About the Measure tool and its limitations¶

The Sonic Visualiser Help reference describes the Measure tool like this:

The measure tool enables you to obtain measurements in scale units (such as time in the X coordinate, or whatever the Y coordinate of the current layer represents) corresponding to certain pixel positions. To measure a region, just click and drag a rectangle covering it, using the left mouse button with the measure tool selected ...
It's important to note that the measurements shown in this way are based entirely on the pixel coordinates of the measurement rectangle, not on properties of the data being displayed.

The measure tool also has the ability to produce an automatic bounding box for a graphical feature (when double-clicked):

The area enclosed by the rectangle is based on the extent of similarly-coloured pixels surrounding the click position: it is entirely graphical, involving no audio analysis, and so depends on the gain and colour scheme in use in the spectrogram.