It’s mid-summer and time to add a little light humor to highly technical discussions about mobile mapping. Here’s my contribution…
Anytime I perceive that I am dealing with something that is expressed as an acronym, I know that I am venturing into mentally challenging territory. It’s either an advanced technology concept that can’t be expressed in a simple, single word like "light" or "earth" – or a government agency. Either way, I know that I will really have to concentrate to understand it.
This week’s article is posted as a public service for those readers who want to be savvy about mobile mapping acronyms. Whether its in the board room with executives or at a cocktail party with business associates, you will at least have a clue when these tricky little word forms surface in conversations. If you can’t memorize the list, the next best thing to do is duck into the restroom, do a quick search on your smartphone, then rejoin the conversation with an erudite attitude.
LiDAR – Light, Imaging, Detection and Ranging
Now here is the acronym of all acronyms. Try a Google search on this word. Sometimes its spelled as LiDAR (lower case "i"), or LIDAR (capital ‘I") or lidar (all lower case). Simply defined, it is laser radar. Which leads us to two other acronyms – laser and radar. So if you understand radar and know what a laser is, you have it made.
For those who do not know, here is a simple explanation of LiDAR. Laser emitters scan an area with focused light beams of a certain frequency. The light beams are reflected back to a sensor after striking objects. The time it takes for the light beams to return is recorded and used to compute distances to the objects. Laser scanners produce millions of x,y,z points, known as point clouds.
GNSS – Global Navigation Satellite Systems
Formerly known as GPS (Global Positioning System), this simpler form is still widely used by hunters, fishermen, and drivers. However, if you are in the midst of a group of industry techie peers, and you drop the G(PS) word, you will immediately be viewed as ignorant and uninformed.
GPS refers specifically to the satellites launched and maintained by the United States of America for defense, navigation, and location purposes.
GNSS is a more inclusive term that covers not only U.S. GPS satellites, but also Russian GLONASS satellites, European Union’s Galileo satellites, and more recently the Republic of China’s Compass satellites. It will include any other new satellite system launched by other nations in the future.
GNSS gives us a fairly accurate horizontal (x,y) position on the earth and a less accurate vertical position (based on a rough model of the earth’s surface).
IMU – Inertial Measurement Unit
IMU’s are typically used to track motion vectors of airborne objects (airplanes, missiles). The vector motion paths are called roll, pitch, and yaw (or attitude).
We need these intricate devices for mobile mapping systems. Simply knowing an x,y,z position on the earth from GNSS is not enough information to align LiDAR point clouds and images to the vehicles trajectory, or path. IMU’s also come in handy when we temporarily lose GNSS signals (underpass, tunnel) or when we have degradation of the GNSS signal due to sky view obstructions (urban canyons, treelines). Since IMU’s operate continuously without interruption, they provide supplemental tracking information.
DMI – Distance Measurement Instrument
DMI ranks high on the list of most frequently used acronyms. Be careful of context here. When talking about computers, DMI means Desktop Management Interface. For mobile mapping, it has an entirely different meaning.
Think of DMI’s as rolling measuring devices attached to the rear wheels of your vehicle. They continuously measure the revolutions of the wheels in feet per second. Knowing the diameter of the wheel with tire, the running distance from a starting point can be calculated. DMI’s can tell the mobile mapping system how far it has traveled at any given point in time. Velocity can also be derived by analyzing the change in rate of measurements over time. By using two DMI’s, we can extrapolate information about direction. As the vehicle turns, the inner wheel turns slower than the outer wheel.
In mobile mapping systems, DMI’s contribute data to the processing of a vehicle trajectory and help in solving any gaps in GNSS by providing motion / path related information.
If you encounter an acronym not listed here, consult allacronyms.com (http://www.all-acronyms.com. I discovered this great resource, and immediately made it a shortcut on my smartphone web browser. Now I have social security for those panicked moments of ignorance.