Why GSD Decides Every Drone Survey
Ground Sample Distance is the single most important number a drone surveyor calculates. It tells you how many real-world centimeters each pixel in your final ortho or point cloud represents. A 2 cm/px ortho is workable for a cadastral re-mapping; a 12 cm/px ortho is useless for anything beyond aerial reconnaissance.
The math is simple but the consequences are not:
``` GSD (cm/pixel) = (sensor_width_mm × altitude_m × 100) / (focal_length_mm × image_width_px) ```
Push altitude up to cover more area in one flight and GSD gets coarser. Drop altitude to get finer GSD and you fly longer, take more photos, drain more batteries, and produce gigabytes more data. Pre-flight planning is choosing the right point on that trade-off, before you take off — not after.
The new Drone Mission Planner on SurveyingPedia does this calculation for you across 20 real drone models, plots the lawn-mower flight pattern on a satellite map, and exports a KML you can import directly into DJI Pilot 2, Litchi, or QGroundControl. Free, no signup. This article is the surveyor's mental model behind it.
Pick the GSD First, Altitude Second
The right workflow is: decide what GSD your client needs → compute the altitude that gets you that GSD on your drone → check the flight time is feasible → fly. Trying to "just fly at 100 m" and seeing what comes out is amateur work.
| Project type | Recommended GSD | |---|---| | Cadastral / property boundary | 2–3 cm/px | | Topographic mapping | 3–5 cm/px | | Volumetric (stockpiles, excavation) | 3–4 cm/px | | Infrastructure inspection (cracks, defects) | ≤1 cm/px | | Vegetation / forestry index | 5–8 cm/px | | Large-area orthophotos (>100 ha) | 5–10 cm/px |
With a DJI Mavic 3 Enterprise (Hasselblad 4/3" 12.8 MP, 12.29 mm focal), a 3 cm/px GSD lands at about 130 m AGL. With a DJI Phantom 4 RTK (1" CMOS 20 MP, 8.8 mm focal), the same 3 cm/px needs about 110 m AGL. The planner does this calculation live as you change the altitude slider — you see the GSD update in real time and the GSD value turns green when it's under 3 cm/px, amber up to 5, red above.
Overlap: The Other Half of the Decision
You can have the perfect GSD and still produce garbage if your overlap is wrong. Frontal overlap is along the flight strip — image N and image N+1. Lateral overlap is between adjacent strips.
The safe defaults the planner ships with are 80% frontal / 70% lateral. This is what Pix4D, Agisoft Metashape, RealityCapture, and WebODM all assume for high-quality output. Anything below 70/60 starts to risk thin tie-point coverage and holes in the output. Anything above 85/80 just wastes battery and time without measurably improving output (diminishing returns).
A few special cases:
* Tall vegetation, water, sand dunes — bump frontal overlap to 85–90% to give the software more tie points on featureless terrain. * Urban, structural — 80/70 is enough because edges and corners give the software plenty to grip on. * Repetitive industrial floors (solar farms, parking lots) — bump both to 85/80, because the floor is too uniform for the software to find features.
Flight Time, Battery Count, Photo Count
The mission planner computes all three from your altitude + overlap + polygon + drone model. The math behind it:
* Footprint per photo: width = sensor × altitude / focal · height same with the other sensor dimension. At 100 m with a Mavic 3E that's ~140 m × 105 m. * Strip spacing: footprint width × (1 - lateral_overlap). At 70% lateral that's 42 m. * Shot spacing within a strip: footprint height × (1 - frontal_overlap). At 80% frontal that's 21 m. * Photos: area / (strip spacing × shot spacing). * Flight time: total path length / cruise speed + turn overhead. Drones can't outrun the shutter — if you push speed above what the camera shutter interval allows, you'll miss shots. The planner caps cruise speed at min(drone max, shot_spacing / shutter_interval). * Batteries: flight time / (single battery time × 0.85). The 0.85 safety factor accounts for wind, takeoff/landing, and the "land at 20% remaining" rule every responsible pilot follows.
When you change a slider, all four big numbers in the dashboard update together — pull altitude down and see flight time go up, batteries go up, photos go up. Pull overlap down and see photos drop. This is the real value of having a live planner instead of doing the math on paper.
Reading the Warning Banners
The planner shows red warnings when it detects a problem:
* Altitude exceeds drone service ceiling — you set the altitude above the maximum the drone can fly. Some regulators also cap drone flights at 120 m AGL by default; the planner doesn't enforce regulatory caps, that's on you. * GSD too coarse — you're trying to map at 10+ cm/px. Almost no surveying use case justifies that. Usually you forgot to lower the altitude. * Area too large — flight time exceeds 5× the drone's single-charge endurance. Split into sub-missions, each ≤2× single-charge. * Area too tiny — you drew a polygon smaller than 100 m². Probably a misclick. Use the "Draw on map" button and click 3+ vertices to define a usable area.
Exporting and Flying
The planner gives you two export formats:
* KML — Google Earth standard. Imports cleanly into DJI Pilot 2 (the RC Pro app), Litchi (route planner mode), Mission Planner (ArduPilot ecosystem), QGroundControl (PX4 ecosystem), and most photogrammetry mission tools. The KML contains the polygon boundary, the flight path as a LineString, and individual Placemarks for each waypoint. * CSV — plain index, lat, lng, altitude. Use this when you have a custom mission system or want to script-import into your own tool.
Both files name the drone in the filename so you can keep multiple missions organized.
What the Planner Does Not Do (Yet)
* Terrain follow — altitude is AGL from the takeoff point. For hilly terrain use your flight app's terrain-follow mode or wait for our SRTM-based update. * Regulatory check — we don't know your country's drone laws. 120 m AGL is the EASA default, 400 ft (~120 m) is the FAA default. Some sites are restricted (airports, military zones). Check before flying. * Wind correction — flight time assumes calm. Add ~15% margin if winds exceed half the drone's rated maximum. * Battery temperature — cold batteries deliver less. Add ~20% safety in winter. * Camera ISO / shutter optimization — you set those at the drone interface.
These are conscious choices. Adding them without doing them properly would mislead surveyors. We'd rather ship a planner that gets the photogrammetric math exactly right and lets you handle the operational details, than a fancy tool that's wrong about both.
Related Tools and Reading
* GNSS Mission Planner — satellite visibility before takeoff * Deformation Monitoring — check site seismic activity * CORS Directory — find your nearest PPK reference station * Coordinate Converter — transform between datums after processing * Embed widgets — drop our maps into your blog or proposal
Open the planner now and bookmark it. Use it before every flight — your client's deliverable will thank you.