Thew Associates was retained in January of 2019 by an engineering firm providing design services for the stabilization of a concrete flood wall on the northerly bank of Rondout Creek in Rosendale, New York. The engineer required bathymetric surveying and mapping of the Creek in the vicinity of the flood wall to facilitate their design. Thew Associates collected bathymetric data utilizing a Norbit iWBMS multi-beam echo sounder mounted on a remotely controlled EchoBoat. In addition to the bathymetry, the multi-beam sonar utilized for the survey was able to confirm areas of significant scouring and undermining of the flood wall footings.
The 1.8-meter EchoBoat USV was utilized due to restricted access (steep banks), swift currents, and variable water depths. The vessel was secured with a tether held by personnel on a bridge adjacent to the upstream end of the survey area. The Norbit iWBMS integrated multi-beam sonar system is compact and lightweight enough to be easily used with the USV. A simplified graphical user interface that communicates with the shore station and operator through a remote connection allows the user to navigate the USV and observe the data being collected. Additionally, the system has the ability to transmit the data in real-time to the office so the Project Manager can also view the data being collected.
Multi-beam sonar was chosen over vertical single-beam sonar because of the ability to provide complete coverage of the Creek bottom and the ability to collect bathymetric data under the flood wall to map the extent of the undermining caused by scour beneath the flood wall footings.
During the survey, raw position and motion data was logged on the survey vessel, while GPS/GNSS observations were logged by a local base station set up over a project control point. This data was combined and post-processed to provide very accurate positioning of each sounding, even in areas where the GPS/GNSS signals are interrupted, such as near the bridge or immediately adjacent to the flood wall. The data was all initially post-processed and validated utilizing Hypack hydrographic surveying processing software. The bathymetric data was subsequently exported as an XYZ file to create a representative bathymetric surface, as well as an LAS point cloud file of all soundings. The LAS file was imported into TopoDOT to extract detailed breakline data for the flood wall, footings, extents of undermining, and adjacent concrete apron. The breakline and surface area was brought into an Autodesk Civil 3D environment to prepare the final bathymetric mapping, including plan and cross-sectional views to illustrate the existing conditions and for use in the remedial design.
Figure 1: Visible undermining of the concrete footing to the left of the image. The vertical section of the flood wall is seen to the extreme left of the image.
Multi-beam sonar can be used to inspect the integrity of many underwater components including:
- Bridge piers/abutments
- Retaining walls (submerged installed sheet pile retaining wall on Buffalo River project)
- Dam structures
- Submerged intakes/outfalls
- Abandoned-in-place infrastructure (submerged roads/bridges, buildings, walls)
- Breakwater armor (stone, blocks: dolos, antifer, tetrapod, etc)
Figure 2: The flat horizontal green line is the exposed portion of the top of the concrete footing. The blue represents the Creek bottom.
Figure 3: The vertical flood and concrete footing is readily visible with the adjacent Creek bottom. Scour was not occurring in this area.
Over the past few years, unmanned aerial vehicles (UAV’s) have become a fixture in the toolbox of surveying and mapping professionals. Light Detection and Ranging (LiDAR) sensors and digital cameras mounted on UAV’s allow surveyors to gather a tremendous amount of data more efficiently than conventional surveying methods, which have made the technologies very popular. Thew Associates was an early adopter of UAV technology utilizing both LiDAR and photogrammetry technologies and this post aims to provide an overview of the strengths and limitations of both technologies.
How the Technologies Work
LiDAR is an active sensing technology. Simply put, the sensor sends out light pulses and measures the time it takes for the light to return. Points are then processed and classified to create a 3D point cloud of the site. Conversely, photogrammetry is a passive technology. Digital photographs are collected with a camera mounted on a UAV and a 3D model is derived by stitching together overlapping images through automated detection of common pixels in each image.
Strengths and Limitations of LiDAR and Photogrammetry
LiDAR and photogrammetry each have their best use cases; therefore, the overall project objective will determine which technology is deployed. Terrain, vegetation, and desired deliverables are just a few of the factors that determine which technology is best suited for your project.
Photogrammetry is best utilized when planimetric information is required. LiDAR’s 3D point cloud is more difficult to derive planimetric information from due to the density of the data set (50 – 100 ppsm); however, intensity values and RGB overlay can be used to produce a visually rich image.
One limitation of photogrammetry is that it can only generate points based on what the camera sensor can detect. Therefore, if topographic information is required in areas where there is vegetation growth, a LiDAR survey is the better method. LiDAR pulses can penetrate tree canopies and other vegetative mats where photogrammetry cannot.
Both technologies have limitations when it comes to weather. Ideal weather for UAV photogrammetry data collection is an overcast day with no precipitation and light to no winds. LiDAR, as an active sensor, can be flown day or night but data cannot be collected with any precipitation or high winds.
LiDAR data processing is much more efficient than UAV imagery processing. Once the trajectory of the UAV has been resolved utilizing post-process kinematic (PPK) algorithms, a three-dimensional LiDAR point cloud can be generated within a few hours, while the stitching together of digital images utilizing software such as Pix-4D can take up to 24 hours for a relatively small site (e.g. 30 acres). The LiDAR data also provides a much more reliable and robust solution for creating bare earth data set for topographic mapping.
Thew Associates is currently testing the ability to derive planimetric information from a LiDAR data set by increasing the amount of overlap between flight lines and flying multiple missions to densify the data set.
LiDAR and Photogrammetry Deliverables
Photogrammetry deliverables can include:
- RGB point cloud
- Digital Surface Model (DSM)
- Digital Terrain Model (DTM)
LiDAR deliverables can include:
- 3D Point Cloud
- Digital Elevation Model (DEM)
- Digital Terrain Model (bare-earth elevation data)
- Triangulated Irregular Networks (TINs)
One of the biggest differences between photogrammetry and LiDAR is the equipment costs. At a relatively affordable price, you can enter the photogrammetry market with the purchase of a DJI drone, a camera, and a subscription to a processing software such as Pix4D.
The cost of entry to performing UAV LiDAR missions is much more expensive due to the technology costs of the LiDAR sensor and the combined with a larger drone platform that is needed to carry the heavier sensor.
UAV vs. Manned Aircraft
The evolution of UAV’s in the surveying and mapping industry has not eliminated the need for manned aircraft LiDAR and photogrammetry. Manned aircraft is still more cost-efficient on larger sites (300+ acres) due to flight times and beyond the line of sight regulations for UAV’s.
There are countless individuals and services firms offering UAV based LiDAR and photogrammetry solutions; however, many of those providers are not professional land surveyors. Regardless of the technology being employed, sound surveying practices continue to be required to ensure the collected data meets the accuracy standards published by the American Society Photogrammetry and Remote Sensing (ASPRS). There is no substitute for robust quality control and quality assurance procedures including sufficient and accurate ground control, an adequate number of verification shots, and statistical analysis to ensure the integrity of the data set.
If procuring a UAV service firm to prepare topographic or planimetric mapping, ensure that a professional land surveyor is providing oversight for data acquisition and processing.