Surveying firms that have been successful in incorporating UAS data in ALTA surveys adopt best practices to ensure precision standards are met.
Plan, execute, and check. In Part One of this three-part series in the September xyHt issue, we looked at the time/cost/precision proposition of using UAS for elements of ALTA surveys. Firms interviewed agreed that while using the UAS yields productivity gains, meticulous care needs to be taken in all phases of the workflow.
The key to building confidence in UAS derived data for surveys with specific precision requirements, according to the firms we interviewed that have been successful in doing so, begins with a well-thought-out workflow.
Derrick Westoby, UAS program lead at PBS Engineering and Environmental, provided an outline of a typical UAS workflow for ALTA (and other) survey requests they receive:
- Receive request and discuss project requirements with team and clients
- Check airspace classification for flight area and perform an initial site assessment to see if the request can be met within the boundaries of FAA Part 107 (faa.gov/uas/commercial_operators). Is it in an area of heavy manned aircraft traffic? Near prisons, schools, stadiums, etc.?
- There is the FAA Before-You-Fly app that helps determine areas with drone restrictions. It is important to note, these No Drone Zones only restrict taking off or landing and do not necessarily restrict flight in the airspace above the identified area. If anything is unclear, contact the FAA.
- In addition, there are a number of great online free portals, typically provided by UAS equipment and services providers, that search local ordinances and laws concerning UAS use within the subject airspace. For example: www.cc4w.net/drone-resource.html
- Become familiar with FAA Remote ID requirements: bit.ly/3JzSh76
But note, that none of these resources will give you the whole picture. If you are flying in a city or county for the first time, it is good idea to call and ask if there are any specific local drone restrictions.
- Consider whether UAS technology can meet the requirements. Is it too big for a UAS? Get a manned aircraft quote for larger sites to compare. If there is marginal benefit from the UAS, most firms will opt for the traditional approach.
- Other site considerations: Is cell service present? Is there significant terrain relief? Photogrammetry vs lidar?
- Work with a PLS and survey team to define the scope of work and distribution between field crew, UAS, and drafting.
- It may be worthwhile to set up a preliminary autonomous mission, to get the lay of the land, and you can gather some of the marketing and other deliverables on the same flight.
- Set up UAS control plan for field crew to establish prior to flight, or UAS operator to establish when arriving on site.
- The UAS pilot (usually a licensed member of the field crew) arrives and develops control per plan, performing a site assessment for UAS in the process. If the operator notices something missed while missions were being set up remotely, they will be adjusted in the field or they will call the UAS office and request the flight plan be adjusted.
- Perform UAS flights.
GCP, RTK, and PPK
The GNSS elements of UAS and integrated IMU have evolved and improved rapidly. Depending on the client requirements or expectations for precision and accuracy, many firms are using RTK enabled UAS, others post-process; namely PPK, and PPK with IMU processed together. Whichever is chosen, in the planning process you should check if there is a CORS or local RTN base nearby, and/or if you can get virtual Rinex. But optimal for UAS operations is to have local base set up on site; on local control, or “cook” some static to get a position through post-processing, or online positioning services like the National Geodetic Survey OPUS, or the multi-constellation post-processing services of local or regional RTN.
Surveyors may use GNSS-only methods for many types of mapping, but when it comes to ALTA and pre-design surveys, those we spoke to all agree you need at least some ground control points (GCP).
“Within the last year and a half we have transitioned to almost all of our drones having RTK/PPK capabilities,” says Seth Gulich, UAS program manager at Bowman and adjunct lecturer for surveying engineering at Penn State University. “It is almost necessary for meeting the strict accuracy requirements of an ALTA survey on larger sites.”
Testing settles the matter. “Through a lot of our own internal testing over the years, as well as some manufacturer recommendations, we determined that RTK and PPK are not a direct replacement for project control,” says Gulich. “It provides us the ability to reduce the required control, but not eliminate it entirely. My view is that it almost becomes a requirement in order to ensure confidence in data as projects increase in size.”
“I put my ground control out at somewhere between a 500-to-700-foot grid,” says Blair Ellison, UAV program manager at Weihe Engineers.
“That’s kind of a rule of thumb. We make sure we have a good set around the perimeter and in the middle. Old-school aerial ground control was a 500-foot spread, and that still works pretty well with what we’re doing.”
The key for ALTA surveys is “relative positional precision.” After all, the surveyor is certifying positions, and not simply “mapping.” One can go down a rabbit hole of evaluating RTK and PPK workflows, but as Ellison says, “They don’t pay us to do lab work. They pay us to do checks. If you use a ground control point, you can actually go back and trace it. I’m all about GCPs—if I don’t have time to put GCPs out, I’m in the wrong business.”
With at least some GCPs, the surveyors we spoke with said the relative positional precision will typically be two centimeters plus 50 parts per million, two sigma. Flying at 150 feet, this comes out to an inch or three quarters of an inch. Surveyors typically checked this against conventional surveys when they first got into UAS and are quite confident in these kinds of results.
One variation on a GCP method, and a way to reduce the number needed, is do checks on hard, highly-visible features. This can include utilities, corners of concrete features, etc. When a base is set up to cook the data for PPK, or broadcasting corrections for RTK, rovers can be used to establish positions on GCP, visible features that can be used as a check, and other check shots. Note that the precision of the RTK solution can factor into this, and if you are not in good RTK conditions, surveyors will often do some or all of the checks with a total station.
Whatever we do, ALTAs or topos, we do cross-checks,” says Ellison. “Vertically across the site with the GPS or total station. We don’t ever set a model or send a model out without cross checking it on the ground. Let’s say it’s a 20-acre field—I’ll run an ‘X’ across the middle of the field with the GPS on the truck or on foot.” The linework of the resultant ALTA can be easy to check, looking for parking lot paint lines and intersections. Edges of building can be more challenging, in part due to the nature of how they are derived photogrammetrically, but also the difficulty of getting the check shots with GNSS. So a total station is often used. But this is changing with the advent of no-calibration tilt compensation for GNSS rovers and now even prism poles.
The next element of the value proposition of UAS for surveys is processing. There is the processing for position (e.g., PPK), orthomosaic generation, photogrammetry, generation of point clouds, and classification and ground strip of clouds. And a new step that is rapidly becoming a privacy requirement for UAS work in many areas: anonymization. This can involve the blurring of faces and license plates. In addition, there are productivity gains that can come from automated feature and linework extraction; either in workstation software or some online services. We’ll examine processing further in the next installment.