Bad weather may be the bane of a lidar operator’s existence. In lidar surveys, minimizing flight time is the key to minimizing costs, so operators carefully optimize parameters like flight altitude, field of view, and pulse repetition frequency to cover the survey area at the desired point density with fewest flight lines. One parameter that they cannot plan for, however, is the weather.
French survey company Société Topographie Informatique (STI) knows this fact all too well. They were contracted by the Communauté Urbaine de Douala (C.U.D.), a regional government body in Cameroon, to conduct lidar and camera surveys for urban planning and flood plain management. Partially funded by Agence Française de Développement (AFD), the survey would cover a relatively wide expanse of 7,000 square kilometers, including its largest city and economic capital, Douala, plus areas with natural land cover and large dense African forests.
Preparing for the survey
In the 2021 acquisition phase, with close to 200 hours of survey flight time planned over a period of two months, the STI team was eager to get the project rolling. They installed their Teledyne Optech Galaxy PRIME lidar sensor and Phase One IXU-RS 1000 camera in their pressurized bi-turboprop Piper Cheyenne II XL aircraft and optimized their flight plans. All that the survey crew had to do at this point was wait for a good weather day to start the project.
The STI team quickly found out that this was going to prove to be their greatest challenge. “The biggest problem we faced came in the form of low cloud cover that seemed to encompass the whole area,” said STI project leader Bogdan Munceanu. Cameroon, located in the Gulf of Guinea in west-central Africa, is often described as ‘Africa in miniature’ because it boasts all the major climates and vegetation of the vast continent. Unfortunately for STI, Douala has one more weather phenomenon to its credit—heavy cloud cover for about 9 months of the year. Helped along with the humidity from the sea, haze, dust, and perennial bush fires, there is a constant low cloud cover around the city and its surrounding areas.
This was a problem because one of the limitations of lidars is that their lasers cannot penetrate dense cloud cover and reach their target; rather, the clouds scramble the beam so that the target of interest (in this case the ground) is not mapped reliably.
Weathering the challenges
Realizing the clouds were not going away, Munceanu was quick to accept that he had to find a work-around for the problem. “With this survey we did not have the luxury to say that the skies are not looking so great this morning so let us fly in the afternoon,” he said. “If we were lucky, we may have gotten a half hour of clear skies on rare occasions, but for the most part it was heavy clouds every day, and all day. We could wait for months and still not see a whole day without dense cloud cover.”
“Our plan was to fly at an altitude of around 3000 ft from the ground,” Munceanu continued. “We recognized early on that the presence of the clouds would not allow us to stick to our flight plan. Once in the air we would have to adjust the altitude depending on the altitude of the cloud. I figured if we stayed at the base of the cloud with varying adjustments of on average 300 ft below the 3000 ft mark, we would be able to achieve the necessary results.”
For traditional fixed FOV sensors, reducing the flight altitude in this way would have been a major challenge because the swath on the ground would get narrower, potentially creating gaps between flightlines that could be a basis for a client to reject the survey data. Instead, the operator would need to compensate for the narrow swath by packing the flightlines closer together, resulting in more flightlines, more survey time, and more cost.
STI, however, had a better option. “What helped us immensely was the Teledyne Optech PRIME’s SwathTRAK feature,” said Munceanu, “which allowed us to keep a regular swath on the ground even when we varied from our flight plan.”
Dynamic field of view to the rescue
To keep the lidar’s swath consistent, SwathTRAK constantly monitors the GNSS and IMU data to determine whether the aircraft is deviating from the flight plan. Such deviations can include both the aircraft’s position (too high, too low, or too far to the side of the flight line) and its orientation (too much roll, pitch, or crabbing). SwathTRAK also constantly monitors the range to the ground reported by the lidar to determine whether the elevation of the ground itself is changing.
By combining this position/orientation and ground elevation data, SwathTRAK can calculate whether these factors are distorting the swath on the ground. If they are, SwathTRAK immediately adjusts the scanner’s FOV to compensate for them and keep the swath on the ground exactly as planned.
Teledyne Optech originally designed SwathTRAK for high-relief terrain, where it could increase the FOV when flying over hills and reduce the FOV when flying over valleys, thereby keeping the swath at a constant width and improving operational efficiency and data consistency. Over the years, however, Teledyne Optech has developed SwathTRAK to handle many other conditions. For example, in corridor mapping applications with rapid left and right turns, SwathTRAK enables the Galaxy scanner to point towards the planned area when the aircraft is flying to the left or right of the planned course.
In this case, STI realized that SwathTRAK could also compensate when the aircraft itself (not the ground) was changing elevation. Thus, when the aircraft had to fly lower due to clouds, SwathTRAK made the
With the flexibility provided by SwathTRAK, STI didn’t have to replan for hourly-changing weather conditions, and were guaranteed that the sensor would deliver the planned swath width and density despite flying lower than planned.
Successful conclusion despite a cloudy start
In practice, STI planned their surveys normally, setting the flight altitude to maximize productivity and cost efficiency, resulting in a 200-hour project. During each survey, the STI field crew (pilot and operator) adjusted their altitude to fly just below the cloud ceilings, relying on SwathTRAK to compensate for the reduced altitude automatically.
That said, the operator did need to work within the hard limits of the technology during the flight planning phase. For SwathTRAK, the maximum scanner compensation is ±10° from the planned FOV, with a hard limit of ±30° from nadir (the Galaxy’s maximum FOV being 60°). For example, if the nominal FOV used is ±25° (a 50° full FOV), only ±5° is left for compensation. If the nominal FOV is ±20° (a 40° full FOV), SwathTRAK can utilize the full ±10° for compensation.
Finally, SwathTRAK only works for the lidar, not the camera. Flying lower than planned still slightly decreases the footprint of the camera images on the ground, which could conceivably cause gaps between them. To avoid this, STI planned for plenty of sidelap between images.
On average, the cloud cover forced STI to fly about 120 m/380 ft below their planned altitudes, but as much as about 170 m/560 ft lower on Douala’s cloudiest days. STI would have consistently lost up to 20% of flight lines every mission without SwathTRAK and would have spent a sizeable amount of time in replanning and post-processing to avoid/explain issues related to gaps and reduction in coverage rate. With SwathTRAK, the operator did not have to waste time remaking the flight plan each morning and the analyst was able to streamline the data, which was of consistent coverage and density.
Despite the challenges posed by the clouds, the team flew every planned flight while maintaining the desired point density and an average absolute accuracy of 8 cm off of ground control. STI’s client, who had accompanied the team on one of the flights and was familiar with the challenges posed by the local weather conditions, was very happy when presented with the final results. STI estimates that without the flexibility of using SwathTRAK it would have taken them twice the time and effort to complete the project. Ultimately, the STI team’s quick thinking allowed them to use the Galaxy PRIME’s built-in SwathTRAK feature to their advantage, coming out of a cloudy situation to complete the project on schedule and on budget.