Mediterranean portolan charts: their origin in the mental maps of
medieval sailors, their function and their early development

A.  ANTECEDENTS

A.1. Possible cartographic antecedents for the portolan charts
A.2. Why is there a widespread assumption in favour of a textual origin for the portolan chart?
A.3. Islamic antecedents
A.3a. The Book of Curiosities
A.3b. al-Idrīsī’s Charta Rogeriana of 1154

A.3b.1. Comparison between the content of the Charta and the portolan charts
A.3b.2. Different purposes of the Charta and the portolan charts
A.3b.3. Why the portolan charts were not used by al-Idrīsī
A.3b.4. Why the portolan charts made no use of the Charta

B.  SPATIAL COGNITION AND MENTAL MAPS

B.1. Evidence for the use of mental maps
B.2. The brain and spatial memory

B.2a. The workings of spatial orientation
B.2b. Is spatial cognition innate in humans?
B.2c. The hippocampus

B.3. Types of mental maps

B.3a. Definition of a ‘Mental map’
B.3b. Western examples of mental navigation maps

B.3b.1. London’s Black Cab drivers

B.3c. Indigenous examples of mental navigation maps
B.3d. When mental maps are essential

C.  NAVIGATIONAL PRACTICE

C.1. Navigation in the Ancient World
C.2. Mediterranean navigation

C.2a. Pre-knowledge of the route
C.2b. The skill set required for navigation
C.2c. Sea signs

C.3. The mental map in the Mediterranean context
C.4. Was sailors’ knowledge the source for the portolan chart?

D.  DIRECTION-FINDING

D.1. Astronomical wayfinding
D.2. The mental wind compass
D.3. The magnetic compass

D.3a. The introduction of the magnetic needle and magnetic compass
D.3b. Adjusting from astronomical to magnetic bearings
D.3c. Where does the magnetic compass fit into the origins debate?

E.  PELAGIC COURSES ACROSS THE MEDITERRANEAN

E.1. 13th-century textual sources: the Liber and Lo compasso

E.1a. The Liber de existencia riveriarum et forma maris nostri mediterranei
E.1b. Lo compasso de navegare

E.2. Could there have been textual antecedents for the pilot books as opposed to mental ones?
E.3. The termini selected for the pelagic statements in the two portolani
E.4. The pelagic statements listed in the Liber
E.5. The real meaning of the Liber’s ‘pelagic’ statements

E.5a. How do we know that the statements could not describe actual pelagic courses?
E.5b. Why the Liber’s pelagic statements must have been copied from a cartographic document
E.5c. How were the Liber’s pelagic statements constructed?
E.5d. The discrepancies between the stated pelagic distances in the Liber and Lo compasso

F.  SOURCING THE PELAGIC DATA

F.1. How were distances measured?

F.1a. Arabic numerals
F.1b. Measuring distance in terms of time or miles
F.1c. Scale(s) of the portolan charts
F.1d. The conceptual issues involved

F.2. The importance of landward visibility
F.3. The ‘sub-charts’ hypotheses

F.3a. The Adriatic and Aegean

G.  WHAT CAME FIRST?

G.1. The ‘chicken and egg’ question of priority: portolano or chart

G.1a. Separate development
G.1b. The parenting of the portolan charts
G.1c. The coastal itineraries
G.1d. Settling the priority question

H. INDICATIONS ABOUT THE DATE AND PLACE OF ORIGIN

H.1. Pointers to the likely period of the portolan charts’ creation

H.1a. The confirmed terminus ante quem

H.2. Dating the portolan chart’s appearance: why the Liber’s account of the Black Sea and the Charta Rogeriana are important

H.2a. The Liber’s Black Sea toponyms
H.2b. Black Sea distances
H.2c. How likely is it that the Black Sea information was taken from a marine chart?
H.2d. Establishing a terminus ante quem for the portolan chart
H.2e. The Charta Rogeriana establishes the portolan chart’s terminus post quem
H.2f. Palaeomagnetism

H.3. Where might the portolan chart have originated?

I.  FIRST THE PELAGIC DIAGRAM AND THEN THE CHART

I.1. ‘Inadvertent triangulation’
I.2. The mental chart takes graphic form

I.2a. gradiens

I.3 What might such a pelagic diagram have looked like?

I.3a. The sailors’ viewpoint
I.3b. The charts’ projection
I.3c. The missing link

I.4. Ways in which the mental map might have been ‘downloaded’

I.4a. What medium might have been used?
I.4b. The downloading process
I.4c. The rectification procedure
I.4d. The role of the scribe

J.  COMPLETING THE PORTOLAN CHART COVERAGE

J.1. The charting of the Black Sea

J1a. The peculiarities of the Black Sea
J1b. 13th-century developments
J1c. Toponymy
J1d. The shape of the Black Sea

J.2. Charting the Atlantic coasts and the final Mediterranean improvements

K.   THE CHARTS’ MAIN ELEMENTS

K.1. Coastal outlines

K.1a. What was the source for the coastal outlines?
K.1b. How were the coasts demarcated?
K.1c. Rivers
K.1d. Islands
K.1e. Charting the Aegean archipelago

K.2. Navigational dangers

K.2a. Earliest mentions
K.2b. The treatment of dangers in the portolani and chart compared
K.2c. How were the dangers recorded and how did those get onto the charts?
K.2d. Fishermen
K.2e. Religion

K.3. Toponymy

K.3a. The Carte Pisane’s toponymy compared to that of the portolani
K.3b. How were the toponymic lists in the early portolani compiled?
K.3c. Toponymic referents
K.3d. Red names
K.3e. Updating the names

K.4. Illustrations and commentary

L.   ORALITY, LITERACY AND GRAPHICACY

L.1. Orality and the portolan chart

L.1a. What needed to be explained orally for the charts’ effective use?

L.1a.1. Aspects of the charts’ information that required explanation
L.1a.2. Information not provided by the charts but necessary for navigation

L.1b. Orally transmitted pelagic knowledge

L.2. Two modern comparisons

L.2a. London’s Black Cab drivers
L.2b. The London Underground Map

L.3. Literacy or illiteracy

L.3a. Would medieval sailors have been literate?
L.3b. In what way could the written word have contributed to medieval navigation?
L.3c. What textual information was included on the charts and what omitted?

L.3c.1. Information aimed at literate users
L.3c.2. Information that could be understood by non-literate users

L.3d. Were the charts initially designed with illiterate users in mind?

L.4. Graphicacy

L.4a. The necessary graphical skills

M.  THE CHARTS’ FUNCTIONS

M.1. The varied purposes of the portolan charts and how they were used

M.1a. Planning a voyage
M.1b. The advantages to a navigator of a chart over a portolano
M.1c. The portolan charts’ limitations
M.1d. Usage by non-mariners

N.  COMMERCIAL ASPECTS

N.1. The portolan chart as an object of commerce

N.1a. How might the market have been established?
N.1b. Not a single lineage
N.1c. Dissemination
N.1d. Apprenticeship
N.1e. Customisation

N.2. The portolan charts and the world of trade

N.2a. The possible contribution of merchants to the charts’ origin and development
N.2b. How important was trade in the charts’ origin?

N.3. Should we divide the portolan charts into two types?

N.3a. Survival

O.  EPILOGUE

O.1. The portolan charts contrasted with the post-1500 surveys outside the Mediterranean

P.  CONCLUDING REMARKS

P.1. A mental map origin
P.2. The charts’ creation
P.3. Purpose and use
P.4. The portolan charts’ place in the history of cartography
P.5. Rating this essay
P.6. The future

POSTSCRIPT (27 August 2023)

Q.  THE CRUCIAL FACTORS THAT EXPLAIN THE PORTOLAN CHARTS’ ENDURANCE

Q.1. Copying discipline
Q.2. The ‘Fidelity Imperative’

Q.2a. Why was fidelity valued so highly when copying the charts?
Q.2b. Why was the choice of copying method so important?

Q.3. The chart as an object of commerce
Q.4. The period before the Carte Pisane
Q.5. How might the earliest chartmakers have interacted?
Q.6. How might errors in the Mediterranean’s coastal outlines have been recognised and corrected?

Future Research
Postscript conclusions