Mobile LiDAR - SO HOW EXACTLY DOES It Work?
This could be more than you want or have to know about the complexity of a mobile mapping system (MMS), but also for those interested, here's a conclusion the average Joe can understand. If you have a Mobile Mapping System arrive on your job site to execute a laser scan, the machine looks like bit more than an elegant little bit of PVC strapped to the top of a 4WD SUV. Have a peek here be fooled by its simple appearance. Inside is really a modern miracle... a fashioning together of independent and amazingly complex systems that, when brought together, produce what could have seemed impossible not long ago. This little white box of wonder, when fired up, begins communicating with as many satellites in space as it can find. Positions are established. Movement and headings are monitored at breathtaking speed with every reading time stamped so that every few seconds, when the computer catches its breath, the independent bits of data are perfectly spliced back together along with complex corrections to reveal an elegant reproduction of the surface that was just driven past. It really is truly a thing of beauty.
Tech-savvy MMS providers use this amalgamation of technologies to provide its clients with mobile laser scanning services that produce an X, Y, Z point cloud in State Plane or similar coordinate systems. Here is a description of the various technologies used and how they tie together to create a precise 3-D description of the areas surveyed:
HOW EXACTLY TO Determine Position - GPS
Mobile Mapping Systems (MMS) utilize the Global Positioning System (GPS) to determine where we have been at a given point in time. The accuracy of the raw GPS data is not adequate for the purposes so we use among three technologies to correct the raw position information. If we've a known control point near the job, we can create a GPS base station over that point and use measurements of GPS position from the satellites to determine the difference between your measured and actual position of that point every second. This error or "differential" is then transmitted by radio connect to the Mobile Scanner and the GPS data collected by the Scanner each second is corrected. This technique is called REAL-TIME Kinematic or RTK. If no control point is available, we can establish one and, using industry standard tools, determine its exact location at another time. Using RTK, we are able to establish our position to within �1 inch in Easting and Northing and �2 inches in elevation.
The next method we use to find out our position is similar to RTK but is termed Virtual Reference Station or VRS. In this method, we subscribe to something that has a amount of base stations (the VRS Network) create in a given area. We use wireless data links to obtain the GPS corrections from this Network and the data from the GPS on the Mobile Scanner is corrected as in the RTK method. The accuracy of this system is comparable to RTK.
The 3rd method we use to determine position uses OmniStar�, an exclusive satellite based correction service. It provides corrections in a way much like VRS except that we work with a satellite receiver to receive the data instead of a radio link. The accuracy of OmniStar� is slightly significantly less than RTK or VRS at � 4 inches. We can mix and match these technologies as necessary - for example we can use RTK to do the scan but define the control point using the OmniStar� system to eliminate the necessity for determining the control point position later.
How To Measure Changes in Direction - Inertial Measurement Unit
Using our GPS technology, we measure our position accurately about once per second. However, we are constantly changing direction. At 5 miles each hour, we travel about 7 feet per second. During that second, we may change our direction of travel (heading) by many degrees. Find more information (IMU) measures changes in scanner heading 100 times per second. The accuracy of the IMU is � 0.1 degrees (~ 0.17 feet far away of 100 ft away). The positioning of the Mobile Scanner is automatically reset when it gets a new GPS position "fix" each second so any small error from the IMU is not allowed to accumulate.
How To Collect Data of the Topography - The Laser Module
Using GPS technology and an IMU, we can keep track of the exact position of the Mobile Scanner at each time. The GPS and IMU are both mounted inside the scanner housing. Beyond your housing, we have a rotating laser module. This scanner includes a distance measuring laser mounted at a right angle to the direction of travel of the vehicle or vessel. The laser measures 36,000 points another, rotates up to 10 RPM and has a range of up to 450 feet. The accuracy of the distance measurement is ~� 2 inches. The distance between points is controlled by the laser RPM and the speed of the automobile. We typically generate a spot cloud with points ~0.5 to 2 ft apart. The info can be thinned to lessen the size of the info sets if desired. If better data coverage is required, the settings in the control software could be changed. The Mobile Scanner can scan anything it really is driven past above, below or to either side of the scanner.
HOW EXACTLY TO Correct for Uneven Ground - Pitch and Roll Sensors
The Mobile Scanner is mounted to a car or vessel. In a perfect world, the ground or body of water would be perfectly horizontal. Since that is obviously false, we correct the data by measuring the pitch (uphill or downhill angle) and roll (left or right angle) of the scanner using pitch and roll sensors. These instruments measure pitch and roll 100 times per second to an accuracy of � 0.01 degrees (~ 0.02 feet far away of 100 ft) Because these instruments measure the attitude of the Mobile Scanner so frequently, we can travel over rough terrain or in rough seas at reasonable speeds without seriously impacting the accuracy of the data.
Summary
Mobile LiDAR or Mobile Scanning since it is sometimes called can offer previously unheard of efficiency and accuracy in many survey applications. Whether you must measure the volume of your stockpiles, do detailed mine planning, measure beach erosion, determine vegetation encroachment on utility right of ways or solve a myriad of survey or engineering problems, Mobile LiDar may provide the answer to your problems.