The speed and intensity with which mobile mapping systems can collect data over large areas has a tendency to relax people about pre-planning data collection runs. It’s so easy to go out and collect data. But if you are in the mobile mapping business for profit, you need to make every minute and mile of your project pay for itself. Here are some tips based on actual field experiences and training of Topcon mobile mapping system operators.
Route planning appears to be a very straight forward process on the surface – define our starting point, our target area, and where do we start, what do we collect and where do we stop. But there are other factors that need to be considered for a comprehensive route plan.
- Locations for static and kinematic alignments: If you are using a Topcon mobile mapping system, Spatial Collect software automatically initiates a 2 +/- minute static alignment before data logging can begin. At the end of the data run, Spatial Collect initiates a 5 minute static alignment. During these periods the vehicle must remain motionless, parked in an open sky area with 6 or more tracked satellites.
After the initial static alignment is completed and the system changes to logging mode, at least three minutes of kinematic alignment should be performed to get the best results from the IMU. This procedure should be done in an area with 6 or more tracked satellites. Vehicle speed should be above 15 mph and the vehicle path should include one left turn and one right turn.
The route plan should designate locations within or near the data collection target where static and kinematic alignments can be performed. Look for areas like parking lots with unobstructed open sky conditions and minimal traffic movement. In dense urban areas, you may need to locate these on the periphery and drive a short distance to the target.
- A well-planned route: The entire course should be drawn out on a map on a logical turn-by-turn basis. Take into account one-way streets. Avoid doubling back on a street already collected unless deliverables require it.
- Fuel stops: If collecting for several hours, it’s better to make a planned refueling stop at a known location than try to find one when the low tank warning light comes on.
- A skillful navigator: The driver should focus on the road ahead and operating the vehicle. The navigator should prepare by reviewing the route mentally several times, and be capable of giving the driver advanced notice about upcoming turns and maneuvers. Monitoring system functions is also part of the navigator’s job.
- Location for GNSS receiver: If you are not using data logged on a CORS or GNSS network reference station for post-processing and choose to set up a local base, you definitely need to consider location and security. Unobstructed open sky conditions are a must. If you have any doubts about security of the location, plan on providing manpower to guard the receiver against theft or vandalism.
- Site Reconnaissance: I highly recommend a reconnaissance trip to the target site before the actual data collection run. You will likely identify possible obstructions that are not visible on paper or online maps. Note traffic patterns at certain times of day that can be a huge impediment to efficiency. Detours and road closings can be put in place overnight, preventing you from acquiring segments of your target. It would be a good idea to check with local road departments to see what is scheduled.
The accuracy of mobile mapping data is directly related to the quality of GNSS signals available at the location and time of data collection. Most mobile mapping projects are likely to involve underpasses, tree canopy, or urban canyons that can cause GNSS outages or degraded signals. The intelligent mission planner will take GNSS availability and quality into consideration to provide the greatest advantage for success of the project.
The number of available satellites and their position in the sky isn’t a mystery. Topcon offers a free GNSS Mission Planning tool that can be downloaded at: http://www.topconsupport.com/documents/view/1467. In addition, you will need to download a GPS+ almanac (for GPS and GLONASS satellites) at: http://www.topconsupport.com/documents/view/2031. The software has a wizard that will guide you in setting up a mission planning project.
Simply enter the geographic location in Lat/Long or pick a city from the global list. Then set the date and time range when you plan on collecting data. Mission Planning will display a graph of available satellites as well as PDOP, an indicator of the 3D positional accuracy that can be expected. This graph shows satellite availability for Birmingham, Alabama on July 19, 2011, from 7:00 AM to 5:00 PM. From this graph it can be determined that the best GNSS conditions will occur between 11AM to 2:30 PM.
This consideration may seem self-evident, but overlooking it can lead to a prematurely aborted project. Upon request, Topcon can provide a chart that lists the data size in megabytes per minute and per hour for all IP-S2.1 and IP-2.2 system sensors. From this, the user can calculate the amount of hard drive storage capacity required. I would plan on adding an additional 20% to the calculated amount as a safety buffer.
Local Laws and Permits
What’s in the legal future of mobile mapping projects? As with other activities that take place on public right-of-ways, government agencies will likely impose regulations and permitting to cover this method of data collection.
Take a look at this very interesting document issued by the Belgian Privacy Commission at: http://www.huntonprivacyblog.com/2011/01/articles/european-union-1/belgian-privacy-commission-issues-recommendation-on-mobile-mapping/. It includes recommendations for mandatory registration of mobile mapping services, public notice of activities, a Privacy Impact Assessment, and blurring of faces and license plates.
If you have knowledge of or experience with any current laws or regulations that govern mobile mapping, please share them with LiDAR News. Send an e-mail to firstname.lastname@example.org. Credit will be given for your contribution.