Beginning in the early 17th century, countless maps depicted California as an island floating just off shore from North America. As described by the Stanford University Libraries webpage California as an Island in Maps, “It was not until Father Eusebio Kino’s map entitled ‘A Passage by Land to California,’ informed by his travels between 1698 and 1701, that this cartographic blunder was exposed.” Yet still—with the myth of California as island so firmly planted in European consciousness—this pervasive cartographic error continued for another 50 years.[i]
Seventeen years into the 21st century, Velodyne LiDAR Strategic Markets Director, Harris Wang is optimistic that a similar shift in consciousness will soon occur related to the use of LiDAR in mapping. The application, which is still in its infancy, has seemingly limitless potential. But Wang says, “the [mapping] market hasn’t developed a full understanding of the technology.” Such shifts in thinking linked to emerging technologies are nothing new to the field of Western cartography.
Maps at the time of Kino—often called the Age of Exploration, between the 15th and 17th centuries—were often beautiful hand-drawn affairs. But they were more artwork than scientific documentation. Early map-makers often relied upon colloquial descriptions by explorers, while coyly glancing at the work of fellow mapmakers. Such creative “lifting” was what allowed for the myth of California as island to persist despite mounting evidence to the contrary.[ii]
Throughout the Age of Exploration, advancements in navigational technology allowed for bold expeditions to correct pervasive mapping inaccuracies. Simple items like the compass, which was invented in China around 2,200 years ago, were improved to allow more precise maritime navigation. Specialized marine chronometers, plus reflective instruments like the quadrant and sextant, were used to make more precise observations of longitude, which had been a lasting challenge for explorers. And as these explorers became more oriented with respect to fixed reference points, the maps they created became increasingly accurate.[iii]
Around the time of World War I, planes began to be used for ground mapping and military reconnaissance. But the earliest aerial cartographers recorded their high-altitude observations in simple drawings and sketches. As the war progressed, so did the technology, and planes were soon equipped with cameras pointed groundward. During the inter-war period, fleets of prop planes combed the earth with increasingly high-tech optics that enabled new precisions in remote sensing.[iv]
Thus, the revolutionary science of photogrammetry was born, which involves measuring ground features from photographs. Today, photogrammetry is still used for map-making and military observations, while other applications include utility inspections, such as evaluating the encroachment of vegetation around power lines. Given the modern climate of daily industry disruption and evolution, it’s natural that some observers are beginning to question what the role of Photogrammetry will be in digital age. Especially when new alternatives, such as drone-based LiDAR surveying, are literally hovering on the horizon.
Harris Wang feels that, right now, remote sensing customers are caught up in “trying to figure out which is better than the other.” Photogrammetry or LiDAR? But Wang believes it shouldn’t be about superiority, because each method has its particular strengths and weaknesses.
Photogrammetry, Wang explains, provides a top-down image that is best produced at midday, when the height of the sun reduces shadows on the landscape. And due to this top-down vantage point, vertical position accuracy—say the height of a building relative to street level—can be imprecise. Because of the volume of data produced by an optical image, data management and 3D modeling can be burdensome and must be done via post processing. But photogrammetry has several advantageous qualities, among them the ability to produce startling and detailed overview images in full color. Such maps are more familiar to viewers and more pleasing to the eye, much like those early maps of California and the world are still recognized as stunning works of art.
Meanwhile, Wang touts several practical features of LiDAR sensing, which sweeps lasers across the landscape to create a true 3D model. To start, it’s not just a top-down view. “With LiDAR,” says Wang, “you’re getting data at all angles.” Plus, LiDAR sensing is not dependent upon the sun and can be performed at any time, day or night. The data set produced by LiDAR observations are an order of magnitude smaller than photogrammetry, which allows for real-time processing. Such continuous processing can allow for an autonomous survey vehicle to make in-field mapping adjustments that lead to more comprehensive data sets. But, of course, the LiDAR output is currently a black-and white point cloud, which can be harder to visually appreciate. “In most cases, you can get away with no color,” says Wang. “Or you can post process and add the color back in.”
Ultimately, Wang believes that photogrammetry and LiDAR won’t be adversaries but partners in the coming mapping revolution. “It’s really about your application,” says Wang. “And what problem you’re trying to solve.” A surveyor might use photogrammetry, during peak sunlight hours, to obtain a visually familiar overview of the survey area. “But at other times of the day, [he] could use LiDAR to fill in the gaps,” and obtain more precise observations that photogrammetry can’t provide.
All told, it sounds a bit like the Age of Exploration—when California started as an island, based on incomplete observations and mythical influence—but was soon firmly reattached to the continent with the help of technological innovations. Or Wang offers another analogy: “It’s like doing math. You can use a calculator, a computer, or plain old pencil and paper.” And, today, firmly planted in the information age, many mathematicians still use all three.
What all of this suggests is that quite soon, survey drones might comb the skies—like the old aerial photography prop planes of the 20th century—each carrying a camera for top-down traditional photogrammetry plus a Velodyne LiDAR sensor to fill in the gaps.
[ii] “Detecting the Truth: Fakes, Forgeries, and Trickery” Library and Archives of Canada. Accessed January 11, 2017