Geometry of the flow relative to the disk and the line of sight. The protostellar disk midplane is shaded in gray, out to a radius r. The line of sight is from the upper left; it is inclined by an angle i to the disk axis. On the left side, a source at height z above the disk is projected inward to a radius of r′, which itself will be compressed to r′cosi due to disk inclination i (see Eq. (1)). The half-sphere on the right shows, for a point at projected disk radius r at a height z and azimuth angle α, the relative geometry between the vertical (parallel to the disk axis), the line of sight to the observer (inclination i), and the flow direction (angle φ away from the disk axis; see Eq. (2)). The spherical triangle shaded in gray is used to calculate the angle between the flow direction and the line of sight.