Technical Insights by Ethan Guzek
Engineering practice must keep up with technological advancements. Spurred by a need to collect high-resolution survey data in difficult-to-access areas, McMillen Jacobs Associates has recently launched an Unmanned Aerial Vehicle (UAV) photogrammetry program. Photogrammetry software uses a suite of geolocated images to produce 3D point clouds with centimeter-scale ground sampling resolution. This data can then be used to create digital surface models or 2D/3D topography that can be imported into Auto-CAD. The spatial data achieved using this method is denser than that derived from most traditional surveys and is also capable of addressing vertical-overhanging slopes and structures, which are sometimes inaccessible even when using rope access techniques. This alternate approach is less expensive than using a surveyor and can produce data sets comparable to LiDAR scans.
Expanding our scope of services is allowing us to improve our current workflows by bringing the quality of field data up to par with the complex analyses that have become standard in the engineering industry. Take a limited-access slope stability investigation for instance. It is necessary to get hands-on data to model material properties, but it is also necessary to have spatial data with enough resolution and coverage to perform global stability analyses. To gather hands-on data on material properties, rope-access reconnaissance is usually the most effective method. For spatial data we are now turning to UAV (aka “drone”) photogrammetry, as it is one of the fastest ways to gain small centimeter-scale spatial data over large areas. Under favorable flying conditions, data for an eight-acre project site can be collected in less than an hour. By processing this information in-house, we are improving assessment turnaround times by completing critical analyses sooner for time-sensitive projects. Also, data collection can be economically targeted to the project needs and issues.
The versatility of this data allows for a variety of analyses across multiple platforms. Spatial measurements—including distances, volumes, and areas—can be made in the photogrammetry software itself using the 3D point cloud. Volume measurements are especially helpful for estimating excavation volumes for bid documents when dealing with complex terrain, and for pre- and postconstruction comparisons. The ability to open the program and measure things like slope height or slope angle simplifies field efforts and reduces the need to revisit the site when additional measurements become necessary. This methodology also can be applied to infrastructure. We recently created a 3D model of a bridge to assist with large-scale measurements of the substructure.
For more in-depth analysis, surface raster data can be exported into Geographic Information System (GIS) software. This allows for the extraction of high-resolution surface profiles, which are a critical input for limit-equilibrium slope stability and rockfall models. Also, this tool facilitates stepping back and assessing the big picture. Because we are often face-to-face with the subject matter in the field, at times we are too zoomed-in to observe the larger controls on global stability.
We are excited about this tool because it has made high-resolution spatial information easily accessible. It is helping us refine our data, analyses, and final products for a variety of projects, and we are able to model increasingly complex geometries. Completed projects, such as those mentioned herein, demonstrate the potential of this technology when used in conjunction with experience, hands-on field data collection, and proper flight planning. Please reach out to us if you have questions or inquiries about applying this tool to one of your projects!
Virtual Site Tours: Use UAV technology for virtual site tours for RFP or pre-bid procurements and avoid the need for mandatory site meetings, a timely option in light of COVID-19.
Ethan Guzek is a Staff Geologist in our Seattle office. For more information contact firstname.lastname@example.org.
Related: Manual vs. Remote Inspection