Satellite monitoring vs. field audits: why both are necessary for credible biodiversity evidence

Procurement teams evaluating biodiversity credit providers often ask which monitoring approach is used: satellite or field. The implicit assumption is that one is more rigorous than the other — that a provider relying on remote sensing is cutting corners, or conversely that field visits are the only thing that counts as real evidence.

Neither position is correct. The question is not which method to use. It is what each method can and cannot establish, and whether the evidence package as a whole addresses the questions an assurance provider will ask. On that test, satellite monitoring and field surveys are complements, not substitutes. Each one covers the limitations of the other.

What satellites provide

Modern earth observation satellites pass over any point on the planet every five days. The Sentinel-2 archive, operated by the European Space Agency, provides continuous multispectral imagery from 2015 onward at 10-metre resolution. For biodiversity monitoring purposes, this capability is transformative: it means any land parcel can be observed continuously, its vegetation condition tracked backward to a pre-intervention baseline, and any disturbance event — fire, clearing, flooding, agricultural incursion — detected within days of occurrence.

The Normalised Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI) derived from satellite imagery are established proxies for vegetation density and photosynthetic activity. Increases in NDVI over time on a restoration site are direct evidence of vegetation recovery. Sudden NDVI drops are evidence of disturbance. Neither can be fabricated retrospectively — the satellite archive is an independent, timestamped record that no party to a credit transaction controls.

At landscape scale, satellite analysis can assess connectivity between restoration sites and adjacent natural areas, detect encroachment at parcel boundaries, and classify land cover transitions from bare ground or degraded habitat to recovering forest or grassland. For programmes covering hundreds or thousands of hectares, there is no cost-effective alternative.

What satellites cannot do

At 10-metre resolution, a satellite pixel covers a hundred square metres of ground. Individual trees, let alone individual plant species, are not identifiable. NDVI reflects aggregate vegetation density — it cannot distinguish between a recovering native forest and a monoculture plantation with similar canopy cover. It cannot measure soil carbon content, record species-level biodiversity indicators, or observe the presence of fauna. It cannot confirm that the ecological change suggested by a vegetation index reflects actual ecosystem recovery rather than some other confounding factor.

What field surveys provide

A field survey conducted by a qualified, independent ecologist provides what satellite imagery cannot: species-level identification, direct observation of ecosystem condition, and measurement of the below-ground and fauna dimensions of biodiversity that remote sensing cannot reach. A skilled field ecologist can assess habitat condition against published reference benchmarks, identify invasive species, evaluate soil health, and record the presence of indicator species whose presence or absence reflects broader ecosystem quality.

Field surveys also perform a critical methodological function: they ground-truth the satellite signal. If satellite imagery shows an NDVI increase on a restoration site, a field survey can confirm whether that increase reflects native species regeneration, encroachment by a fast-growing exotic, or natural succession in a previously degraded area. The satellite detects change; the field survey interprets it.

What field surveys cannot do

Field surveys are expensive. A qualified ecological survey of a single parcel of a few hundred hectares costs thousands of euros and takes several days. Programmes covering tens of thousands of hectares cannot be surveyed in their entirety on a regular basis. Field surveys are also snapshots: they record conditions at the time of the visit, not continuously. A site that was surveyed in good condition in Year 1 could deteriorate in Year 2 without detection until the next scheduled visit. Field surveys alone cannot provide the continuous monitoring record that assurance providers require to confirm that a restoration intervention is ongoing and the site is not reverting.

The assurance standard

Why ISAE 3000 assurance requires both

Assurance providers reviewing biodiversity evidence under ISAE 3000 are asked to conclude whether a positive biodiversity action disclosure is not materially misstated. To reach that conclusion, they need to answer two distinct questions, and neither method alone can answer both.

The first question is whether the site is being monitored continuously and whether ecological condition is changing in the expected direction over time. This is a question about the ongoing nature of the intervention — whether the restoration is active, whether the site has not been cleared, whether any disturbance has been detected and addressed. Satellite imagery answers this question. It provides the timestamped, independent, continuous monitoring record that demonstrates the intervention is real and ongoing.

The second question is whether the satellite signal corresponds to genuine ecological improvement, and whether the biodiversity indicators claimed in the disclosure actually reflect conditions on the ground. This is a question about the ecological validity of the monitoring data. Field surveys answer this question. An independent ecologist's assessment confirms that the NDVI increase recorded by satellite reflects native species regeneration, that the habitat condition index has improved against the baseline benchmark, and that the species composition data is consistent with the claimed outcome.

Providers that rely on satellite data only can demonstrate that vegetation is recovering but cannot confirm what is recovering or whether the ecological interpretation is sound. Providers that rely on field surveys only can confirm ecological condition at specific points in time but cannot demonstrate continuous monitoring between visits. Neither passes the full evidence test that assurance providers apply.

biodiversity.earth methodology

How biodiversity.earth combines both methods

Every biodiversity.earth credit is backed by an integrated monitoring programme that uses satellite coverage and independent field surveys as complementary evidence streams, not alternatives.

Sentinel-2 imagery is processed for each parcel on a continuous basis, generating an annual monitoring record that covers vegetation density, canopy formation, disturbance events, and landscape connectivity. The archive extends from 2015, providing a pre-intervention baseline that predates credit issuance. This record is available to purchasers and their assurance providers as independently accessible raw data — not filtered through a programme portal.

Independent field surveys are conducted by qualified ecologists at the pre-intervention baseline stage and at the five-year outcome assessment milestone, with interim visits scheduled as ecological or monitoring conditions require. The field surveys document species composition, habitat condition index, and soil condition using standardised methodology that is consistent between baseline and outcome measurements — so that the comparison is interpretable and auditable.

The combination produces the complete evidence record that assurance providers require: a continuous satellite monitoring archive demonstrating change over time, validated by independent field surveys confirming ecological interpretation, with a pre-intervention baseline that predates the credit transaction and an outcome comparison that is methodologically consistent with it. Both evidence streams, together with the credit registry record and full audit documentation, are included in every biodiversity.earth credit package.