Guidelines for mobile mapping system

GNSS signal considerations

• A sufficient number of satellites must remain visible during the progress of the survey to continuously support the precise localization of the vehicle, so the conditions that are beneficial for a survey with GNSS are also beneficial for a survey with the MMS instrument; in particular, the time of day must be carefully chosen, and it must also be considered that urban canyons limit the acquisition signal.

Survey route planning

• The survey route should be planned by reducing as much as possible abrupt maneuvers and avoiding forward and reverse car directions on the same mission.

• Since the combination of sharp-edged road curves (e.g., 90° or almost) and GNSS occultation due to high buildings can result in trajectory estimation errors, it is advisable to plan the route by reducing the number of necessary turns or to turn only in areas with good GNSS visibility; so it was decided to pass through crossroads by continuing straight ahead whenever possible.

• To better handle the resulting data and to better manage the processing time, it is preferable to carry out several missions for short stretches rather than a single mission for long stretches of road.

Data quality and coverage

• Acquiring the same area two or more times is helpful for the data processing phase (better if the path is in closed rings), specifically during the multi-pass adjustment of the trajectory .

• To get a homogeneous result in the final point cloud, the car should be located as much as possible in the middle of the road, thus having symmetry of acquisition volumes with respect to the left and right sides of the road.

• To aid in the trajectory reconstruction phase, it may be useful to identify significant architectural points (e.g., manhole corners) whose coordinates can be measured by other systems (e.g., a GNSS receiver) and which can be used as control points; it is appropriate to plan the survey by making sure that these points are measured by the instrument and that they are not hidden or obstructed (e.g., by parked cars).

Additional insights

• Post-processing is important: The iterative trajectory refinement phase (using multi-pass and target-based methods) is important for reducing errors

• Control points are essential: Using pre-surveyed control points will correct trajectory errors, especially in areas with GNSS signal loss.

• Filling data gaps: Missing data, for example on building facades at sharp corners, can be filled by surveying the area twice with opposite trajectories or by integrating data from other systems like Terrestrial Laser Scanning (TLS).

  • The MMS data is optimal for ground-level mapping but cannot capture roofs. For a complete model, integration with other systems like UAVs or Aerial Laser Scanning (ALS) is required.

• Managing point density: Vehicle speed directly impacts point density. A lower speed increases density but also increases data volume and processing time/resources.

References

• Treccani, D., Adami, A., Brunelli, V. et al. Mobile mapping system for historic built heritage and GIS integration: a challenging case study. Appl Geomat 16, 293–312 (2024). https://doi.org/10.1007/s12518-024-00555-w