This ticket follows on from the recent addition of the basic "line-extraction" a/k/a "profile across an image" tool to Firefly.

When the "profile measurement" a/k/a "line extraction" tool is used, on an image with a WCS, the information in the resulting plot becomes significantly more scientifically interesting if the coordinates of the points along the line are available in world coordinates as well as in pixels, and especially if the angular distance along the line is also available.

This would go from 0 at the "tail" of the arrow to whatever the length of the arrow is at the "tip" of the arrow. This is the exact same computation as is used for the ordinary line tool!

I would suggest that you do the computation in arcseconds, and report it explicitly with those units (the correct VOUnit string is "arcsec"). This will be the most useful in the common use cases of performing a measurement across a star or galaxy.

In addition, you should always compute and include in the resulting table the offset in pixels from the tail to the tip of the arrow, in units of pixels. This is just sqrt((x - x_tail)^2 + (y - y_tail)^2).

Therefore, for an image with a WCS, the table columns should be (with VOUnit strings in parentheses):

• offset (arcsec) [DEFAULT X AXIS of plot]
• ra (deg)
• dec (deg)
• pixOffset (pixel) <-- note this is a double, not an int
• x (pixel)
• y (pixel)
• flux (or whatever the pixel value is) [DEFAULT Y AXIS of plot]
• values from any other HDUs...

For an image without a WCS, the columns should just be

• pixOffset (pixel) [DEFAULT X AXIS of plot] <-- note this is a double, not an int
• x (pixel)
• y (pixel)
• flux (or whatever the pixel value is) [DEFAULT Y AXIS of plot]
• values from any other HDUs...

Using the offset (or pixel offset) along the line as the default horizontal axis of the plot produces a substantially more quantitatively useful plot even when there is no WCS, because it's got a proper invariant scale, instead of having the scale effectively depend on the angle at which the line was drawn, and having the default horizontal axis be either pixel X or pixel Y, as currently implemented, depending on the angle of the line.

Mirrors ticket FIREFLY-868.