Construction of narrow spiral masks, illustrated with NGC 628. The top panels show the original (left) and new narrow mask (right). The bottom panel illustrates the construction of the narrow mask on the north-western spiral segment of NGC 628 (green colour). The ridge of peak emission for each tracer (CO, Hα) is identified along successive cuts perpendicular to the analytic log-spiral function (grey arrows), and limited to the original spiral mask footprint. The ridge is dilated to provide a narrower mask, which is by construction always a subset of the original mask. The final mask is the union of the narrow masks based on the CO and Hα peaks.