Recently we garnered some attention from the Charleston, WV press as well as LiDAR News regarding a project we are currently working on for the City. Although we are months away from completion, our Mobile LiDAR field activities are complete, so I find this an opportune time to provide a broad overview of the complexities of an urban survey. In past postings on Bakers Mobile LiDAR blog, Ive discussed the importance of planning. Project planning comprises a myriad of mission-critical activities including the development and implementation of best-practices, such as those currently being examined by ASPRSs Mobile Mapping Committee. These are a vital component to a projects overall success. Although you may spend hours or days planning and applying best-practices, there are still challenges to performing a large survey.
Combine tall buildings, rolling terrain and one lane, tree-lined mountain roads, and you have the makings of the perfect GPS storm. We use Trimble Planning software to determine satellite visibility and PDOP given a certain elevation mask. However, the obstructions encountered due to the varying conditions have a dynamic impact on the collection. Additionally, we approached the project as a survey-grade solution; where accuracy matters. Knowing we would encounter conditions not conducive to ideal GPS conditions, our staff developed a GPS control network to utilize in post-collection adjustment. The network of 800 control points distributed throughout the city, were utilized to spatially adjust captured point cloud data to ground control observations. A local survey firm was utilized to establish the control network (via GPS surveying techniques) based on Bakers derived control plan, prior to the LiDAR scanning, and to further accelerate the project schedule.
A Google Earth KMZ depicting the control point layout was developed by our staff and provided to the local survey firm for use in establishing ground control.
LiDAR scanning activities in urban/congested areas are highly efficient when performed during the night or early morning hours; a common practice utilized to mitigate local traffic conditions. This however, is not always an option for projects that also require the simultaneous capture of digital ground based photography; as was the case for the Charleston project. In an effort to diminish the impact of traffic on travel speeds and data clean-up/processing, our crew performed collections during off-peak traffic hours, and categorized the road network throughout the city into two groups: 1) Central business district (downtown); and 2) residential. The downtown areas were scanned during the weekend, where traffic would be at a minimum. Conversely, the crew utilized the weekdays, during normal business hours, in the residential areas, anticipating residents would be at work and off the local roads.
Each SBET (sometimes a few a day due to weather) were exported to a Google Earth KMZ to perform cursory review of status.
One of the primary reasons for the initial press was to introduce the residents to the presence of a mysterious vehicle roaming the streets of their city. In the past, our crews have spent a good bit of time explaining to local residents that flag them down, (and occasionally law enforcement officials that havent been briefed by their supervisors) what it is that they were doing roaming neighborhoods. Imaginations run wild when a strange looking vehicle with out-of-state tags comes driving by – perhaps the result of news articles about privacy rights and recordation of wireless networks and personal information play a small part. Even with the press release and public service announcement, our crew was still approached, albeit dramatically reduced in volume, by pedestrians, homeowners and other citizens regarding the purpose of our activities. In comes the last line of defense to maintain the production schedule the elevator pitch. Developing a standard, concise elevator pitch is a must to keep the system rolling that is, unless you dont have to worry about budgets and schedules. If used properly, and of course delivered in a professional manner, the inquisitor receives the pertinent information they need to feel satisfied, and the crew gets to continue their work with minimal interruption.