REPRINTED WITH PERMISSION BY SPAR
July 30, 2014 || Mobile mapping systems (MMS) offer many benefits when acquiring very accurate and precise field survey data. One of the biggest benefits offered by mobile mapping is the drastic reduction in exposure of survey field crews to the hazards of a project site and any live traffic from the general public. Additionally, all of the survey data is collected from the safety of a moving vehicle that is operating at or near the posted speed limit. This speed and efficiency in collecting large volumes of data has the potential to drastically reduce timelines on most any transportation design project. For example, an efficient conventional two-person survey field crew is able to collect on average of about ¾ to 1 mile of topographical survey data per day in an urban environment. However, the MMS is able to generate anywhere from twenty to sixty miles of complete topographical survey information per day in an urban environment (and possibly even more), which drastically reduces the time needed for the field data collection. This is a very important benefit, as transportation is seeing a greater push into the design-build realm from the more traditional design-bid-build approach.
Additionally, an MMS has the ability to collect topographic survey data that achieves the accuracies and tolerances needed for engineering design. Because the MMS can collect very precise and detailed information efficiently and expeditiously, you can deploy it at any time during the lifecycle of an engineering design project. This means the technology can be leveraged during the initial planning or feasibility phase, during the design phase, throughout the construction phasing, or after the construction has been finished for the final as-built of the project.
The successful use of mobile mapping in achieving engineering design accuracies is dependent on a few factors:
- Mission Planning – The project team must spend a lot of time and work in great detail during the planning phase of the project. Initial planning is completed in the office using desktop software such as Google Maps, Google Street View, and Certainty 3D’s TopoMission. Also, any existing files or maps that are shared digitally by the client should be utilized. In addition, if warranted, a MMS project manager will do a site visit for any additional information prior to the deployment of the MMS. However, even when all this planning is performed, there will be some exposure of field crews to the project site when using MMS.
- Control Target Placement – To achieve design accuracies, survey control points are needed near the vehicle’s traveled paths. Companies that provide MMS services have developed project control guidelines that are used on every project. These guidelines help to avoid potentially dangerous target placement sites while maintaining an adequate spacing of the targets, which allows for the analysis and statement of accuracy that each project requires. These guidelines often start with a published specification from one of the Departments of Transportation around the country.
- Survey Control – The establishment of the survey control is vital to the overall success of any mobile mapping project, but the placement and targeting of the control network is often a topic of heated discussion. In order to achieve engineering design tolerances, the minimum horizontal accuracy standard of the utilized control points must be located with second order survey, and the vertical accuracy standard must be third order. This ensures that the LiDAR point cloud data will be surpassed in accuracy by the control points it relies on, allowing it to achieve the accuracies that can be used for final design purposes.
- Data Acquisition – MMS acquisition for engineering design will require the placement of a GPS base station within 5 miles of the vehicle when operating throughout the project area. On large scale projects, the use of multiple base stations are a good practice to keep a maximum base line of 5 miles between the base station and the MMS vehicle throughout the project. Keeping the baselines at a minimum between the MMS vehicle and the base stations will ensure that the overall data quality and the integrity of the data remain optimal across the entire project area.