Why the term needs a definition
When scientists measure an object from close range, or sometimes in direct contact with it, they may use terms such as proximal sensing or proximal remote sensing. The term appears across many fields and disciplines, including soil science, crop research, vegetation monitoring, inland-water observation, snow and ice studies, and urban environmental monitoring.
However, proximal sensing is not always used in the same way.
In soil science, the term often includes sensors in contact with or close to the soil. In crop research, it may refer to handheld, vehicle-mounted, tower-mounted, or other field-based sensors. In ecosystem science, proximal remote sensing often describes cameras and optical instruments installed near or within vegetation canopies. Sensors may also be mounted on low-flying drones, on floating platforms or buoys, or beside water bodies. In other fields, similar systems may be described as in situ sensing, near-surface observation, close-range monitoring, or field instrumentation.
The scientific purpose of proximal sensing may differ among fields and even among individual studies. Yet many technical challenges, workflows, and procedures overlap. Different fields and traditions have developed their own terms and definitions, but there is still no single operational definition that works equally well across soil, vegetation, inland water, snow and ice, and urban environments.
A broader definition should identify the properties of the measurement rather than prescribe a particular platform, sensor, distance, or discipline.
How different fields define proximal sensing
Different research communities have developed their own definitions of proximal sensing, usually in response to the measurement problems most important in their field.
In proximal soil sensing, the definition is often explicit and distance-based. Viscarra Rossel and colleagues describe proximal soil sensing as the use of field-based sensors to obtain signals from the soil when the sensor is in contact with, or close to, the soil. In this tradition, “close” is often operationalised as within approximately two metres of the soil surface. The definition can include sensors inserted into the soil, sensors moved across the surface, and non-invasive instruments such as electromagnetic induction systems, gamma radiometers, visible and near-infrared spectrometers, and ground-penetrating radar. The emphasis is on detailed, field-based measurement of soil properties and their variation in space and time.
In precision agriculture and crop sensing, proximal sensing is commonly used for instruments placed close to plants or soil. These can include handheld sensors, tractor-mounted systems, field cameras, and fixed or mobile sensor platforms. Here, the term is often contrasted with airborne and satellite remote sensing. The practical focus is frequently on observing within-field variability, supporting crop management, and linking sensor signals with plant, soil, and environmental conditions.
In ecosystem science, the related term proximal remote sensing is often used for optical instruments installed near or within ecosystems. These may include tower-mounted cameras, spectral radiometers, thermal sensors, fluorescence instruments, microwave systems, and terrestrial laser scanners. The emphasis is often on continuous, high-temporal-resolution observations that link plant- and canopy-level processes with eddy-covariance measurements, airborne surveys, and satellite products.
Across these traditions, the details differ. Soil science may include sensors in direct contact with or within the target. Ecosystem science often focuses on non-contact optical observations from towers. Agricultural applications may use vehicle-mounted, handheld, or drone-mounted systems.
Yet the underlying idea remains similar: measurements are made close enough to the target to provide detailed observations under conditions that can be designed, controlled, and documented by the observer.
The proposed definition below does not replace these established uses. It identifies common measurement conditions that allow proximal sensing to be recognised across fields.
A proposed definition
When proximal sensing is considered in terms of its physical implementation, three principles recur across fields, traditions, and instrument types:
- The sensor observes or measures the target at sufficiently short range that atmospheric correction is not required.
- The observation can resolve sub-metre spatial detail of the target.
- The observer can control and document the sensor-target geometry.
A cross-domain definition of proximal sensing
Proximal sensing is the observation or measurement of an environmental target at sufficiently short range that atmospheric correction is not required, sub-metre spatial detail of the target can be achieved, and the observer can control and document the sensor-target geometry.
The definition is independent of platform type. Proximal sensing may involve contact or non-contact measurements and can be conducted using handheld, fixed, tower-mounted, vehicle-mounted, robot-mounted, drone-mounted, or other field-based sensor systems.
This definition is deliberately based on measurement conditions rather than on the name of the platform. A sensor can be handheld, mounted on a tripod, installed on a tower, attached to a vehicle, placed on a robot, carried by a drone, or integrated into a fixed field system.
It may observe a target without contact, or it may be in contact with or embedded in the target where that is part of an established disciplinary practice.
The definition is not intended to erase existing uses of the term. It aims to provide a practical common ground for discussing a diverse family of observations.
Short range is not one distance
No universal distance can define proximal sensing.
A few centimetres may be a long distance for a leaf spectrometer and a negligible distance for a camera observing a crop canopy. A drone at ten metres above vegetation may be close enough for high-detail observation with minimal atmospheric influence, while a tower camera at the same distance may have a different field of view, target area, and measurement purpose.
The relevant question is not simply how far the sensor is from the target. It is whether the sensor-target path is short enough that atmospheric correction is not required for the intended measurement.
This does not mean that air between the sensor and target has no effect. Temperature, humidity, aerosols, haze, turbulence, and path length can influence measurements under particular conditions. It means that atmospheric effects are not a primary correction step required to interpret the observation in the way they often are for more distant optical remote-sensing measurements.
The phrase “sufficiently short range” is therefore a physical and methodological criterion. It must be evaluated in relation to the sensor, wavelength, target, measurement path, and required accuracy.
Spatial detail matters
Proximal sensing should make it possible to resolve the target at sub-metre spatial scale.
This criterion reflects the close relationship between proximal sensing and detailed observation. It distinguishes measurements intended to characterise leaves, plants, canopy patches, soil features, water-surface structures, snow patterns, objects, or local urban conditions from observations that represent large mixed areas.
Sub-metre spatial detail does not mean that every measurement must be an image with centimetre-sized pixels. A soil probe may characterise a small measurement volume. A spectrometer may observe a limited footprint. A camera may resolve fine target structure. A mobile sensor may produce dense point observations along a transect. The shared principle is that the measurement can address local target variability rather than only broad, aggregated areas.
This criterion also creates a useful link to scaling. Proximal observations often help interpret what is happening within a satellite pixel, across a drone-mapped field, or around a tower footprint.
Control of geometry
Proximal sensing usually gives the observer meaningful control over sensor-target geometry.
The observer can choose where to place the sensor, when to acquire data, which part of a target to observe, the viewing direction, height, field of view, distance, and sometimes the illumination or measurement environment.
Control is not absolute. Field conditions remain variable. Clouds move, vegetation grows, water surfaces change, snow falls, wind shifts branches, and practical constraints limit sensor placement.
However, the observer can normally design, adjust, repeat, or document the observation geometry in ways that are not possible for a satellite overpass.
This control is a major source of both opportunity and responsibility. It enables high-detail, repeated observation. It also means that poor positioning, undocumented changes, unsuitable fields of view, and inconsistent acquisition schedules can become important sources of error.
Platform does not decide the category
A platform is a way of carrying a sensor. It is not, by itself, a definition of the measurement.
A sensor in contact with soil may be considered proximal sensing within soil science, even though it is not a non-contact optical observation. A tower-mounted camera may be described as proximal remote sensing because it observes vegetation without touching it.
A drone can support proximal sensing when it observes a target at short range, resolves fine detail, and operates within a controlled acquisition geometry. The same drone can also support airborne remote sensing when it surveys larger areas from greater height, with a longer atmospheric path and a less direct relationship to local target conditions.
A field camera, handheld spectrometer, soil probe, thermal sensor, terrestrial laser scanner, or ground-penetrating radar system may all belong within the wider proximal sensing space, depending on the measurement conditions and purpose.
The category should follow the observation, not the hardware label.
A field between fields
Soil scientists may focus on field-based sensors and spatial mapping. Crop researchers may focus on management zones and within-field variability. Ecologists may focus on phenology, canopy function, and ecosystem processes. Hydrologists may focus on water level, discharge, and water quality. Cryosphere researchers may focus on snow and surface change. Urban researchers may focus on environmental conditions at human scale.
The targets and measurement physics differ. The shared methodological questions are familiar:
- What is the target?
- What does the sensor record?
- How stable is the geometry?
- Which processes influence the signal?
- How is the measurement calibrated, validated, documented, and interpreted?
Proximal sensing is therefore not simply a compromise category between in situ measurement and remote sensing. It is a field of observation in which proximity, spatial detail, and controllability make particular methods possible.
A practical boundary
The proposed definition should be used as a guide, not as a gatekeeping device. Some systems will sit near the boundary. Some established communities will continue to use narrower or broader definitions for good reasons. The purpose of a shared definition is to make those differences visible and discussable.
For practical work, the definition suggests a short checklist:
☑ Is the target observed at sufficiently short range?
☑ Is atmospheric correction unnecessary for the intended interpretation?
☑ Can local target detail be resolved?
☑ Can the observation geometry be designed, controlled, and documented?
If the answer to all of these questions is broadly yes, the system belongs within the proximal sensing space.
The value of the term is not that it settles every boundary dispute. Its value is that it focuses attention on the conditions of observation. That is where the scientific work begins.
For further information:
- Viscarra Rossel, R. A., Adamchuk, V. I., Sudduth, K. A., McKenzie, N. J., and Lobsey, C. (2011). Proximal soil sensing: An effective approach for soil measurements in space and time. Advances in Agronomy, 113, 237–283. https://doi.org/10.1016/B978-0-12-386473-4.00005-1
- Adamchuk, V. I., Hummel, J. W., Morgan, M. T., and Upadhyaya, S. K. (2004). On-the-go soil sensors for precision agriculture. Computers and Electronics in Agriculture, 44(1), 71–91. https://doi.org/10.1016/j.compag.2004.03.002
- Pierrat, Z. A., Magney, T. S., Richardson, A. D., Runkle, B. R. K., Diehl, J. L., Yang, X., et al. (2025). Proximal remote sensing: An essential tool for bridging the gap between high-resolution ecosystem monitoring and global ecology. New Phytologist, 246(2), 419–436. https://doi.org/10.1111/nph.20405
- Adamchuk, V. I., and others. (2018). Proximal soil and plant sensing. In Precision Agriculture Basics. American Society of Agronomy. https://doi.org/10.2134/precisionagbasics.2016.0093
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