An example of the measurement accuracy notation used for a baseline observation is which of the following?

• A. ±(5mm + 2ppm)
• B. ±(2mm + 5ppm)
• C. ±(1mm + 1ppm)
• D. ±(10mm + 3ppm)

Answer: (A)

Baseline measurement accuracy is expressed as a combination of a fixed error term and a length-dependent term, written as ±(A mm + B ppm). The fixed part accounts for instrument-related and setup uncertainties that don’t change with distance, while the ppm part captures how smaller, random errors accumulate with longer baselines. The example with 5 mm as the fixed component and 2 ppm as the proportional component clearly shows this format: a constant 5 mm plus a small, length-dependent term that scales with the baseline. For a longer baseline, the ppm term adds additional uncertainty in proportion to the distance, so the total error reflects both components. Other options, while still fitting the same form, use values that are less representative or would imply less balanced typical contributions, making the 5 mm + 2 ppm example the cleanest illustration of the notation.