What is LiDAR? LiDAR is short for “Light Detection and Ranging” and is a sensor that uses pulses of light (non- visible wavelengths at about 1550 nm and 1064 nm) to determine the distance between the sensor and the object. These pulses are sent out rapidly (up to about 150.000 pulses per second) and “bounce” off of objects to return to the sensor. LiDAR then measures the duration of time it takes for that light to come back to determine the distance of the object. Using light as a tool for measurement provides highly accurate data, as the speed of light is nearly constant. To calculate the distance, LiDAR multiplies the time it takes for the light to hit the object and come back, and divides it by 2. This quantity is divided by 2 to account for the distance the light must travel on its way back to the sensor. Using these light rays, LiDAR can compute the relative distance from one object to the other. Because LiDAR sends so many pulses in a short amount of time, the distances that it calculates is relatively accurate, but in the case in which two distances adjacent to one another do not coincide or are significantly different to each other, LiDAR will also interpolate the data to provide a smoother transition between data points. LiDAR can be used for a large variety of purposes and can measure distances laterally on the ground, aerially from above, and even through water down to the sea floor.
What is Photogrammetry? Photogrammetry is the use of producing a map or large image or model using photographs of a designated area. To produce a product from photogrammetry, images must overlap at least 50% in order to piece together a fluent image. There are two main types of photogrammetry: Terrestrial and Aerial. Terrestrial uses a handheld camera and can either take images around a central axis to produce a first-person view of a location, or can also take photos around a focus object to produce a model or map of the focus object. Ordinary cameras can be used to do this, including the cameras from our iPhone. Aerial photogrammetry uses a mounted camera on an aircraft to take medial pictures of the ground surface. This is often used with drones and UAVs, and is most commonly known to be used in Google Maps. However, aerial photogrammetry can also be used for other commercial purposes.
Advantages of LiDAR? Elevation measurements are a lot more accurate with LiDAR. Because photogrammetry uses triangulation with images to discern elevation, certain interpolation points can be difficult to obtain and not very accurate. This becomes more apparent for layouts that include a lot of vegetation or generally any overhead obstruction. LiDAR is more desirable for cases like these. Another advantage is LiDAR’s ability to still function in the dark, or any bad weather for that matter. Photogrammetry could still potentially be used with setting up lights, but when this happens the quality of the pictures decrease and accuracy becomes even more of an issue. Under proper conditions with good lighting, photogrammetry is a very appealing tool but when it comes to more complex imaging, it may make more sense to use LiDAR.
Advantages of Photogrammetry? Although LiDAR can be accurate, it can still be difficult to obtain high resolution like one would be able to with photogrammetry. Because images are used in photogrammetry, depending on how big of an area one would like to survey, different lenses could be utilized to obtain more detail. LiDAR typically offers an accurate resolution to about one meter at the moment. In addition to the topic of survey area size, although LiDAR does a wonderful job obtaining precise measurements, it is a lot cheaper to use photogrammetry. Where LiDAR could potentially provide more quality data, it costs a lot more to run LiDAR over large areas than to take pictures. Depending on the type of product one wants to obtain and how much they are willing to spend on it, photogrammetry may be a better option for potential users.
Sources Davies, A. (2018, April 26). What Is Lidar, Why Do Self-Driving Cars Need It, and Can It See Nerf Bullets? Retrieved from www.wired.com/story/lidar-self-driving-cars- luminar-video/ Karpowicz, J., & Higgins, S. (2017, October 14). Aerial Surveying with a Drone and Photogrammetry vs LiDAR. Retrieved from www.expouav.com/news/latest/figuring-aerial-surveying-drone-instead-arguing- photogrammetry-vs-lidar/ SBL. (2016, December 19). A Brief Introduction to Photogrammetry and Remote Sensing ~ GIS Lounge. Retrieved from www.gislounge.com/a-brief-introduction-to- photogrammetry-and-remote-sensing/ What is LiDAR and How Does it Work? (n.d.). Retrieved from www.3dlasermapping.com/what-is-lidar-and-how-does-it-work/