The ability to convert photographs into CAD models made it possible to repair an induction furnace fume collection ductwork system with one shutdown rather than two, preventing several hours of additional down time.

DeChant Consulting Services, Inc., (DCS) is a five-year old company that specializes in close-range photogrammetry to support industrial projects. Clients include process plant owners, construction contractors, consulting engineers, and municipalities. The firm's principals have a combined twenty years of experience with photogrammetric measurement techniques, surveying and conventional measurements. Advances in computer technology, and photogrammetric modeling software in particular led them to found the company and bring these benefits to industry.

Retained by Industra Service Corporation in January 1998, DCS was hired to provide close-range photogrammetric services at a facility owned by American Silicon in Wenatchee, WA. A badly deteriorated system of induction furnace ductwork required replacement to sustain plant operations. The challenge was to minimize the duration of the furnace shutdown while retrofitting it with new ductwork. This could only be accomplished by prefabricating the replacement ductwork and readying the fabrications for an intense construction effort during shutdown of the furnace.

Instead of shutting down the furnace twice, once to take the measurements and again later to install the new ductwork, the initial shut down was avoided by using close-range photogrammetry instead of taking traditional contact measurements. The photos were converted into AutoCAD models using PhotoModeler Pro from Eos Systems, Inc., Vancouver, BC. Unlike competing programs, PhotoModeler requires no subordinate measurements so there was no need for a survey crew to validate discrete reference locations. The CAD models derived from the photos were highly accurate, and no rework was required when the new ductwork was installed.

Using Available Information

Plant drawings of the original ductwork were available, but were not deemed reliable to support the fabrication of new ductwork. Traditional methods of verifying the as-found configuration of the ductwork had drawbacks. Not only was the ductwork at an inaccessible height, but the operating temperature was in excess of 600ÝF. Contact methods of measurement would have required an initial furnace shutdown of one day to cool the ductwork and another day and a half for two people to take measurements. This approach also would have compromised acceptable safety practices. Close-range photogrammetry was selected as the most viable solution for acquiring the as-built dimensions of the existing ductwork.

Fig. 1 Field photography of furnace ductwork. The 8-ft. long precision scale can be seen laying on the catwalk in the upper left of the image.

Photogrammetry Technique

Three hours were spent taking 70 photos of the 30-foot x 40-foot x 10-foot area of ductwork using Kodak 420 digital camera (Fig. 1). The camera's 1.5 million pixel resolution is satisfactory for industrial applications.

Fig. 2 After loading the image from the digital camera into the software, discrete point and cylinder markings are made on the image.
An established coordinate system was unnecessary to satisfy the requirements of this project, and this was a key benefit of using PhotoModeler instead of other competing programs. Other programs would have required the establishment of three or more reference coordinate control measurements, which are typically adhesive-backed targets placed on the project site. The locations of the targets are validated with a theodolite and photo-modeling software references these locations when it creates the 3D model.

Fig. 3 PhotoModeler discrete point markings are indicated on the ductwork.
With PhotoModeler, this processing step was unnecessary. An arbitrary photogrammetric coordinate system was used and scaled by use of an eight-foot long precision scale bar that was horizontally placed on the catwalk of the project (upper left in Fig. 1). That spared the time and expense of having a theodolite crew come to the site. After downloading the pictures onto a computer hard drive, PhotoModeler was loaded, a new project was created, and a pre-saved calibration standard for the camera used to take the photographs was selected. Calibrations, which include focal length, lens distortion, image aspect ratio, and principal point coordinates are routinely used as part of the PhotoModeler software package.

Fig. 4 Second view of the induction furnace ductwork. Again, discrete point markings are indicated on the ductwork.
The photographs of the plant were then read into the software. As the images appeared on the PC monitor screen, the user marked features such as pipe joints and cylinders that appeared on multiple photographs (Figs. 2 - 4). This step took about 16 hours. Over 200 discrete points and numerous cylinder diameters in this particular project were marked to meet the engineering retrofit requirements.

Fig. 5 Screen capture from the PhotoModeler software combining the views in Figs. 1 and 2.
The PhotoModeler software package then processed the images using an advanced algorithm that works with the input data (Fig. 5). The software calculated the position of the camera for each photo, the intersection of light rays for each of the photo's marked points, and created a 3D model of the ductwork. In all, the process of transforming digital photos to a 3D model took about two and a half days.

The 3D photogrammetric data was supplied to Industra Service Corporation who in turn created the 3D model in DXF format. The data was used to prepare orthographic drawings of the ductwork. Having the information in this format helped speed the production of the drawings. Preparing drawings from manually collected data would have taken approximately 40% longer and would not have been as dimensionally accurate. Once the drawings were finished, they were issued to a remote shop for duct fabrication. Due to time constraints, the ductwork sections were shipped to the American Silicon facility as soon as they were finished.

According to Steve Tidyman, Industra Service Corporation's project manager, "The installation was completed with no unusual difficulties. I was surprised at the accuracy of the fit, particularly with our tight schedule that did not permit pre-fit assembly at the shop." Some allowances were made during fabrication to provide a method of adjustment during the on-site installation, according to standard operating procedure. No actual rework was required at the site, which minimized furnace downtime.

Other Applications

The same close range photogrammetric process has been used on a number of other projects. For example, one study examined a small section of a metal processing plant that was going to be dismantled and rebuilt. The harsh environment made a traditional survey difficult. In addition to being toxic, the environment experienced heavy vibrations that did not permit stable readings with optical transits during plant operations. Transits were used during plant shutdown to establish a plant reference coordinate control network. Photogrammetry is immune to vibration, so to accommodate this environment, three overlapping digital images of the site were taken from an upper level deck during normal plant operation. The result of this project produced measurements accurate to an 1/8 inch, which met or exceeded customer requirements.

Different tests performed by DCS obtained models that were accurate from 1:6000 to 1:20000 of overall project measurement size. The difference in accuracy depends on the method of marking points on the photos. The greater accuracy can be achieved with a feature called "sub pixel target marking" using adhesive-backed targets.


From these experiences, DCS has determined PhotoModeler to be an excellent tool for accurate field measurements. The company archives the results of its project and can recall the information at anytime in the future for additional project measurement needs, without relying on repeat trips to the project site. In the case of the piping retrofit, photogrammetry proved to be an excellent tool for minimizing down time while maximizing safety and measurement precision.

For information on PhotoModeler Pro and/or training workshops, contact the author at DeChant Consulting Services-DCS Inc., P.O. Box 3261, Bellevue, WA 98009-3261. Phone: 425.637.1865 or Email to Interested readers may also contact Eos Systems, Inc. 101-1847 West Broadway, Vancouver, BC, V6J 1Y6. Ph: 604-732-6658, Fax: 604-732-6642 or visit the web site at