Field Report: How the Agras T100 Became My Quietest Survey
Field Report: How the Agras T100 Became My Quietest Survey Partner on a Vertical-Build Site
META: Marcus Rodriguez explains why he swapped a purpose-built camera drone for the Agras T100 when documenting a 42-floor tower in Chongqing’s old town, and the battery trick that kept him airborne during golden hour.
The first time I flew an Agras T100 over a construction site, the safety officer thought I’d lost my mind. “That’s a crop-sprayer,” he said, pointing at the carbon-fiber arms still stained chartreuse from the last rice cycle. He was technically correct; the T100 ships with a 40-litre tank and no gimbal. But the moment I showed him the RTK Fix rate holding at 99.8 % while the aircraft hovered 28 m above the rebar lattice, he stopped laughing and started taking notes. By the end of the week the superintendent was using my orthomosaic to settle a concrete-pour dispute that had already cost the programme four days.
This is not a conversion story about “re-inventing” an ag drone. It is simply what happened when I needed centimetre precision, a 45-min hover window, and IPX6K weather insurance on a vertical-build project wedged between Chongqing’s Yangtze cableway and a 16th-century temple. The T100 was already on the rental shelf, so I treated it like any other tool: strip it to the essentials, test the edge cases, keep what works.
Why I Even Considered an Agricopter
The brief sounded innocent: produce a weekly progress model of a 192 m tower, deliverable 6 a.m. Monday, 3 cm GSD, no road closures, no night flights. The catch was the only viable take-off point: a 6 × 6 m platform cantilevered over an active tourist wharf. Anything bigger than a suitcase would block pedestrian flow and trigger a permit nightmare.
I ran the numbers. A standard quad-mapping rig could do it on three batteries—if the wind stayed below 7 m s⁻¹, which, in a Yangtze gorge, is fantasy. A hexacopter with extended arms had the lift but sounded like a swarm of hornets; the temple monks had already complained about vibration during foundation blasting. The T100, on the other hand, swings 4 × 130 cm prop discs at only 900 rpm when loitering. Decibel meter: 58 dB at 30 m, roughly a conversation in a café. More importantly, it carries two 14S 18 000 mAh smart packs that can be hot-swapped without rebooting the flight controller, so the clock keeps running while the tourists keep walking.
Stripping the Tank, Keeping the Redundancy
Removing the spray system took 23 minutes: eight M4 bolts, two quick-disconnect dry-break couplers, one Molex plug for the peristaltic pump. The aircraft drops 9.4 kg and gains 8 cm of ground clearance—handy when you hand-launch from a barge deck. I left the original boom tubes in place; they double as landing skids and protect the downward vision sensors from stray rebar.
With the tank gone, the T100’s stock payload bay opens to 280 × 310 × 200 mm—plenty for a 42 MP APS-C mapping camera with mechanical shutter. I mounted it on a 3D-printed wedge that clips into the same dovetail used by the chemical hopper, so the centre of gravity stays inside DJI’s published envelope. The wedge also angles the lens 12° off nadir to eliminate prop blur without needing a gimbal; you correct the oblique in photogrammetry software later.
RTK, But Treated Like Survey Gear
Construction sites are RTK graveyards: tower cranes bounce signals, sheet-metal scaffolding creates multipath ghosts. Out of the box the T100 holds a Fix rate above 95 % in open farmland, but urban canyons are a different religion. I borrowed a trick from total-station workflows: set the base on the roof of the temple, 37 m above the wharf, giving clear line-of-sight to the whole block. The base broadcasts corrections through a 35 MHz radio instead of 4G; the temple’s thick granite walls had turned the cellular modem into a heater. Result: Fix rate averaged 99.4 % over 42 flights, and the one time it dropped to Float the aircraft auto-initiated RTH instead of soldiering on—a safety response I now trust more than my own reflexes.
Swath Width vs. Golden Hour
The client wanted facade texture, not just roof progress, so I flew an offset grid: 28 m altitude, 12 m s⁻¹, 80 % front overlap, 70 % side. With the APS-C’s 35 mm equivalent lens the ground swath is 44 m; multiply by 1.3 for the oblique wedge and you still cover a full tower face in three passes. The whole mission fits inside the 15-min window when the rising sun grazes the glass curtain wall but hasn’t yet thrown hard shadows on the cast-in-place core.
Battery management becomes choreography. I land with 25 % reserve, swap packs while the camera is still writing to the SD card, and relaunch within 90 seconds. The trick: pre-heat the spare batteries to 35 °C using a portable sous-vide stick plugged into the site canteen’s kettle outlet. Li-ion internal resistance drops just enough to squeeze an extra 2.5 min hover time on the second pack—long enough to re-fly a strip if a tourist ferry wanders into frame. I learned this on a rice terrace in Guizhou where mornings start at 8 °C; the same physics works on a Chongqing winter site at 11 °C.
Spray-Drift Lessons Applied to Dust
Ironically, the agronomic heritage of the T100 is what keeps the lens clean. The original nozzles are spaced at 50 cm and calibrated for 50–500 µm VMD. Replace the diaphragms with blanking plugs and you get four pressure-balanced air jets that blow grit away from the camera port whenever the payload bay door opens. Think of it as an inverse crop-duster: instead of pushing chemicals out, it pushes dust away. After 61 flights I found only two microscopic pits on the UV filter—both occurred the day I disabled the jet rig to save 120 g of take-off weight. Lesson learned: the weight penalty is cheaper than a filter replacement.
IPX6K and the Typhoon That Wasn’t
Chongqing’s spring weather arrives like a drunk contractor: unpredictable, loud, occasionally violent. One Friday the harbour master issued a Force 6 warning at 14:00; by 14:30 the anemometer on the crane cab hit 13 m s⁻¹ gusts. I had just landed the T100 when the sky tore open. Horizontal rain, 28 °C drop in ten minutes. The aircraft sat on the skids for 45 minutes while we sheltered under the barge’s tarp. Water streamed over the fuselage, but the IPX6K rating—tested at 100 L min⁻¹ from a 12.5 mm nozzle at 100 kPa—meant the flight controller never lost its heartbeat. I wiped the GNSS antennas, relaunched for a quick stability check, and still finished the mission before civil twilight. Try that with a consumer quad and you’ll be shipping a brick back to Shenzhen.
Data Chain From Prop Wash to BIM
Back in the site container I off-load 1 847 RAW frames, each geotagged with RTK-corrected coordinates. Processing is vanilla Agisoft: align, build dense cloud, mesh, texture. The only tweak is a custom python script that reads the T100’s flight log and embeds yaw, pitch, roll into the EXIF—handy when the facade team asks why a shadow moved between weeks. The resulting 3 cm mesh drops straight into Navisworks alongside the BIM cube; clash detection caught a 22 cm misalignment between the mechanical sleeve and the curtain-wall anchor before the steel left the workshop. Savings: one redesign iteration, three weeks off the critical path, enough budget to pay for the entire season of drone surveys.
Regulatory Side-Note: Why the Tank Still Matters
Civil Aviation Administration of China (CAAC) rules classify any UAV by maximum take-off weight, not by mission. With the tank removed the T100 clocks 38.6 kg—just under the 40 kg threshold that triggers extra paperwork for urban ops. Had I left the dry-hopper on, even empty, the bird would have tipped the scale at 42 kg and required a one-off airworthiness inspection that costs more than the rental. Always weigh the aircraft after modification; the difference between 38.6 and 42 is the difference between Friday delivery and Monday bureaucracy.
One Last Thing About Tourists and Prop Noise
Remember the Chongqing flower-sea story that made headlines last month? Ten thousand mu of rapeseed planted on terraced hills, visitors queuing for balloon rides and helicopter selfies. The operator there told me his R44 burns 180 L of Jet-A per scenic loop—noisy, thirsty, and dependent on refuelling trucks that crawl up switchbacks. Watching a T100 document a skyscraper at 58 dB while tourists eat fish-head hotpot 20 m away made me realise the same math applies: low-altitude flight doesn’t have to be loud to be profitable. The only difference is my payload is photons, not passengers.
If you’re curious about the wedge drawings or the sous-vide battery table, I keep the open-source files on a Telegram channel—ping me on this WhatsApp thread and I’ll send the link. No sales pitch, just CAD and thermal logs.
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