The Roman Map of Britain RatiV staqerioV p.e. The River Taff , Ratis statio Cardiff / Caerdydd (perhaps more specifically Gabalfa)

RatostaqhbioV potamou ekbolai Ratostathybius, var. Ratostabiou Ratostabius (Ptolemy II 3 2)

    As a defining point in Ptolemy's outline of the coast of southern Wales, RatostaqhbioV is positioned one degree east of the mouth of the River Towy and fifty minutes west of the mouth of the River Severn. All four points defining the southern coast of Wales from the Octapitarum promontory (St. David's Head) to the mouth of the Severn are listed at the same latitude. 

    Camden, employing the variant form, placed Ratostabius at Caerdydd/Cardiff. He reasoned that -tabius represented the old name of the River Taff, and that Ratos- survived in the name of the neighboring parish "Rath," called by the English  Roath." [Y Rhath/Roath ST1977] Carl Müller's preference for Ratos-tabius was furthered by Rivet & Smith in PNRB pp 444-5.

    I would suggest another look at the form
RatostaqhbioV, and point out that the Ptolemy didn't always transliterate names. Pterwton stratopedon Pteroton stratopedon, KainoV limhn Kainos limen and many others represent partial or complete translations of place names. A search of Greek forms quickly yields staquroV, the slightly later form of staqeroV ' standing fast, firm, fixed.'  The only hurdles to equating staqeroV with staqhbioV are scribal confusion of b for r (or B for P) and the substantivization of the form, neither of which is without precedence. The best Latin equivalent of staqeroV is probably stativa, identical in meaning and a substantivized form common to contemporary Roman military histories. Stativa is from castra stativa 'permanent camp'. Unfortunately stativa implies more of a fortification than the period or location warrants.

    An alternative to stativa, with less baggage and a broader variety of meanings, is statio '1. In milit. lang., a post, station 2. Transf., in gen., a station, office, position, in government, etc. 3. Naut. t. t., an anchorage, roadstead, road, bay, inlet 4. A place of residence, a post, station of the fiscal officers of a province 5. A post-station, post-house 6. A religious meeting, assembly of the Christians' (Charlton T. Lewis, Charles Short, A Latin Dictionary). If you are at all familiar with the Roman coastal sites along the Bristol Channel you can probably see where this is going. Cold Knap was the site of a ferry, as was Sudbrook. So, why not a site between them, at Cardiff? Cardiff is at a potamou ekbolai 'river mouth', and neither Cold Knap, nor Sudbrook are. Rath and Rato- would simply be L. ratis 'raft'. Examples of large rafts dating back to the late bronze age have been found along the Humber and the Ancholme, at Dover and near Chepstow. On the Taff just west of Roath is Gabalfa (ST1678) W.y geubalfa 'the ferry-place', ceubal 'ferry'. 

    Why did Ptolemy translate the second element and not the first? He simply didn't recognize it. Rate, his own entry for Leicester, and continental sites with similar first elements effectively obscured the true meaning.

 *found earliest in Comm. Math. by the 4th century Syrian philosopher Iamblichus of Apamea (c. 300-330 CE) 

Some information on British rafts:

See McBain's for Irish
raithneach , raineach 'fern' Gaulish ratis
rath 'prosperity'
rāth 'raft'
rāth , rāthan 'surety'
rāth 'a fortress, residence' Welsh rhath, cleared spot; borrowed from Gaelic? (Rhys).
rāmh 'an oar'