To help clarify the identity of the objects in the Gum, RCW, Sharpless and BFS catalogs, I've taken the locations given in those catalogs and overlaid them on top of a false colour version of Douglas Finkbeiner's all-sky hydrogen-alpha map.
The astronomical catalog repository Vizier contains electronic versions of the Sharpless and RCW catalogs. Vizier does not contain the Gum catalog, so I have created a spreadsheet with most of the Gum data here. (Note that I calculated the l and b galactic coordinate columns in the spreadsheet from Gum's 1900 epoch right ascension and declination coordinates. I have not included Gum's own l and b coordinates in the spreadsheet as they were based on the old Lund Pole system.)
The catalog overlays are available through the Milky Way Explorer. As a starting point, here are the overlays for the region around the Lagoon nebula:
It is important to note that the catalog coordinates are not always very accurate, especially for the Gum and Sharpless catalogs. Given that Gum was measuring nebula locations on tiny photographs taken using a 100 mm (4 inch) Schmidt camera, the errors in his catalog are not surprising. What is surprising is that the coordinates for many of the objects within the first 35 entries in the Sharpless catalog (including the Lagoon nebula region) are incorrect. A possible reason for the Sharpless errors is described below.
Gum subdivided 9 of the nebulae in his catalog into smaller subnebulae and designated them with lowercase letters. In the original paper these were given "Stromlo" designations after the location of Gum's observatory, but the scientific literature quickly switched to using Gum's name instead (eg. Gum 64a, 64b and 64c). The RCW catalog similarly subdivides 12 of its nebulae into smaller subnebulae but unfortunately describes these subdivisions only in its notes (not in the main tables) and does not designate them. On the overlay maps, I have followed the tradition in the scientific literature and assigned the RCW subnebulae lowercase letters as well, based upon the order that the subnebulae are described in the RCW notes.
In all but four cases, the RCW catalog provides a width, a height and a central coordinate for each nebula. For RCW 1, RCW 54 and RCW 63, the catalog does not contains a central coordinate but instead lists a series of coordinates in the region. When Raymond Bruton created the electronic version of the RCW catalog for Vizier, he supplemented the data in the original catalog with estimated central coordinates for these three nebulae but admitted that the results were "crude". On the overlay, I have used Bruton's data but designated these nebulae as RCW 1*, RCW 54* and RCW 63* to recognise that this data was not in the original RCW paper. Finally, the RCW catalog provides no coordinates or extent for RCW 116 at all. Instead it merely states in the notes that RCW 116 is a "Concentration inside 113". Presumably this was an oversight.
The positions of a number of H II regions, many of those from S3 to S32 especially, are in error in the Sharpless catalog.
Catalog of CO radial velocities toward galactic H II regions, Blitz, Fich and Stark, 1982
Unlike the small telescopes used to produce the Gum and RCW catalogs, the Sharpless catalog was based upon photographic plates produced by the giant 48 inch Schmidt camera on Mount Palomar. For this reason, one would expect that his results would be considerably more accurate than those from the southern hemisphere. And indeed, in his first 1953 paper, Sharpless states that "The co-ordinates of the center of each nebula were measured to 1 minute of arc with respect to a near-by star ..". And yet, in his second 1959 paper, which included the final revised catalog, the 1 minute accuracy estimate has been dropped and many of the objects within the first 35 entries of the Sharpless catalog have coordinates which are significantly more than 1 minute of arc in error.
The Czech astronomer Pavla Maršálková gave a credible explanation of Sharpless's mistake in his extremely useful 1974 paper, A comparison catalogue of HII regions. In order to derive the nebula coordinates, Sharpless measured their distances from nearby stars with well known positions. These positions were taken from the great Bonner Durchmusterung (BD) and Cordoba Durchmusterung (CD) star catalogs. These catalogs were published in segments over 5 decades, from 1886 to 1932. Since these observations were taken over time, the positions given depended upon the year. This is because the stars appear to change their position in the sky over time because of their own proper motion but also and more importantly, because of a wobble in the Earth's axis, called precession.
Sharpless, of course, was well aware of precession and in his first 1953 paper, he noted that "the coordinates ... are given for the same epoch as the corresponding section of the BD and CD catalogue". This inconsistent epoch was not very satisfactory for a widely used catalog, however, and in the final 1959 paper he announced that the coordinates had been revised to the common 1900 epoch.
However, the calculations to revise the coordinates were not done correctly! Maršálková suggests that the calculations were done on the assumption that the star observations were all done in 1855. This would have been approximately correct for the northern hemisphere BD catalog, but not correct for the southern hemisphere CD catalog, which was completed decades later. As a result, the positions for many of the first 35 nebulae in the Sharpless catalog, which are all southern nebulae, are incorrect.
The BFS catalog - Sharpless revised
The Sharpless catalog is now becoming widely used for amateur CCD imaging and the coordinate errors have caused some confusion and misidentifications for some astrophotography available online. Fortunately, Leo Blitz, Michel Fich and Antony Stark released a revised version of the Sharpless catalog in 1982. This is available through Vizier here. This revised catalog should be used instead of the original Sharpless catalog if possible.
Blitz, Fich and Stark dropped Sh 2-6, Sh 2-52, Sh 2-71, Sh 2-80, Sh 2-95, Sh 2-116, Sh 2-207, Sh 2-267, Sh 2-274, Sh 2-290 and Sh 2-313 because these objects had all been identified as planetary nebulae after the publication of the Sharpless catalog. They also dropped Sh 2-103, Sh 2-221, Sh 2-223, Sh 2-224, Sh 2-240, Sh 2-244, and Sh 2-248 because they had been identified as supernova remnants and Sh 2-14 because they were unable to detect it.
In addition, the BFS catalog corrected the coordinates of the remaining Sharpless objects and added 65 new nebulae, now identified as BFS 1 through BFS 65.
The original BFS catalog was not without its own mostly small errors. These were corrected in the online Vizier version. The Vizier catalog mysteriously adds two unidentified nebulae both labelled simply "BFS" and located near Sh 2-182 and Sh 2-202. I left these in the overlay and hope to clarify their nature in the future.