Drones have the potential to completely change the data acquisition industry and are, in fact, already doing so at this moment. It’s important to understand both the capabilities and limitations before deciding to incorporate them into a data collection plan.
By Caleb Cass, PE
Drones are the latest, greatest and coolest technology to hit the solid waste industry in a long time. They have the potential to revolutionize data collection on your site. This is a very exciting time for the industry. As we see these cool new technologies come on the market, it’s important to understand that although the tools may change, the underlying principles and best practices of data collection mostly stay the same. A professional who has experience in topographic surveying, data processing and volumetric calculations could use many different instruments and techniques to collect and analyze data in a way that is accurate—both relatively and absolutely—repeatable and efficient. Just because one has a piece of cutting edge technology, it does not mean that he will be able to produce quality data. It is likely that he will be unable to produce professional quality data if he does not have experience in the geospatial industry and an understanding of the foundational principles that govern all data collection jobs.
What many owners, operators, engineers and designers are discovering is that any new tool is only as good as the hands that are operating it. Drones (also known as UAVs and UASs) have the potential to completely change the data acquisition industry and are, in fact, already doing so at this moment. As with any new and innovative technology, the list of specifications and capabilities can quickly become nearly nonsensical to a first time buyer. There are a large number of hardware, software and service providers to choose from. It’s important to understand both the capabilities and limitations of drones before deciding to incorporate them into a data collection plan.
Probably the most asked, most vaguely answered and most misunderstood question regarding different data capture technologies is this: “How accurate is the data?” We all try to boil this question down to a single number or range like “one to two tenths of a foot” or “three to four inches.” The real answer depends completely on what question is truly being asked; that is, the purpose for which the data will ultimately be used.
In general, geospatial data must have two types of accuracy to provide the most value for the owner, client or purchaser. If data is going to be compared from survey to survey (calculating cut and fill volumes), combined with design CAD (calculating constructed remaining airspace), integrated with other surveys, or used for permitting, planning or design, it must be accurate in each of two ways: 1) Relative to itself and 2) when compared to a real-world coordinate system (Absolute).
Relative accuracy has to do with the scale, shape and size of the dimensions that the data represents. Are the dimensions and shapes of things correct? Is the scale of the data correct so that all the measurements are true to the real world? If you measured a building wall with a tape measure very precisely and then measured that same wall in the data, would it be the same? When images taken with a drone are geotagged, it means that the approximate location of the drone when each picture was taken is embedded into the photo file. All professional mapping drones and some consumer grade drones have this ability. Using geotagged photos and a good photogrammetry software, a decent amount of relative accuracy can be achieved from the photos alone. This is true for small areas and one-time volume calculations for things like stockpiles, but this does not work for large areas or where data needs to be reproducible. And although the dimensions, scale and shape of this information is correct for a small area, the location of the resulting data will only be as accurate as the internal GPS of the drone, which is usually around 2 to 3 meters. To be truly accurate, the data still needs to be shifted, rotated, scaled and leveled to match the exact site coordinate system, which is where Absolute accuracy comes in.
Absolute accuracy is the question of whether the data is positioned correctly in a 3D model environment, when compared to the known coordinates of the site. Do the coordinates of the data match a physical point on the ground whose coordinates we have? In other words, if I painted a big white “X” on the site benchmark and then found the Easting, Northing and Elevation of the center of that “X” in my data, would it match the recorded coordinates for the benchmark? This might seem like a given, but it’s not always the case. Many new service providers are selling asset owners on the Relative accuracy of their drone systems. It’s not until after the drone is purchased or the fieldwork is complete, that the data is compared to previous surveys or design CAD that it becomes very clear that nothing is tied to the local site grid.
Implementing Drones in Your Operation
There is, of course, a correct way to use drones for performing topographic and aerial surveys that involves localizing the data to the site grid, and not just using the on-board GPS for positioning. This localization is usually done one of two ways. First, by using ground control points (also called GCPs) that have been surveyed based on the site benchmark just like a traditional aerial flyover. GCPs need to be placed flat on the ground in open areas that are not likely to be disturbed, buried or destroyed. The second, but less common method, involves setting up a base station and using the drone as a rover that receives real-time position corrections from the base. The base station can be set up on any physical point that has known coordinates, like the site benchmark, survey control points, aerial chevrons, targets, etc., and the drone has to be specifically made to receive and use these position corrections for geotagging the photos that are taken.
The benefits of using drones for gathering information are undeniable, and the excitement around this technology is reaching a fever pitch. One of the main benefits that has largely been out of reach of ground-based surveys like GPS and laser scanners is the ability to provide an orthorectified aerial image of the site from the day of the flight. In the drone world, these photos are “stitched” together from hundreds or thousands of individual photos, hence the term “orthomosaic”.
These are some of the basic technical considerations that any service provider needs to understand and be able to discuss with their clients. It’s smart to test new technologies and to see how the different technologies can make your operation more efficient, more productive or generate cost savings. However, it is prudent to make sure that the service provider has a history in the geospatial industry, they have professional staff, and they are able to discuss the pros and cons of different data capture methods. As you are evaluating different technologies, different drones and different methods, be sure to ask questions about the work they are producing. It’s the provider’s responsibility to make sure that you understand the value and integrity of the product you are paying for. | WA
Caleb Cass, PE, is a civil engineer who lives in San Antonio, TX and works on projects throughout the country. He started his career as a field inspector for one of the 10 largest construction contractors in the U.S. and then went on to spend five years performing engineering design for a national consulting firm. Caleb now is Director of Engineering for Firmatek 3D Mapping (San Antonio, TX) and has spent the last six years performing engineering, mapping and consulting work primarily for mining, solid waste and engineering clients. He can be reached at (915) 539-4198 or via e-mail at firstname.lastname@example.org.