Multispectral survey
The multispectral survey is a precision-agriculture cornerstone. DroneField computes NDVI, GNDVI, NDRE, EVI and VARI from imagery captured with DJI Phantom 4 Multispectral (P4M) and Mavic 3 Multispectral (M3M).
What does “multispectral” mean?
Section titled “What does “multispectral” mean?”A normal RGB camera records 3 spectral channels (Red, Green, Blue). A multispectral camera records 4–5 channels:
| Band | Wavelength | What it’s good for |
|---|---|---|
| Blue | 450 nm | P4M only; reconstructed for M3M |
| Green (G) | 560 nm | Chlorophyll content |
| Red (R) | 650 nm | Chlorophyll absorption |
| Red Edge (RE) | 730 nm | Chlorophyll saturation, canopy health |
| NIR | 840 nm | Leaf structure, canopy density |
Vegetation indices (NDVI, GNDVI, NDRE, EVI, VARI) are computed from these bands (see Vegetation indices).
Why are multispectral images black & white? A 3-minute video explainer:
P4M vs M3M
Section titled “P4M vs M3M”| Property | P4M (2018) | M3M (2022) |
|---|---|---|
| RGB camera | 2 MP (small) | 20 MP |
| MS bands | 5 (B, G, R, RE, NIR) | 4 (G, R, RE, NIR) |
| Auto-exposure | Manual (sensor) | Automatic |
| RTK | Yes | Yes |
| Flight time | ~25 min | ~45 min |
| Take-off mass | 1.5 kg | 0.95 kg |
The M3M is significantly more modern — faster, longer flight, much better RGB. For a fresh purchase the M3M is the right call.
P4M’s Blue band absent from M3M: DroneField reconstructs the Blue band from the JPEG — with limited accuracy, but enough for the classical indices.
Planning
Section titled “Planning”Altitude
Section titled “Altitude”- 60–80 m AGL for typical agricultural NDVI
- Higher (100 m+): the alignment noise between bands can grow
Overlap
Section titled “Overlap”For multispectral, high overlap is even more important — each band is a separate image, and band-to-band registration has to be tight:
- Along-track: 80% (or more)
- Cross-track: 75% (or more)
Weather
Section titled “Weather”Overcast is mandatory (officially recommended). Why:
- Direct sun casts shadows — uneven reflectance
- Thin clouds change moment-to-moment, introducing differences between neighbouring images
DroneField corrects for changing light with the drone’s sunlight sensor (see below), but it can’t fully compensate for shadows and fast-changing cloud cover.
Time of day
Section titled “Time of day”10:00–14:00 (Europe). The sun’s perpendicular position reduces shadow length.
Calibration panel and sunlight sensor
Section titled “Calibration panel and sunlight sensor”DroneField computes reflectance from the image metadata: the sunlight sensor (irradiance) readings on top of the drone, the camera’s vignetting correction and the per-band sensor gain adjustment — no panel images are needed for that. Make sure the sunlight sensor is clean and unobstructed.
Photographing a calibration panel before / after the flight is still good DJI practice (e.g. for other software or your own checks), but DroneField does not use panel images — keep them out of the import folder.
Image grouping
Section titled “Image grouping”DJI M3M captures 5 images per exposure:
DJI_0001.JPG— RGBDJI_0001_MS_G.TIF— Green bandDJI_0001_MS_R.TIF— Red bandDJI_0001_MS_RE.TIF— Red Edge bandDJI_0001_MS_NIR.TIF— NIR band
DroneField groups them automatically by prefix. Don’t rename the files!
Processing time
Section titled “Processing time”For a 100-image Mavic 3 M RTK RGB survey:
| Configuration | GSD | Time |
|---|---|---|
| Intel i9 / 16 GB RAM / Windows | 6 cm | < 40 seconds |
| Mac M4 / 24 GB RAM / macOS 26 | 5 cm | < 30 seconds |
Multispectral processing adds roughly 40-60% to the runtime (separate rendering of 4-5 bands).
Reading the result
Section titled “Reading the result”What matters on NDVI / GNDVI / NDRE maps are the relative
differences, not the absolute values. An NDRE of 0.55 alone
says little — but if the field average is 0.62 and there’s a
patch around 0.40, that’s stress or weakness there.
Typical interpretation mistakes
Section titled “Typical interpretation mistakes”- “NDVI is above 0.85 everywhere — depleted” — no, that’s saturation, no differentiation across the dense canopy. NDRE differentiates.
- “Banded NDVI” — weather change, not the plant.
- “Low NDVI in shadowed area” — tree, wire and cloud shadow all lower the index. Not stress, just illumination.
When to re-survey?
Section titled “When to re-survey?”Index changes should be tracked at 2–3 week intervals during the season. Within a “survey season”:
- Spring, growth phase — first survey
- Before flowering — second survey
- Before harvest — third survey (if relevant)
Save these as separate projects in DroneField, not the same one.