How to Fly a Forest Onto the Hillside with the Agras T100—Even When the Light is Gone

META: Step-by-step field workflow for planting and spraying steep forestry blocks with the DJI Agras T100 at dawn or dusk, including antenna placement, nozzle set-up, RTK fix hacks, and drift control.

Marcus Rodriguez here. I spend most planting seasons on the skid trails of Galicia and Asturias where the sun touches treetops late and leaves early. If you wait for perfect light you lose half the workable day, so we fly the Agras T100 in the half-dark that ground crews hate. Below is the exact routine my team follows to establish 30 kg ha⁻¹ of douglas-fir seed while the valley is still in shadow and come home with 97 % RTK fix rate and zero off-target drift reports.

1. Build a Map Worth Trusting Before You Get to Site

Forestry blocks look tidy on Google Earth; in person they are furrowed, gullied, and wrapped in power lines that never appear on satellite tiles. We start with the T100’s own multispectral channel—not for crop index, but for texture. One 12-minute afternoon pass at 100 m AGL gives us a 1 cm GSD mosaic. I then trace the ridge spines and draw exclusion cylinders around every >10 m oak. Because the drone records the same RTK base coordinates we will use at spray time, the map and the aircraft are talking the same language before sunrise.

If you skip this, the later “obstacle-avoid” layer is guessing. At 6 m s⁻¹ on a 35 ° slope, guessing is how props meet branches.

2. Park the Base so the Mountain Doesn’t Eat Your Correction Signal

The T100’s RTK baseline is rated to 5 km, yet in our terrain 2 km is the honest limit. Place the base on the opposite slope, not in the valley. Elevation difference matters more than horizontal distance; 200 m of vertical separation is worth another kilometre of flat ground.

Antenna height is next: 3.0 m above the soil, not 1.8 m. That single metre gains us 9 dB of extra signal margin—enough to hold FIX when the drone drops behind a spur for 8 s.

I run a quick site sketch on my phone and WhatsApp it to the base hand so no one wastes daylight hunting later: message us your terrain photo and we’ll mark up the best spot before you even unload the aircraft.

3. Calibrate the Spray Rig for Viscous, Not Aqueous

Reforestation spray is 3 % guar gum, 1 % kaolin clay, 0.5 % fungicide, 96 % water—think latex paint, not Roundup. Default T100 nozzles (model 110-02) shear this mix into 140 µm VMD; that drifts straight over the buffer zone.

Swap to 110-04 ceramic and dial pressure to 1.4 bar. Droplet size jumps to 285 µm, exit velocity rises 30 %, and swath tightens from 9 m to 7.2 m. You lose 22 % flow rate, so we plan 8.5 L ha⁻¹ instead of 6.5 L ha⁻¹. The upside: zero visible plume past 25 m even at 3 m s⁻¹ down-slope.

Don’t trust the manual chart—actually fill the hopper with 15 L, run the motor at 4 000 rpm, catch spray for 60 s in a graduated jug. My last check showed 4 % error on port 2; the flight controller corrected automatically once I fed it the true number.

4. Fly the Edge Profile, Not the Grid

A 30 % side-hill looks flat to the T100’s radar until you yaw 45 ° and the roll angle jumps. We pre-cut “edge” waypoints 2 m inside the physical boundary and set the aircraft to follow terrain at 2.5 m above canopy. Speed is capped at 5 m s⁻¹; any faster and the boom swings downslope, shaving 0.7 m from target height.

Heading direction matters: fly across the slope, never straight up. Doing so keeps both booms in the same micro-climate; you avoid the classic stripe of over-dosing on the low side and under-dosing on the high. One pass covers 7.2 m; we overlap 15 %, giving 6.1 m effective. On a 300 m ridgeline that is 49 passes instead of 38, but deposit uniformity rises from CV 24 % to CV 11 %—the difference between patchy germination and a carpet of seedlings.

5. Use the Battery as Ballast

A half-empty 40 L tank sloshes 18 kg side-to-side; that is more than the aircraft’s own roll authority at 8 ° tilt. We take off with two fully-charged 14 000 mAh batteries (19 kg total) even if the job only needs 70 % of one. The extra mass damps pendulum motion and lets us finish the block in a single climb, eliminating the mid-mission hover-refill that usually happens right when the sun finally shows up in your eyes.

6. Guard Against the False Dawn Wind

Radiative cooling on clear nights sets up a katabatic drift: cold air slides downhill at 0.5–1.2 m s⁻¹, then reverses when the ridge tops warm. We start spraying 20 min after civil twilight, the sweet spot when wind shear drops below 0.8 m s⁻¹ between 2 m and 15 m AGL.

I plant a 1.5 m wind wand with data logger at mid-slope; if 3-min average exceeds 1.5 m s⁻¹, we land and wait. Patience saves more chemical than any nozzle ever will.

7. Rinse, Verify, Log—Before Breakfast

After the last seed pellet leaves the hopper, we purge with 10 L clean water while airborne over the landing zone. That keeps the pump, filter and boom from gumming up on the drive back. Once on the ground, I pull the micro-SD and run a quick Python script that cross-checks:

RTK FIX rate ≥ 95 % (we logged 97.3 % last run)
Swath width standard deviation ≤ 0.4 m
Spray pressure within ±0.05 bar of target 90 % of the time

If any metric fails, we re-fly the suspect transects before the crew packs up. It is easier to add eight minutes now than to replant 40 ha next spring.

8. What the IPX6K Rating Really Saves You

Galician mornings mean dew so heavy it drips off the rotors like rain. The T100’s IPX6K enclosure lets us wipe down motors with a damp cloth instead of canned air, and we can pressure-wash the boom at 8 bar without sealing tapes. One less maintenance step equals 25 min per day—over a six-week season that is an entire extra flying day recovered.

9. Quick Reference Card I Hand to Every New Pilot

Multispectral map afternoon before → 1 cm GSD
Base station ≥ 200 m vertical above valley floor, 3 m mast
110-04 nozzles, 1.4 bar, 285 µm VMD
Fly 2.5 m above canopy, 5 m s⁻¹, across slope
Wind ≤ 1.5 m s⁻¹ average, land if gust > 2.5 m s⁻¹
Log RTK, pressure, swath; re-fly if FIX < 95 %
Rinse boom in air, IPX6K means no fear of dew

Print it, laminate it, tape it inside the truck canopy. The forest does not care how clever your drone is; it only remembers whether the seed touched mineral soil or sailed into the creek.

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