Agricultural sensor glossary

Measurement and instrument performance

Agricultural sensor specifications combine measurement terminology, agronomic quantities and communications terms. The meaning of a value depends on the measured quantity, unit, conditions and instrument configuration. This glossary groups related terms so that a specification can be read alongside its measurement method.

Measurand

The quantity intended to be measured, such as soil water content at a specified location and depth.

Accuracy

Closeness of a measured value to the true value of the quantity. It is distinct from agreement between repeated readings.

Precision

Agreement among repeated measurements under stated conditions.

Resolution

The smallest change in the measured quantity that produces a perceptible change in indication.

Measurement uncertainty

A parameter describing the dispersion of values attributed to the measured quantity. Its interpretation requires the evaluation method and stated coverage.

Calibration

Establishing a relationship between reference values, indications and their uncertainties, then using it to obtain measurement results under specified conditions. Adjustment changes the instrument response and is a separate operation.

Drift

A change in an instrument's indication over time associated with its measurement characteristics.

Repeatability

Measurement precision under specified conditions that retain the procedure, system and operating circumstances over a short interval.

Soil and irrigation quantities

Volumetric water content

Water volume divided by the volume of the soil sample or sensing region, commonly expressed as m³/m³ or a percentage. A value of 0.25 m³/m³ equals 25% by volume.

Matric potential

The component of water potential associated with the soil matrix. Instruments may report a negative potential or a positive suction magnitude; the sign convention must accompany the value.

Field capacity

A soil water condition following substantial drainage after wetting. Its value depends on soil characteristics and the method used to define it.

Available water

The water held between specified upper and lower soil water limits. The amount accessible to a crop also depends on rooting depth and distribution.

Electrical conductivity (EC)

A measure of electrical conduction, often reported in dS/m or mS/cm. Bulk-soil, pore-water and extract measurements describe different measurement contexts.

pH

A logarithmic quantity describing hydrogen-ion activity. A solution measurement and a soil-extract measurement require their respective sampling methods.

Flow rate

Volume passing a point per unit time, such as litres per second or cubic metres per hour.

Water level

The vertical position of a water surface relative to a stated reference. Radar distance becomes level through the configured mounting reference.

Pycno Terra and Soil Mini illustrate different soil-system specifications. Terra uses explicit Master and Node configurations; Soil Mini offers variant-specific interfaces for a compatible acquisition system. The soil-sensor comparison explains these architectures and alternative instruments.

Weather and crop quantities

Relative humidity

The ratio of water-vapour pressure to saturation vapour pressure at the same temperature, expressed as a percentage.

Vapour-pressure deficit

The difference between saturation and actual water-vapour pressure. Air-based and leaf-based calculations use different temperature assumptions.

Evapotranspiration

Combined water loss through evaporation and plant transpiration. Reference evapotranspiration is a modelled reference quantity; crop evapotranspiration requires additional crop and condition information.

Leaf wetness duration

The time during which a sensor's configured wetness criterion is met. Surface position, threshold and sampling interval affect the result.

Growing degree days

Accumulated temperature above a selected developmental base, subject to the calculation method and any temperature limits.

PAR

Photosynthetically active radiation, conventionally associated with wavelengths from 400 to 700 nm.

PPFD

Photosynthetic photon flux density, commonly measured in µmol/m²/s.

Daily light integral

Accumulated photosynthetic photon exposure over a day, commonly expressed in mol/m²/day.

NDVI

Normalized difference vegetation index: near-infrared reflectance minus red reflectance, divided by their sum. It describes a spectral relationship rather than a unique diagnosis.

Interfaces and data systems

Data logger

A device that acquires and records observations, often with a programmed measurement sequence.

Gateway

A device connecting supported instruments or networks to another communications system.

SDI-12

An addressed serial interface at 1200 baud used by environmental instruments to transfer measurements to recorders.

RS485

An electrical signalling specification used by differential serial links. It does not define the meaning of an instrument's registers.

Modbus

A communications protocol used to access addressed data and functions. Register interpretation remains device-specific.

LoRa

A radio modulation technology used in long-range, low-power communications.

LoRaWAN

A networking specification for compatible devices, gateways and servers using LoRa radio technology.

GNSS

Global navigation satellite system; the category includes GPS and other satellite positioning systems.

RTK

Real-time kinematic positioning, using carrier-phase observations and correction information to support precise relative positioning.

ISOBUS

Agricultural machinery communications based on ISO 11783. Supported functionalities determine compatibility between specific components.

Sampling interval

Time between successive observations. It may differ from the interval between uploads.

Quality flag

A recorded indicator identifying conditions such as a missing value, invalid measurement or substituted observation.

Units and data interpretation

A value should retain its quantity and unit through export and analysis. A change of 0.01 m³/m³ equals one percentage point of volumetric water content; it is not necessarily a 1% relative change. One dS/m equals one mS/cm. A rainfall depth of 1 mm over one hectare represents 10 m³ before interception, drainage or other losses.

Range, resolution and accuracy describe different properties. The operating-temperature range of an enclosure does not establish the accuracy of a temperature observation across that range. Likewise, a digital display with additional decimal places does not establish improved measurement performance.

Development of terminology

The International Vocabulary of Metrology provides a common framework for measurement concepts. Environmental interfaces developed alongside electronic acquisition, with SDI-12 originating in 1988. Agricultural machinery standards and remote sensing added terminology for spatial data and automated operations.

Frequently asked questions

Are soil water content and soil moisture the same term?

Soil moisture is used broadly. A specification should identify whether the reported quantity is water content, matric potential or a relative instrument response.

Does a sensor's EC value measure fertilizer concentration directly?

Conductivity reflects the combined electrical contribution of dissolved ions and the measurement context. Identifying individual nutrients requires a suitable analytical method.

What information should an exported sensor record contain?

Device and channel identity, original observation time, quantity, unit and quality status provide the minimum context for interpretation. Calibration and configuration records should remain associated with the instrument.