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Instrument and observation

The device that measures, and the record of what it measured

An instrument is the device that measures; an observation is a distinct referent recording what was measured, never the device itself.

RSM ID Learning Center3 min read

An instrument is a physical or digital device that measures something: a soil probe, a flow gauge, a weather station, a laboratory analyser. An observation is a recorded result of measuring: this moisture value, at this depth, in this field, at this time. The two are different referents with different lifetimes. A probe may be installed, recalibrated, moved, repaired, and finally retired over several years; each of its readings is fixed the moment it is recorded, and remains what it was even after the device is gone.

Here instrument is used in its plain sense. Document 08 registers the naming kind instrument without a published definition, and the RSM model's rsm:Instrument covers a broader family of constructs, so how a physical device is classified semantically is an open question (SPEC-10); the naming kind and the semantic class are separate either way. Keeping device and reading apart lets each carry its own provenance. A reading points at the device that produced it by RID, and the device's calibration history, deployment, and custody are separate records related to it. Whether every single reading needs its own RID, or whether readings are grouped into identified datasets, is a granularity choice for the space that keeps them; a relationship that needs its own lifecycle, provenance, or permissions is eligible for an RID of its own (§7.4).

Example

Soil Probe 17, 451.EPT1QX0PDW4YF6KG, measures moisture in North Field. Its 2031 season of readings is published by the Valley Soil Observatory space as a dataset that refers to the probe and to the field by RID. When the probe is recalibrated mid-season, the calibration is recorded as an event about the device; the readings before and after keep their own provenance, so an analyst can tell which ones a correction applies to. When the probe moves to another field, its RID does not change — but its later readings are about the new field, not North Field.

A common misconception

“The sensor's data stream is the sensor.” Telemetry is a series of observations about a place, produced by a device. Treating the stream's identifier as the device's identity means a replaced probe silently inherits its predecessor's history, and a retired probe's readings lose the device they came from.

Observations become evidence for claims, each with its provenance. The Journal Instruments, materials, and observations follows Soil Probe 17 through its whole life, and the Fieldnote A sensor is not its reading works through the hardest distinctions.

In the specification

Resources include recorded observations: Document 08 §2.1. Relationships by RID: §7.4. Registered kinds: §18.3.