Read what each deformation instrument measures

Ask what the instrument measures

A record says “50.” Before comparing it with another record, ask for units, direction, location, reference and interval. Fifty microradians of tilt is not fifty centimeters of uplift. A tiltmeter measures a slope change; GNSS tracks positions and displacement; satellite measurements require their own geometry and interpretation.

The USGS deformation-methods explanation describes how these complementary methods constrain changing ground shape. Deformation can reflect moving magma and fluids or fault motion. No instrument label alone uniquely identifies the subsurface cause.

Convert an angle without calling it uplift

One microradian is one millionth of a radian. A 50 microradian change is approximately 0.002865 degrees. Although that angle is small, an instrument may detect it. The conversion changes angle units; it does not create a height measurement.

Use the separate original beam illustration. If the beam has a 10 m horizontal span, an angle of 50 microradians corresponds to endpoint height difference 10 tan(50 × 10⁻⁶) m, approximately 0.0005 m or 0.5 mm. For this tiny angle, tangent is approximately the angle expressed in radians.

Draw the beam and label both its horizontal span and angle. This is hypothetical geometry, not uplift inferred at Sand Hill. The historical record supplies neither this beam nor a uniform local slope over that length. Real deformation varies spatially, so the same conversion cannot be applied indiscriminately to a volcano.

Compare translation with changing separation

Two fictional stations begin 1000 m apart along an east-west baseline. First move both 20 mm east. Their coordinates change, but their separation stays 1000 m. A common translation is not extension of that baseline.

Next move station A 10 mm west and B 10 mm east. Their relative displacement is 20 mm east and their separation becomes 1000.02 m. Reverse those motions and separation becomes 999.98 m, a signed change of −20 mm. Finally move A 10 mm east and B 30 mm east: the same 20 mm extension now accompanies a common translation.

Predict each result before inspecting the static worked ledger. There are no extra geometry sliders and these are not observed station positions. A magnified arrow makes displacement visible but is not drawn at the same scale as the true 1000 m baseline.

Relative displacement magnitude is not always length change. If B moves 20 mm north while A stays fixed, the initially east-west baseline grows by only about 0.0002 mm. Its first-order axial change is zero. The perpendicular movement changes direction much more than baseline length.

Preserve the historical measurement qualifiers

In the January–February packet, reported tilt change exceeded 60 microradians near Uēkahuna and Sand Hill. GNSS motion southwest of the caldera was up to 20 cm, while the agency’s satellite interpretation included uplift up to about 50 cm for a stated January 31–February 1 interval. These are different quantities and coverage, as described in the February 9 notice.

Convert up to 20 cm to up to 0.20 m without dropping the qualifier. The GNSS statement does not say every station moved that amount or that all motion was vertical. The tilt summary lacks an exact common interval/component for its total; do not assign it to one day. Interpreted satellite uplift is not a raw line-of-sight pixel value.

Transfer: repair an unlabelled comparison

A report compares “50” from one instrument with “20” from another and concludes that the first location rose more. What information must be restored, and why might even correctly restored numbers answer different questions?