (unmanned aerial vehicles) with IR imagers are being tested by the Bureau of Customs and Border Protection to patrol U.S. borders. However, it takes about five people to operate a UAV, so it's expensive.

SAR (search and rescue) operations are often rush jobs where conditions are less than ideal. Aerial IR SAR is better than ground-based SAR in most instances, but it's still overrated. People targets don't want to be seen or may be hard to detect because they can't move to a visible area or are trying to stay warm by insulating themselves. The key is to narrow the search area and image out the side of the aircraft, not down.

Roof moisture surveying. No application is better suited for aerial IR than the predictive maintenance activity known as roof moisture surveys. The ravages of sun, wind, rain, snow, hail, ice, chemicals, leaks, and time cause every flat or low-sloped roof to fail. Roof problems manifest themselves in two ways: leakage and entrained water contamination.

Leakage is simple, although the leak inside the building rarely reflects the exact spot on the roof where there is a hole or tear in the membrane. Because most insulation absorbs water, it's harder to find the entrained water contamination because the roof may not show a leak until the insulation has absorbed all the water that it can hold.

Owners can use one of three nondestructive tools to find subsurface moisture: nuclear gauges that count neutrons, capacitance meters that measure resistance, and infrared, which shows heat patterns. Both nuclear gauges and capacitance meters allow a technician to take a sample reading on a 5' X 5', 10' X 10', or 20' X 20' grid on the roof. When plotted on a roof plan, these measurements are used to extrapolate where the water is. They work marginally on roofs that don't gain or lose much solar energy and therefore don't lend themselves to IR.

IR is the preferred method for roof moisture surveying. During the day, the sun radiates energy onto the roof into the roof substrate, and then at night, the roof radiates the heat back into space. This is known as radiational cooling. Higher mass (wet) areas absorb and dissipate heat at a different rate than the lower mass (dry) areas--they radiate heat for a longer period of time at night because they take longer to cool. IR cameras can detect these sources of heat and see the higher mass (wet areas) during this window of uneven heat dissipation. An infrared roof moisture survey allows a building owner to assess the condition of a roof at all stages of its service life (figure 3).

[FIGURE 3 OMITTED]

Aerial IR imagery is far more useful than on-roof IR imagery because the images are at plan view (straight down), which enables the accurate marking of wet areas on CAD drawings and because large areas can be seen all in one image. Even the slightest nuances of temperatures can be traced to the source and outlined. IR images of the roof, no matter how spectacular, are only signatures of heat. There are many causes of heat (or apparent heat) on a roof. This is why professional verification by roof consultants is important. The aerial IR report should be reviewed and the printed data taken on the roof to aid in visual examination and further testing.