Welcome to the May 12, 2005 edition of the Spatial Resources E-Letter.
The hottest new technology for the creation of 3D models and accurate as-built plans of real world objects and environments has actually been around for a long time. The idea of making measurements from overlapping photographs is not new, even the idea of making 3D models from photos is not all that unusual. The principles of close-range photogrammetry have been well known since the advent of photography. However, the level of accuracy, speed and ease of use now available has been unthinkable until recent advances in computerization and digital cameras have made it a practical tool.

Close-Range Photogrammetry is an accurate, cost effective technique of collecting measurements of real world objects and conditions, directly from photographs. Photogrammetry utilizes digital images to obtain accurate measurements and geometric data of the object or area of interest, in order to provide spatial information for engineering design, spatial surveys or 3D modeling. The benefits of close-range photogrammetry over other field procedures are purported to be: Increased accuracy; complete as-built information; reduced costs; reduced on-site time; and effective for small and large projects.

Applications are abundant. The hull of a ship, the curve of a fender, the direction of a cell tower antenna - are no problem to create accurate CAD models in MicroStation or AutoCAD from digital photographs taken from different perspectives with a consumer grade camera. Close range photogrammetry is being used in process and manufacturing, petrochemical, power, pulp and paper, equipment design and installation, shipbuilding, construction, forensics, civil engineering, architecture, and topographic surveys.

In the cell tower example, it is critical to determine the tilt and azimuth of cell tower antennas, within half a degree in all directions, to determine their coverage and effectiveness. It is also critical to determine that the correct gauge steel has been used in construction, even the appropriate bolts need to be confirmed. But how to do all that without climbing the tower, from information gathered by the field technicians that already work for the company and inexpensively for numerous towers?

My primary contact for this article, Jan Van Sickle, 40 year mapping, surveying, GIS veteran, and author of three books on GPS and surveying believes close-range photogrammetry can fulfill all those requirements. Mr. Van Sickle was recently asked by a major civil engineering firm to determine the effectiveness of using close-range photogrammetry to solve some of the aforementioned cell tower problems.

Traveling to Fort Worth Texas to conduct a close-range as-built survey on a subject cell tower, Mr. Van Sickle’s objectives were to: Analyze the orientation of cell tower antennas in tilt and azimuth to 1 degree or better; analyze the dimensions of the tower itself in three dimensions; create an as-built representation of structural details for the analysis of the components of the tower - with all analysis derived from close range photogrammetry.

Now close-range photogrammetry is not a black-box, black magic answer. As with any technological solution, there are parameters and procedures required in order to achieve desired results.

The first technical procedure is the camera calibration with the use of auto targets. This refines the definition of the camera’s fixed focal length. It determines the actual relationship between the lens and the picture plane and determines the radial and tangential distortion parameters of the lens. All this information is stored in a camera calibration file on the computer.

The next step is to establish control. Coordinate, scale, and reference information can be created by surveying using a Total Station or GPS or other means depending on the georeferencing accuracy required. At least three 3D coordinates must be visible within the scene. Coordinate, scale, and reference information can be created by placing three or more auto targets in the area of interest and by referencing them to known coordinates or distances from the control survey.

Photography for close-range photogrammetry in this case required photographing the site using a compact, hand-held, digital camera with known characteristic (lens focal length, imager size and number of pixels), taking multiple, overlapping images from different perspectives. By indicating three object points in two images of the same scene, and indicate a known dimension, other 3D points in the images can be determined and enable the production of accurate as-built measurements and 3D models.

To provide an idea of the accuracy available to close-range photogrammetry, there are several components. One of the most important is the size of the pixels relative to the length of the fixed focal length lens and the distance from the camera to the object. For example, looking at the top of the cell tower, the following resolution can be obtained: A 135mm (longest) lens produced 6/100ths in/pixel; a middle sized, 85mm lens generated 2/10ths in/pixel; and a small 24mm lens created 7/10ths in/pixel. Based on this process, the best accuracy available is 1/2 pixel.

At this point, it is important to discuss the software that pulls all these components into a system that automates the close-range photogrammetry functions and makes this technology practical. Mr. Van Sickle used Vexcel Corporation’s (Boulder, Colo.) FotoG version 5.2 software for close-range photogrammetry because of “its ability to provide measurements without physical contact of the object, its speed and accuracy, using straightforward data gathering with inexpensive equipment in the field and typical CAD programs in the office.”

The FotoG close range photogrammetry process allows the creation of 3D CAD models and extraction of precise measurements from film or digital photography. Recent advancements include automatic targeting functionality that eliminates tedious manual tasks, dramatically automating the process of creating the 3D model from as-built, real world objects.

Building the Model
Photogrammetric analytics: The creation of the Photogrammetric Database and Block, the process of taking the digital images and manipulating them to ultimately create 3D models and as-built designs, is performed in FotoG.

Block Layout
The photos of the tower are graphically connected into overlapping pairs, called stereo models, which are the basic photogrammetric working unit.

Orientation of the Photo Pairs
Features are extracted from the pairs by matching the relative orientation of tie points between images, both features and coordinated control points (targets). These common matched points in stereo-pairs are used to calculate the relationships of the photos themselves.

Block Formation:
All the photos in the block are transformed into an arbitrary image coordinate reference system. The origin of this coordinate system is based on the first photo in the block and the orientation of each subsequent photo is determined by the tie points previously chosen.

Absolute Orientation:
This arbitrary image coordinate reference system is transformed into the real world using the coordinates established by survey which are made to harmonize with the image coordinates of the targets and tie points.

3D Modeling and Analysis
Once the Photogrammetric Database and Block has been completed a 3D virtual model of the tower is available. Points, lines and planes from that model are imported into the three-dimensional space in CAD. These components are then drawn together and the actual 3D model is realized.

Conclusions:
The objectives at the outset were to analyze the orientation of cell tower antennas in tilt and azimuth to 1degree or better, analyze the dimensions of the tower itself in three dimensions and create an as-built representation of structural details for the analysis of the components of the Tower. Close Range Photogrammetry provided the fundamental digital information to meet the requirements. The information made available in CAD allowed for the construction of a realized 3D model of the tower; the tilt and azimuths of the antennas were thereby determined to an accuracy that exceeded the specifications: and, a 3D digital model of the tower in CAD makes it easy to derive any dimensions of the tower and accurate measurements of the components of the structure.

Close-range photogrammetry removes the need to physically access each point at which a measurement is required, replacing field measurements with image measurements. Basically, if you can see it, the exact location can be determined from the photography. Photogrammetry allows the field measurements to be done at the office. This reduces the required on-site period to the time required for obtaining photography of the area of interest. Close-range photogrammetry is a fast, cost-effective, accurate means to create 3D models, as-built designs and an alternative to traditional surveying techniques.

To learn more about close-range photogrammetry and how it can benefit your spatial information needs, or just for general information, contact us: On the web, www.spatialresources.com; by email, info@spatialresources.com; or call one of our advisers at 720-934-2482.

Until next time.

Cheers!

Roland Mangold