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| Ptolemy Geographia - decyphering Hibernia (Ireland) |
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| Background |
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Ptolemy was an astronomer and mathematician who probably worked in Alexandria between 127AD and 148AD, he wrote a number
of Astonomical, Mathematical and Astrological books, and his Geographica (about 150AD) that describes the
locations of towns, tribes and rivers across the Empire.
This document shows a way to decipher and understand the logic of the latitude and longitude measurements given in
Claudius Ptolemy’s Geographia for the Island of Hibernia which is modern day Ireland.
Many people have tried to understand this, but as far as I know no one has yet used the following method.
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| My Hypothesis ▲ |
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That the latitude and longitudes quoted in the Geographia are not true latitudes and longitudes, but
are calculated from a few known fixed locations.
This would mean that the variation in shapes from the real modern maps can be explained, and hence Settlement locations
that have not yet been identified can be shown on modern maps.
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| The Area I am interested in and why ▲ |
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I am interested in the areas of Hibernia and Britannia because if they are plotted directly as latitudes and longitudes,
they show marked differences from real latitude and longitude measurements, and also because I live in the South of
England which has an unidentified Roman port of Novus Portus and would like to know where this was located to prove
another hypothesis.
Ireland(Hibernia) is slightly offset, the southern part of Britannia shows a larger offset and Scotland appears to be
rotated 90 degrees thus making this analysis more complex, so we will start with Hibernia(Ireland).
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Image of Hibernia from Tabula Prima d'Europa, 1482AD
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| Background to Claudius Ptolemy’s Geographia ▲ |
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The Geographia is the first known description of locations known by the Romans, this appears to have been compiled by
Claudius Ptolemy of Alexandria in about 150AD. The text covers Europe, Africa and Asia showing that the Romans knew a
high percentage of the Northern Hemisphere in which are shown towns, rivers and peninsulas'.
It is very likely that the text was produced for the Roman Military, and was also for identifying trading routes, and
that the map was based on earlier maps, possibly by Marinus of Tyre or maybe earlier by the Carthaginians.
Ptolemy's work probably originally came with maps, but none have been discovered. Instead, the present form of the map
was reconstructed from Ptolemy's coordinates by Byzantine monks under the direction of Maximus Planudes shortly after
1295AD.
The illustration below is the map produced by Ruysch in 1508AD, and graphically illustrates the extent of the text and
the knowledge of the Romans from that period and was based on plotting the latitude and longitudes given in the text
onto the world map.
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Extract from Wikipedia
The Geography (Ancient Greek: Γεωγραφικὴ Ὑφήγησις, Geōgraphikḕ Hyphḗgēsis, lit.
"Geographical Guidance"), also known by its Latin names as the Geographia and the Cosmographia, is a
gazetteer, an atlas, and a treatise on cartography, compiling the geographical knowledge of the 2nd-century Roman
Empire.
Originally written by Claudius Ptolemy in Greek at Alexandria around 150 AD, the work was a revision of a now-lost atlas
by Marinus of Tyre using additional Roman and Persian gazetteers and new principles.[1] Its translation – Kitab Surat
al-Ard – into Arabic by Al-Khwarismi in the 9th century was highly influential on the geographical knowledge and
cartographic traditions of the Islamic world.
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| Ptolemy Data Format ▲ |
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The first thing to notice about the Ptolemy data is that the decimal points after the main number go to a maximum of .55
so it would seem logical to assume that each East/West segment was divided into six. Hence a section between 58 and 59
North and 10 to 11 East would consist of a 6 x 6 matrix (36 individual points) to provide granular accuracy, and if the
entry went to .05 would allow even finer granularity (144 individual points).
However it would also seem that if a whole number with no decimal places is shown for example 11 East, that it could
either be along the 11 line, or somewhere between 11 and 12 implying that the accuracy was not as good.
Summary:
If a number is displayed with a .05 accuracy it would probably mean that they were more confident of the data, if .1
then less so and finally if both were whole numbers then even less so.
This would mean in turn that the locations we should assume are the most accurate are those containing .05 entries,
followed by those with .1 finally whole numbers.
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| Analysis by other parties (Bill Thayer) ▲ |
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There are a number of other people who have been interested in this subject and have tried analysing Ptolemy’s data
with differing ways of plotting the variance between his data and current maps.
The very first of these modern plots that I have found in the early 2000’s was created by Bill Thayer in his massive
LacusCurtius
website, which I show below.
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Bill plotted a number of maps from the Geographia, but decided that they were not particularly productive as most show
significant differences from the real world, implying that this was too time consuming and not very useful for others
looking at this subject.
The theory was that Ptolemy plotted real world latitudes and longitudes and these could be directly mapped.
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| Analysis by other parties (R Darcy and W Flynn ) ▲ |
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A later version of Bills analysis was carried out by R Darcy and W Flynn and published in 2008 and can be found here below, and shows an offset plot of Ptolemy’s data overlaid
with the modern map.
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As you can see from the map, it still looks incorrect as per Bill Thayer’s version, as it uses the same premise that
the latitudes and longitudes in the Geographia are the same as our modern ones.
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| My Analysis ▲ |
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So it looks like plotting the latitude and longitudes directly onto the map is a waste of time, so lets plot the known
places onto a map and then analyse in reverse. This may prove difficult as a number of Ptolemy’s locations are not
easily identifiable, but we will give it a try and Hibernia does not have too many data points, which simplifies the
analysis.
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This is the modern map of Hibernia (Ireland) from Google My Maps
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| Ptolemy Raw Data ▲ |
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| The following are the locations quoted by Ptolemy for Hibernia,
I have excluded those for the Islands north of Ireland, the Isle of Man and
also Anglesea. |
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| A description of the north coast, beyond which is located the Hyperborean ocean: |
| Boreum promontory |
11.00 |
61.00 |
(Bloody Foreland) |
| Vennicnium promontory |
12.50 |
61.20 |
(Malin Head) |
| mouth of the Vidua river |
13.00 |
61.00 |
(River Foyle) |
| mouth of the Argita river |
14.30 |
61.30 |
(Glengehesk river) |
| Rhobogdium promontory |
16.20 |
61.30 |
(Torr head) |
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| The Vennicni inhabit the west coast; next to them and toward the
east are the Rhobogdi |
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| A
description of the west side, which borders on the Western ocean from
the |
| Boreum promontory |
11.00 |
61.00 |
(Bloody Foreland) |
| mouth of the Ravius river |
11.20 |
60.40 |
(River Earn) |
| Magnata city |
11.15 |
60.15 |
(Cullenamore) |
| mouth of the Libnius river |
10.30 |
60.00 |
(River More) |
| mouth of the Ausoba river |
10.30 |
59.30 |
(River Corrib) |
| mouth of the Senus river |
9.30 |
59.30 |
(River Shannon) |
| mouth of the Duris river |
9.40 |
58.40 |
(Inagh river) |
| mouth of the Iernus river |
8.00 |
58.00 |
(River Maine) |
| Southern promontory |
7.40 |
57.45 |
(Ducalla) |
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| The
Erdini inhabit the coast next to the Vennicni, and between these are the
Magnatae; then the Autini; and the Gangani; below whom are the Vellabori |
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| Description
of the south coast, which adjoins the Vergionius ocean, |
| Southern promontory |
7.40 |
57.40 |
(Ducalla) |
| mouth of the Dabrona river |
11.15 |
57.00 |
(River Lee) |
| mouth of the Birgus river |
12.30 |
57.30 |
(River Barrow) |
| Sacrum promontory |
14.00 |
57.50 |
(Carnsore Point) |
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| A
description of the east side touching the ocean which is called Hibernian, |
| Sacrum promontory
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14.00 |
57.50 |
(Carnsore Point) |
| mouth of the Modonnus river
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13.40 |
58.40 |
(River Slaney) |
| Manapia town
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13.30 |
58.40 |
(Wexford) |
| mouth of the Oboca river
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13.10 |
59.00 |
(River Liffey) |
| Eblana town
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14.00 |
59.30 |
(Drogheda) |
| mouth of the Bubindas river
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14.40 |
59.40 |
(Newry River) |
| Isamnium promontory
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15.00 |
60.00 |
(St John's Point) |
| mouth of the Vinderis river
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15.00 |
60.15 |
(Comber River) |
| mouth of the Logia river
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15.20 |
60.40 |
(River Lagan) |
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| Next to this
is the Robogdium promontory |
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| On
the side next to the Robogdi dwell the Darini, below whom are the Volunti;
then the Eblani; then the Cauci; below whom are the Manapi; then the Coriondi
who dwell above the Brigantes |
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| Map showing identified locations ▲ |
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This map shows the points from Ptolemy data as above, I have plotted them using my interpretation of their details, and
in places where I had difficulties referred to other online sites to allow me to corroborate the data points. There will
be some wrong due to transcription errors over time, and also location misunderstandings.
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The plot of known locations on Google My Maps
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| Overlay Latitude(North/South) lines ▲ |
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Now if I take all the latitudes(North/South) recorded in the Geographia and join the ones with the same latitude I get
the following plot, these latitudes are not parallel with our modern ones, but this is interesting, well it is to me. To
the left of the latitude lines are the Ptolemy latitudes.
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| Background ▲ |
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The plot of latitudes on Google My Maps
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| Overlay Longitude(East/West) lines ▲ |
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Now to build up the blocks lets add the Longitude(East/West) lines running at 90 degrees to the north lines. The numbers
at the bottom of the screen marked 8 - 14E shows Ptolemy degrees East. This grid seems to give quite a a good
correlation with the Ptolemy points already plotted on the map.
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The plot of latitudes and longitudes on Google My Maps
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| Celtic main roads in Roman times ▲ |
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This map is now overlaid by the five main Celtic road routes in Ireland in Roman times, these are north to south, Slighe
Mhidhluachra, Slighe Assail, An Tslighte Mhor, Slighe Dhala and Slighe Chualann.
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| Celtic Hillforts and their relationship to Roman place names ▲ |
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I have plotted the following inland towns after creating the grid, as it was difficult identifying these as modern
locations, and have plotted these onto hillforts that are in the area as these are the most likely places for settlement
locations.
All the hillfort information comes from the Atlas of Hillforts from
Oxford University.
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| Inland Towns ▲ |
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following are the probable locations for the inland towns: |
| Regia |
13.00 |
60.20 |
(NI0813 Clogher, Tyrone) |
| Rhaeba |
12.00 |
59.45 |
(IR0990 Castleruby, Roscommon) |
| Laberus |
13.00 |
59.15 |
(IR1029 Garristown, Dublin, Tara) |
| Macolicum |
11.30 |
58.40 |
(IR0710 Garrangrena Lower, Tipperary) |
| another Regia |
11.00 |
59.15 |
(IR0737 Rahally, Galway) |
| Dunum |
12.30 |
58.45 |
(Rock of Dunamase, Portlaoise) |
| Hibernis |
11.00 |
58.10 |
(IR0801 Mooghaun, Clare) |
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| Final Observations ▲ |
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This map shows the Irish latitude/longitudes, and also some of the Britannia lines using the same logic.
The line taken from Waterford, Dublin or Tara goes down to Tarifa where one of the east west lines from Richborough in
Britannia appears to join.
If this is the case then the fixed point in the Mediterranean would be Southern Spain.
Please Note:
This may just be a co-incidence but brings the possibility that the measurements for Hibernia and the south of Britannia
originate from Tarifa(Isla de Tarifa) in the South of Spain, probably a fixed known location for the Romans.
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| Conclusion ▲ |
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From the above argument it would seem that it is possible to fairly accurately map Ptolemy's Geographia data, and that
it is very likely that the origin of the latitudes and longitudes was based on calculation from a few fixed locations.
In the case of Albion and Southern Britannia that these were calculated from Tarifa(Isla de Tarifa) in the South of
Spain
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