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Thermal Map Infrared Surveys
 
  (click to enlarge)
General

Aerial thermal mapping of your facility, complex, campus, military base or city every few years will reveal leaks in all types of systems, like steam and condensate return lines, hot water lines, chilled water lines, supply water mains, distribution pipes, storm water drains, building heat loss and moisture leaks into your roofs. Aerial photographs should be taken every few years as well. They are inexpensive and can be a great asset when discussing future building additions with management, planning utility repairs and improvements, drawing CADs of the facilities and for uses as simple and handy as 'showing' outside contractors where not to park.

The methodology for taking infrared (IR) thermographs is similar in many ways to taking aerial photographs. To collect the data, the aircraft flies over a given area with a camera mounted to the airframe and oriented looking straight-down (NADIR) to the ground. Oblique or lower angle shots are taken out the side of the aircraft by pointing the camera at the desired angle. The imagery is stored on film or a computer hard drive and later copied it to a convenient deliverable, such as a DVD. Obviously, aerial photos are taken during the day because the sun provides brilliant visible light so one can see features on the ground, like buildings, bridges, roads, etc.

Where aerial infrared thermography differs from aerial visible photography is the time of day when the survey occurs and the wavelength of the imagery that the detector collects. IR thermography of ground objects is performed at night because the sun and its effects on objects is a tremendous distraction in the imagery. Thermography reveals sources of heat and the relative differences in heat from one object to another. Infrared imagery is a grayscale picture whose scales (or shades of gray) represent the differences in temperature and emissivity of objects in the image. Objects in the image that are lighter in color are warmer and darker objects are cooler. No object in an IR image is detected via visible light wavelengths (400-700 nanometers) rather, only from thermal infrared wavelengths in the 3000-5000 nanometers or 8000-14000 nanometers range. Lights and other relatively hot objects are very evident, but as a result of their heat emissions, not their light emissions. Collected IR imagery may then be modified in a number of ways to enhance its value to the end user, such as digitally adjusting the imagery to find particular anomalies and zooming in on different areas of interest (see figure 1). These post-processed images can then be used to prepare predictive maintenance reports on the various systems.

Thermal Mapping and Ortho-Rectification
Using an aerial high-resolution imager, surveying a couple of buildings or a mile or so of underground lines can be done by flying over and locating the target(s) in the imagery, saving the data and putting it together into a report. Scale information can be gathered by taking off the existing CAD drawings or having someone walk over the area with a tape measure. This works fine for small areas, but it is not possible to make precise thermal maps of a whole complex, campus, military base or city without ortho-rectification of the imagery.


Figure 1) Ortho-rectified, geo-TIFF mosaic thermal image of a small city. (click to enlarge)

In order to produce ortho-rectified thermal maps, the ultimate [most useful] product, much more information must be gathered and tagged to the IR imagery. During the flight, the aircraft flies straight, smooth lines on a pre-planned grid, allowing 25% side lap of the imagery. The IR operator manages the sensor data-acquisition (see figures 2, 3) following a structured checklist for orderly data file management. The imagery must be collected with a precise direct-digital timing system, a 3-axis ring-laser-gyro and an inertial navigation system (INS), which is tightly-coupled to a real-time differential GPS satellite positioning system that provides x, y, z positioning of the aircraft at all times. After imagery is collected and QC is verified, the digital infrared imagery is then processed into a series of ortho-rectified image tiles, which are stitched together to create a giant mosaic image. An on-board computer system puts all this information together using a digital elevation model (DEM) of the scene that consists of a uniform grid of point elevation values and the position and orientation of the camera with respect to a three-dimensional coordinate system. The result is presented as a high-resolution IR image in the form of a geo-TIFF, which is compatible with any GIS software such as ESRI ArcView, AutoCAD 3-D Map, Global Mapper, MapInfo, etc.

Once high quality digital thermal and photographic ortho-rectified maps are created, these can be added as layers to existing CAD and GIS systems and to other data sets. By post-processing the imagery, many wasteful conditions can be found and reported. The maps and reports help facility managers keep up with their assets in a very efficient manner. Below, some of the low-hanging fruit of back-end post-processing of the information is discussed.


Figures 2, 3) Aircraft and installed data acquisition systems. (click to enlarge)

The farther one can get from the subject of any imaging survey, while maintaining the resolution to achieve the needed image quality, the more useful the data becomes. This is the aerial advantage. But, one needs to obtain very high resolution imagery in order to survey large areas.

It is true that a picture is worth a thousand words; so get the big picture of your facilities and start speaking volumes about its condition.

Three uses for Thermal Infrared Map Data
(Click Links for Examples and Explanations.)
A) Steam Leak Surveying
B) Roof Moisture Surveying
C) Liquid Leak Surveying of Water Utilities

 

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Stockton Infrared Thermographic Services, Inc.
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