Metric Oblique Imagery
Written by Art Kalinski
GIF 2010 Volume: 8 Issue: 5 (July/August)
Technique Is Growing In Popularity Because It
Greatly Speeds Perception Of A Common
Operational Picture, Even By Untrained Users.
There has been a growing buzz regarding metric oblique imagery and its offspring, 3-D models. Since the invention of the camera, photographers have been taking pictures from oblique angles. The first reconnaissance photos taken from WWI biplanes were oblique pictures taken with handheld Graflex box cameras.
As the need to catalog, reference and match imagery grew, the complexity of dealing with oblique images proved overwhelming. Ortho imagery—with its straight-down view and perfectly square or rectangular shape—proved much easier to work with, so most photogrammetric work has been in the ortho environment. The advent of geographic information systems only reinforced that ortho-centric environment, since most GIS software was designed to operate with rectilinear projections or square view data and imagery.
An oblique image, with its trapezoidal projection, will crash most GIS software. So for decades, ortho imagery from aircraft and satellites became the standard, even though it took highly trained photogrammetrists and analysts to understand what the imagery was showing. It also took time to do the analysis and get the results into the field.
Advancements in optics, digital imagery, GPS, inertial navigation systems and computers now make oblique imagery easier to capture and significantly easier to use. The new-generation oblique imagery is also precisely measurable and accurately geo-referenced so that GIS data can be overlaid on the images perfectly.
The key reason metric oblique imagery is growing in popularity is that it greatly speeds perception of a common operational picture, even by untrained users. As a bonus, it also includes powerful measuring and analysis tools that are changing the nature of photo reconnaissance and visual intelligence for tactical use.
OVERLAPPING IMAGES
Metric oblique imagery has been used extensively by local governments and commercial users since 2003. Unlike a traditional ortho image, metric obliques provide overlapping oblique imagery from four cardinal directions plus an ortho image. Another advantage of shooting imagery from multiple directions is the effect of sun angles, which highlight details that would not normally be visible with ortho imagery.
The photos, taken from low flying aircraft, are clear and positionally accurate because GPS and inertial navigation systems are used during the computerized capture. There is at least a 66 percent overlap of imagery from each direction, so every spot on the ground generally appears in at least 15 images, each image from a slightly different aspect.
Because no ground resources are needed during the capture, the imagery can serve as a geo-referenced base map on which to register other sensor data. If digital elevation data is available, it can be used to enhance the accuracy of the images. Additionally, the process is very fast and efficient since it is computerized from capture to final delivery.
As a result, typical county projects produce tens of thousands of overlapping five-way images delivered at a price below traditional single-shot ortho imagery. Managing the thousands of images would be a nightmare if it was not for the viewing and analysis software that is provided with the imagery. The software, which can be easily learned in two hours by users with no GIS experience, manages the imagery, imports GIS data and overlays the data on the ortho and oblique imagery.
The software also has tools to accurately measure length, width, areas, angles, irregular distance, ground elevation and, unique to oblique imagery, the heights of objects in the image. Additionally, GIS data layer attributes can be queried, the images can be annotated and the annotation can be saved as ESRI shapefiles for inclusion in the agency’s GIS.
The ability to accurately measure objects in the imagery is possible because of a patented process that scientifically captures the geographic location of each pixel in each image. This rich data set permits even more sophisticated analysis and processes beyond just simple measurements, including the fast creation of interactive 3-D models.
3-D models are a natural extension of oblique imagery because they are derived from the imagery and provide even more versatile tools for rapid visual perception. Since the location of each pixel is known, a 3-D model can be derived from multiple images, similar to old “stereo pair analysis.”
Once the surface model is created, the same imagery that was used to create the model is then draped as a photo-realistic and photo-accurate “skin.” The models can be created in any popular format. Because the process is semi-automated with computer processors doing most of the work, generation of the models can be done in hours.
MISSION PLANNING
The 3-D models have been used extensively in Army helicopter and mission planning, and rehearsal simulators such as Lockheed Martin’s Top Scene and LM Safe. A recent example of mission rehearsals using 3-D models was last year’s presidential inauguration.
The 3-D models are geo-referenced and fully measurable, as a result of the fact that the models and viewing software are designed to understand the topology and relationships of building geometries.
Since the capture process is so efficient, imagery can be processed for field use within hours. A real-time downlink capability is being tested by the Federal Emergency Management Agency for use during the next hurricane season. This system permits the near real-time download and field use of the imagery as georeferenced measurable images.
Providers of oblique imagery include Pictometry International, which has a domestic fleet of 52 aircraft and an image library that covers 90 percent of U.S urban areas and key locations in more than 140 other countries, and Woolpert, which provides oblique imagery with the SmartView system using a continuous push broom technique. GeoSpan and Fugro Earth Data also offer oblique imagery.
In a joint effort, Pictometry and Lockheed Martin have established an online service that provides direct access to all Pictometry imagery via a trusted cloud— secure but unclassified and a potential classified SIPERNET and JWICS service. The servers will provide viewing and download capability for the 2 Petabyte Pictometry library.
Most important, work is in progress to place Pictometry cameras and technology into the hands of operational forces in-theater, including areas with limited or denied accessibility.
An in-theater capability could solve a growing problem affecting our operational forces. There is such a flood of ortho imagery and raw un-geo-referenced video that bottlenecks occur because trained analysts can’t keep up and provide the monitoring and analysis that is needed. Oblique imagery and 3-D models are so intuitive that they can be used almost immediately by ground forces, even without the benefit of trained analysts. ♦
Art Kalinski, GISP, is military projects director for Pictometry. This e-mail address is being protected from spambots. You need JavaScript enabled to view it
