Field Notes: How Drone Survey, Orthomosaic, and Point Cloud Mapping Inform Project Development

Field Notes is a recurring series where impactful aspects of projects are brought into focus from the field, along with the people who make it possible.

Land surveying is an ever-evolving profession, with data acquisition and measurement tools continuously becoming obsolete as they are replaced with faster, more accurate, safer, and more efficient tools. One such tool involves the use of an unmanned aerial vehicle (UAV), or what’s more generally known as a “drone”. When paired with a sensor (aka payload) the drone becomes an unmanned aerial system (UAS), capable of capturing unique perspectives from otherwise unreachable vantage points.

In the survey and mapping industry, drones have grown to be safer, less expensive, and easier to operate. Gone are the days of creating your own drones, control systems, payloads, and controllers to tailor an unmanned aerial system (UAS) to a project’s particular needs. The result of a drone survey today can provide aerial imagery, point cloud data, and ultimately data that allows for map creation and digital terrain modeling. Just as important, a drone survey also allows us to obtain a perspective of the overall context of a site in a way that is immediately understandable by non-surveyors.

In this piece, we’ll examine exactly what a drone survey is, what it’s capable of, and how it leads to more informed projects from design through construction.

Read on or skip ahead:

• What is a Drone Survey?
• Why Use a Drone Survey?
• The Drone Capture Process
• Taking a Multidisciplinary Approach to Complete Surveying

What is a Drone Survey?

Drone surveying is the art of mapping areas of land using unmanned aerial vehicle (UAV) technology. Drones carrying cameras or sensors (known as “payloads”) are used to capture data that can be turned into images ranging from the visual spectrum to infrared or point clouds utilizing LiDAR (Light Detecting and Ranging).

Modern drones can be deployed quickly and safely, providing images and videos in different formats, perspectives, and angles. Drone images and videos can be used to create maps, orthomosaics (aka “aerial photos”), and help users better view and understand view corridors.

The Science of Turning Data into 2D and 3D Models

In processing what’s captured during a drone survey, flat, 2D photos are used to create 3D models and obtain a digital terrain model of a site. How? Through a technique called structure from motion (SfM).

Structure from motion is a photogrammetric technique for estimating three-dimensional structures from two-dimensional image sequences. It is based on the principle that if we know the position and orientation of a drone’s camera when images are taken, we can use corresponding features to compare overlapping images as the drone has moved to reconstruct a 3D structure of area.

The SfM process works by first finding matching features in different images of the same scene. These features can be anything from corners of buildings to leaves on trees. Once the matching features are found, the software can use them to estimate the position and orientation of the camera for each image. With the camera positions known, the software can then reconstruct the 3D structure of the scene.

Drone Survey and Geographic Information Systems (GIS)

A drone survey also naturally lends itself to Geographic Information System (GIS) applications. For the practice of mapping and analyzing geographically referenced information, the perspective and data obtained from a drone can significantly streamline that process.

The ability to produce this accurate and information-rich view of a given area has widespread benefits for a variety of projects. For instance, UAV captured data can be used in GIS for anything from modeling post-fire debris flow potential or avalanche terrain risk studies to view shed studies for planning and architecture projects. It can also have major applications in environmental design including the analysis of river morphology and water turbidity, plant species and land cover classification, advanced wetland identification modeling, and many more.

[Interactive] Slider in ArcGIS Showing Water Level Change Over Time

Why Use a Drone Survey – An Essential Component of Upkeep, Planning, and the Design Through Construction Process

The world isn’t a static place, and neither are the uses for drone imagery. While the prevailing usage is to create a map or aerial photo of a site once, mapping how that site changes over time is another very valuable application. Whether it be monitoring how plantings are growing, or tracking how woody debris and boulders may move seasonally in a stream restoration project, recording change over time reveals how well the project stands up to the elements of nature and time.

As the field has evolved, many manufacturers are building UAS specifically for land surveying uses – “metric” cameras for high accuracy, high resolution photography and LiDAR to name a couple. Advances in battery life, carrying capacity, and overall capabilities have rapidly accelerated while costs have come down and piloting has become more intuitive. 

Accuracy

Drone surveys can provide highly-accurate data with a level of detail that is not possible with traditional surveying methods over wider areas. This is because drones can fly closer to the ground and take more photographs, which can then be stitched together to create a detailed 3D model of the survey area.  Whereas traditional surveying methods require intention to map every specific item with a survey field crew, with a drone, you get all the items in your photo and then decide what to include in your map.

Cost-Effectiveness

Drone surveys are typically less expensive than traditional surveying methods, as they require fewer personnel and less equipment. Additionally, drone surveys can be completed much more quickly than traditional surveys, which can further reduce costs.

Efficiency

Drone surveys may be completed much faster than traditional surveys. This is because drones can cover large areas of land in a short period of time. The approach also opens the door to a wider variety of project sites as drone surveys can be conducted in areas that are difficult or dangerous to access using traditional methods.

Safety

All work done with drones for commercial purposes must be flown or overseen by a Remote Pilot in Charge having their FAA Part 107 UAS Remote Pilot License. Drone surveys are much safer than traditional surveys, as they do not require surveyors to put themselves in dangerous situations. For example, drone surveys can be used to survey areas that are otherwise inaccessible. This could be due to steep terrain, failing infrastructure, dangerous lake or river crossings, or areas with hazardous materials among a host of other reasons.

Drawbacks to Drone Survey: Airspace, Weather, and Logistical Limitations

The drawbacks of surveying with drones generally come down to airspace restrictions. There are limitations on where and how high drones can fly in areas around secure locations such as airports. Weather also comes into play. Clouds must not be too close to the ground, and wet ground may be too reflective and shiny to create acceptable imagery for mapping.

While the costs associated with surveying – as well as drone technology – continue to reduce, the process still involves highly-skilled individuals using very advanced and expensive equipment. Each survey crew has a sophisticated setup, including their truck/mobile office – and for this reason, survey field crews are costly. They also often need to be scheduled weeks if not months in advance. Unfortunately, this doesn’t allow for rapid deployment to capture an ephemeral situation or event, such as a flood, moving slide, or traffic conditions. However, there are situations where a decent drone and a Part 107 pilot can be deployed to a site rapidly if needed, at a much lower cost than a traditional field crew. 

Does this mean traditional ground mapping is no longer needed? Not at all. In addition to the limitations noted above, ground cover, tree canopy, and the ability of an experienced survey crew to discern the existence and location of objects that might be under a few leaves, or a thin layer of sand or mulch won’t be replaced anytime soon.

Orthomosaic Mapping and the Drone Capture Process

Orthomosaics are large, dimensionally correct photos made up of hundreds or even thousands of individual photos of a site. Utilizing software to create these images, they provide a surprisingly accurate view of planimetric features.

When using drones to create a mapping product, there are a number of steps to ensure that the resulting product meets the accuracy needs of project.

[Watch] Drone Survey Flight Footage at Manley Road

Mission Planning

To maximize the resulting imagery from a drone survey, there are numerous factors to consider during mission planning. Critical aspects often include determining the best time of day, anticipating low wind conditions, cloud cover, and less congestion from parked vehicles or pedestrians. Ensuring the controller and UAV are up to date on software, firmware, and airspace maps, and that the drone is physically in tip-top condition with charged batteries is vital prior to every flight. Determining the optimal height above ground for the UAV to fly, the overlap and number of images, the specific UAS to be used, and the direction and rate of travel of the drone while acquiring images all add to the considerations when planning a flight mission.

Establishing Ground Control Points (GCP’s)

Small marks are made throughout the site, visible in the drone images, that have specific coordinates and elevations associated with consistent locations on these marks. These may be “T”’s or “L”’s or “X”’s made with tape, or circles with the classic survey pattern of opposing quarters. Depending on the site size and elevation change, there may be 7 to 10 of these used to control the data, and 3 to 4 that are used to check it. The resulting map can only be as accurate as the least accurate control point, quality survey equipment and great care in this process is essential to establish these stations.

Unmanned Aircraft System (UAS) Flight

If all the planning was done properly, the flight is mainly about safety, safety, and safety. Immediately prior to a mission, the pilot must plan ahead to mitigate distractions, disruptions, and disputes that may arise while the mission is underway.

While a UAS can be programmed to literally fly itself , the pilot is on hand to at least ensure safe flight operations from the moment they arrive on-site to the time they depart. While the UAS is in flight, the pilot needs to give their full attention to the safe operation of the drone. Pilots need to look out for other craft entering their airspace and track that the drone is acquiring the correct number and type of images that it is programmed to capture.

Post Processing

The images, sometimes as many as two thousand, are loaded into software. What’s known as a photogrammetry module (both the module and software package can run thousands of dollars) is designed specifically for the kind of mapping needed on a particular project. There are a variety of options when it comes to post processing software packages with some better designed for different types of drone survey applications.

Mapping Creation

The process can also be taken a step further by creating a photographic point cloud, which allows us to create a digital terrain model and obtain the heights of items above the ground such as buildings, wire elevations, and trees. Point cloud data may be used simply for map quality control or to supplement field surveying with 2D items such as paint striping or concrete patterns. But it can also be used to create mapping in inaccessible areas or areas that would be dangerous to map with traditional means. While most surveying won’t rely solely on drone data, it can be an essential tool in all these areas.

Structure from Motion is used to create a 3D model of everything within the images. This is then cleaned up and items that are irrelevant to the project are removed. The clean data can then be used to create linework and digital terrain model (DTM) features (i.e., objects found on the surface area) which are transferred into AutoCAD for final mapping and drafting.

Drone Project Photography

Many times, land surveying work is performed to create a basemap of a location so engineers and architects can design improvements or new uses for that site. But what if the goal is to simply have a record of what is there rather than to immediately improve it? This is where mapping that may require a lower accuracy than “survey grade” may be what is needed.

In simply documenting a project visually from a unique perspective, drone-based photography and mapping really shines. When overseen, controlled, and quality checked by a land surveyor, the resulting mapping from a drone mission will be as accurate and reliable as possible. This data can be used to create GIS datasets that track locations of site improvements, utilities, and other features.

Ultimately drones give surveyors the ability to visualize unique project perspectives, capture otherwise unobtainable viewpoints, and document site conditions in a way that can’t easily be accessed.

A Multidisciplinary Approach Supported by a Complete Survey Team

While the Otak UAV program is hosted through our land surveying group, the use of drones extends much further. From marketing and comprehensive project management to water resources engineering and construction management and inspection, the full breadth of multidisciplinary expertise can benefit from data obtained through drone imagery and video.  Drones are an increasingly valuable part of our industry, and their usage and value will continue to be discovered and utilized for years to come.

While an exciting tool that provides capabilities not before available to land surveyors, UAV-based mapping is still just one of several advanced tools Otak has to map features above, on, and below the surface of the earth. Whether a project requires UAVs, terrestrial scanning, bathymetric mapping, traditional mapping, or nearly any other mapping means, Otak’s land surveying and GIS personnel are equipped with the tools and expertise to get the project done right.