Monday, February 17, 2014

Airborne vs. Mobile Mapping LiDAR

Both systems are similar in nature, technological structure and final data, but offer their own unique sets of benefits and constraints. When used together they bring together a combined dataset that yields an unparalleled base for architectural, civil and structural engineering projects.

An airborne LiDAR and digital imaging system, typically mounted in a fixed wing aircraft, but also suitable to rotary wing platforms, is cable of yielding amazing detailed base maps using a combination of photogrammetric data and methodologies merged with amazingly accurate elevation data, typically comparable to traditional field run topographic surveys. Visibility from the aircraft, which can fly over an area as many times is as necessary to produce the density of data that is required, is nearly unimpeded. Voids in data are usually minimal, and limited to areas of impenetrable ground cover, under bridges and other structures, and in some cases, around tall or closely grouped buildings. The density of the LiDAR scan pattern enables penetration of dense vegetation and tree canopy, regardless of the time of year, and yields data to be collected on the ground far in excess of the grid a field surveyor would have to fight to collect, at great expense. Airborne LiDAR also allows mapping of areas that are otherwise in accessible or have severe restrictions on access, such as railroad and highway corridors, industrial areas, marshes, wooded areas or other restricted areas. Effective use of forward, rear and side lap capabilities of the system also allow mapping and collection of features on building walls, embankments and other vertical features. Airborne LiDAR point densities are typically counted in single digit to tens of points per square meter, which is more than sufficient for most purposes, and far in excess of what would be collected by even the most detailed surveyor on the ground.

Mobile mapping also enjoys high accuracy, high scan density and rapid collection of data. While an airborne system can collect multiple passes easily, higher density for topographic feature collection quickly reaches a point of diminishing return. On the other hand, mobile mapping data is already much denser than be collected from an aircraft, but the features it excels at collecting are most often areas of interest that are obscured from the mapping corridor of an airplane, for example tunnels, under bridges and overpasses, building facades, narrow road corridors in urban environments, or cluttered streetscapes with many small points of detail. Mobile mapping point densities are often counted in the hundreds or thousands of points per square meter, obviously yielded stunning levels of detail of its environment. Shortcomings in using mobile mapping for a project are few, but can be notable. The first is directly linked to the detail collected. Dataset sizes can be staggering, necessitating effective management of collection strategies, data storage, editing assets and project workflow. A second less obvious problem is the proximity of the mapping unit to the surface of interest. Data voids can be problematic, as any object in the path of the LiDAR beam essentially stops it transmission, which means substantial data voids which mean multiple passes must be made to collect additional data or terrestrial LiDAR must be employed to fill in the shadows. These areas of shadow include downslopes of embankments, behind sound walls, vehicles, trees, people, structural members and other very common features.

The most effective mapping approach for many projects is a carefully planned combination of the laser scanning technologies. General topographic mapping of wide areas is best left to airborne systems, with detailed corridor and related data gathered from the safety of a vehicle mounted system. Features that need to be located with pinpoint precision, such as structural elements, piping or other critical items can be accomplished through the judicious employment of terrestrial laser scanners, which excel at collecting highly precise and accurate data for use in structural calculations, modeling, deformation monitoring or planning placement of new elements in an existing structure.

If you have a project that has these types of mapping and modeling challenges, you can rely on AXIS GeoSpatial to guide you in the employment of the proper personnel, methodologies and technology to produce the results you need, on time and on budget. We employ all of the technologies discussed, and have an experienced professional staff that draws on years of experience in this ever changing field, and look forward to your measurement challenge.

-Bill Derry, Prof. LS
Director of Surveying Technologies