Data Collection / Surveying Basics

Digital film: A new era of sensors for aerial surveying and mapping

April 5, 2014
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I know that sensors are not a typical subject for columns in POB, but there are some big things happening with sensor technology that will directly impact everyone who uses geospatial imagery and data for aerial surveying and mapping. For many of you, sensors are a behind-the-scenes technology and you probably don’t spend a lot of time thinking about them. You are naturally more interested in what the sensors capture, so a column like this might feel like walking into a coffee shop and where the barista wants to talk about the espresso machine. You’re interested in getting a tasty latte (high-quality geospatial imagery and data), but the person behind the counter wants to talk your ear off about the machine’s levers and buttons and cool milk steaming nozzle. Bear with me, though, because we are entering a new era for sensors that will reshape the quality of data that geomatics and surveying professionals have to work with—for the better.

It’s probably a safe bet to guess that all of you have been on a plane, but I doubt that many of you have been in the belly of a plane. In fact, I would venture to guess that none of you have spent more time than I have in the belly of planes (or the underbellies of helicopters, for that matter). Specifically, the bellies and underbellies of aircraft equipped with high-resolution cameras and geospatial sensors that collect the data that is the lifeblood of our industry. It’s often cramped and hot and full of places to bang your head or elbows on things made of metal. But it’s where everything starts for our industry: cameras and sensors strapped to the bottom of tons of metal hurtling through the sky over cities and towns and agricultural fields and natural areas and everything else that people want to see from above. So how have sensors changed since I first started squeezing into tight spaces in the bellies of aircraft?

 

The Long Reign Of Film

To be perfectly honest, geospatial sensors have changed relatively little since I started my career years ago. If I could have hopped into a time machine back at the beginning of my career and zoomed ahead to today, I wouldn’t recognize iPhones and the latest Toyota models, but I would feel very at home in the bellies of most aircraft that capture geospatial data. That is because in many fundamental ways sensors are largely the same as they were 20 or 30 years ago, particularly the ones that are predominantly used to capture aerial imagery and data for the surveying and geomatics work that POB readers focus on. Film is usually king of the hill, and it has been for a long time.

Many of the geospatial cameras that capture imagery for your day-to-day work still use film just like in the old days. There’s a good reason for that: The advancements that have been made with sensors over the past decade or two are great for some applications, but they have a number of downsides for precision geomatics and mapping. Digital sensors have a lot of positives, but for the most part they have not been a great fit for your work. In the debate between digital and film, the only sensible choice for precision mapping has typically been to go with traditional technology.

That tension between film and digital is about to come to an end, though, because sensors are entering a new era that offers the best of both worlds. I call it the “digital film” era.

 

The Era of Digital Film

Film has had a long life in this industry for good reason. It’s not inertia and resistance to change. It’s simply that film has advantages that make it ideal for surveying and mapping. One of its strongest attributes is its ability to achieve 0.6 base-to-height (B/H) ratio for engineering-quality precision mapping. Base to height ratio is the distance on the ground between the centers of overlapping photos, divided by aircraft altitude. In traditional photogrammetric registration using aerial photos, B/H ratio between 0.35 and 0.75 is needed for manual photo interpretation, and an overlap ration range from 55 percent to 60 percent is recommended for good results. In a stereo model, base height ratio is used to determine vertical exaggeration. Film cameras do a wonderful job of delivering high-quality imagery that meets those specifications. In contrast, large-format digital cameras provide about 0.3 of B/H ratio, which creates challenges for GIS professionals and makes it a less than optimal choice.

However, that either/or choice between film and digital is disappearing with the new generation of geospatial sensors, which will have tremendous benefits for surveying and mapping professionals. My engineering team has put a special focus on eliminating the divide between film and digital, and after several years of development we have brought to market sensor solutions that achieve the B/H ratio of 0.6, which delivers the specifications that surveying and mapping professionals are looking for while also delivering the benefits of digital sensors. Simply put, you no longer have to choose: the newest generation of sensor technology makes it possible for you to have your film and digitize it, too, to paraphrase Marie Antoinette.

 

Sensors That Capture The Best Of Film and Digital

Visual Intelligence’s engineering team has developed the first camera system that has the metric performance of the venerable film cameras. With a 0.6 b/h ratio and sub micron residual accuracies, our iOne Stereo technology is unique in the industry. These sensors generate one foot contours with engineering metric confidence, and their true color technology (non-pan sharpening) provides color radiometric fidelity not achievable by monochrome pixels that get painted by RGB cones.

The key to these sensors is their accuracy, which is so crucially important to surveying and mapping applications. Visual Intelligence’s patented ARCA design uses synchronously operating camera module heads to form a single virtual central-perspective image. The geometric accuracy of ARCA system is achieved from laboratory calibration as well as calibration flight. First, each camera module head is calibrated to define the camera module model. Then, the entire ARCA arrays are calibrated to obtain the relative position and orientation of the camera modules. After the laboratory calibration, a single Virtual Frame image is formed. The residual of calibration of a single camera module head and the Virtual Frame is less than 1 µm. The geometric accuracy of iOne Stereo System is obtained by examining the image residuals from aerial triangulation of a test flight.

The iOne Stereo system is a scalable medium to large format system consisting of n-camera (6, 9 or 15) modules in the patented ARCA architecture. Each camera module in the system can be an 11, 16 or 29 MP CCD sensor unit. Together these camera modules form a single virtual frame  that can be 21,460 pixels in along-track direction and 7,438 pixels in across-track direction (7 kps or kilo pixel swath) using 135 mm lenses. Scalability allows for larger virtual frames such as 12,000 x 12,000 and upward of 30 kps. The camera system collects RGB color images in 12 bit format.

 In many ways, this concept of “digital film” is what the geospatial industry has been building towards for 30 years, and I am proud that our engineering team is playing an important role in making that vision come true. The reason this advancement is so significant is that it is a “geoimaging information fusion” that finally combines the advantages of digital without losing what makes film so valuable. With the quality of film and the technological advantages of a digital system, this new generation of sensors unites those two technologies in a way that represents a major milestone for aerial surveying and mapping. 

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