Orthomosaics in the Wet Tropics: planning around canopy and weather.
Most orthomosaic work in the Wet Tropics gets shaped less by the drone and more by two things: the canopy underneath the flight, and the weather window above it. Plan for those two and the rest tends to follow.
The Wet Tropics coast around Cairns runs into closed-canopy rainforest within minutes of leaving most coastal sites. That changes a few assumptions baked into standard orthomosaic flight planning - assumptions that work fine over open paddock or a construction pad, but quietly break down once the ground beneath you stops being visible.
§ 01Canopy first, drone second
For terrain analysis or any deliverable that needs a digital surface model, the canopy is the surface. That's not a problem in itself - clients working in conservation, land management, or vegetation monitoring usually want it that way. The issue is when the brief sounds like a ground-level survey but the site is anything but.
Before flying, the question worth asking is: what is the deliverable representing? Top of canopy, ground, or something in between? The answer changes flight altitude, overlap, and how the data gets processed downstream.
- Top of canopy - higher altitude, standard overlap, fastest turnaround.
- Mixed terrain - tighter overlap (80/70 or higher) to handle parallax under partial canopy.
- Bare-earth derivation - paired with ground control, and a clear conversation with the client about the limitations.
§ 02The two-hour weather window
Wet season changes the maths. Most days through the wet, useful flying time isn't a daylight problem - it's a convection problem. Cumulus builds through late morning and afternoon storms can roll in faster than a survey grid takes to complete.
The job isn't to fly the perfect grid. It's to come back with usable data, on time, without putting the aircraft or anyone on the ground at risk.— Standard operating note
Practically, that means the early morning window from about 7am to 10am does a lot of the work through summer. Mobilise the day before where remoteness allows it. Have the grid pre-loaded. Treat anything after midday as a bonus rather than a plan.
For larger sites, splitting the capture across two adjacent mornings is often more reliable than pushing one long flight into deteriorating conditions. The orthomosaic stitches without issue if lighting is reasonably consistent across the captures.
§ 03Ground sample distance and the deliverable
Ground sample distance gets quoted up front in most scopes, but it's worth treating as an output of the brief rather than an input. A 2cm GSD over a 40-hectare site sounds precise until you're flying it three times because storms ate two of the windows.
For most environmental and land mapping work in this region, a GSD between 2 and 4cm covers what's actually being asked. Tighter than that needs to be earned by the deliverable - and almost always means more flights, more battery turnarounds, and tighter weather windows.
§ 04Ground control, lightly
Ground control points are still the most reliable way to lock spatial accuracy. In dense rainforest, they're also the hardest to deploy. Pragmatic compromises: use accessible canopy gaps, established tracks, or pre-existing infrastructure as reference where the survey-grade alternative isn't feasible. Flag the limitations in the deliverable.
Closing note
None of this is unique to the Wet Tropics - the principles apply anywhere with canopy and weather. But the combination here is unusual, and underestimating either one is a quick way to turn a one-day capture into a three-day program. Plan around the canopy and the window. The rest is just flying.