The Roman Map of Britain  Medieval Latin Orthography and Errors in Transmission
with heavy reliance on the works of Bischoff , Sidwell , and Schnetz
See also W. M. Lindsay, An Introduction to Latin Textual Emendation
and Stephen R. Reimer, Course Notes, Manuscript Studies - Medieval and Early Modern

Medieval Latin Orthography

e for ae and oe
    Cesaromago for Caesaromago

i is written for e and vice versa.

o is written for u and vice versa.

y is written for i and vice versa.
    Pilais for Pylais

ci for ti and vice versa

b and u(v) are confused.
    Bindogladia for Vindocladia
    Boluelaunio for Ueluelannio

di (or ti) before a following vowel or diphthong is written z .
Canza for Cantia
Melezo for Meletio

nct is written nt [and an editorial correction of Punctuobice for Pontibus?]

x (and cx, cxs, xs) is written s and vice versa.
    Deva Victris for Deva Victrix

g is written for c and vice versa.
    Bindogladia for Vindocladia

t is written d and vice versa.
    Stadio for Statio

ct is written for th.

h is omitted or inserted at the beginning of words.

False aspiration : h has c added to it (ch).

Gemination and Simplification: single consonants are often doubled, and doubled consonants are often made single.

Errors in transmission

Text dividers (more properly virgulae Lat. 'little twigs')
   Until the 12th century word division was often haphazard or entirely lacking. I believe dividers ( | ) were added to prepare an early example of the Ravenna Cosmography for copying, or were used in place of the more familiar stops ( : and . and ·). A later copyist, while aware that dividers were employed, might not recognize all as such and occasionally take one to be a letter such as  i,  l, long s, etc. Conversely, a letter resembling a divider might be taken as such, i.e. an l taken as /, creating a wrong division. Further, the copyist may have decided that a letter resembling a divider was wrongly placed and ignored it altogether, i.e. Ventaslurum for Venta Silurum, a conflation.
    PNRB p.187 chooses Staurinis for Taurinis Turin to illustrate an imagined fault of the cosmographer. Staurinis is simply /taurinus where the text divider has been transmitted as a long s. Schnetz, the most recent editor of the Cosmography felt the S- came from the last letter of the previous entry Fines.
    A text divider may also interact with an adjacent letter. Manulodulo for Camuloduno can be explained as /C read as M, or better as //C read as M.
    For more on text dividers, see what it the most comprehensive work on the subject of punctuation, M.B.Parkes Pause and Effect: An Introduction to the History of Punctuation in the West (1993).

Wrong division
    Camulo dono for Camuloduno
    Scadu namorum for Isca Dumnnoniorum

Conflation : the combining of two words into one.
    Ventaslurum for Venta Silurum

Metathesis : transposition of letters, sounds or syllables of a word.
    Durbis for Dubris
    Condecor for Conderco

Dittography: repeating a syllable within a word.
    Durnonovaria for Durnovaria?

Haplography : procope, syncope, and apocope (respectively, the loss of initial, medial, and final syllable).
    Fanocodi for Fanococidi?

Assimilation : the transformation of a spelling to a more familiar form.
    Lectoceto for Letoceto (L. lectus, It. letto 'bed')

Recognition errors : confusing a letter or letters, for another letter or letters.

Aggregation: consolidating two letters into one letter.
    devionisso statio for clevionisso statio

Disaggregation: breaking a letter into two other letters.
    Daroecla for Daroeda (for Doerada)

Misread Abbreviations
    lm for r (Iacio Dulma) superscript r broken into l ^~ (~  = m or n)
    a or o from û (-um)
    canted ~ for l, Manulodulo for Camuloduno

Absence of context and unfamiliar names : Context often allows the intended meaning and/or spelling of a word to be discerned by its relationship to the remainder of the text. Continental scribes were unfamiliar with the vast majority of the place-names recorded for Britain. Manulodulo colonia (R&C 99) is recognized as Camuloduno colonia because we are familiar with both the correct form and the presumed geographical context.
    The transition between narrative text and place names in the Ravenna Cosmography is not always clear. There appear to be instances where a portion of the narration has been mistaken for a place name and vice versa.
    Overall the transcription of the Ravenna Cosmography must have been a fairly mind-numbing experience for the scribe. Seemingly endless lists of unfamiliar place names compounded by at best haphazard word division have taken their toll on the surviving manuscripts.