The Enigma of the Origin of Portolan Charts

A Geodetic Analysis of the Hypothesis of a Medieval Origin

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The sudden appearance of portolan charts, realistic nautical charts of the Mediterranean and Black Sea, at the end of the thirteenth century is one of the most significant occurrences in the history of cartography. Using geodetic and statistical analysis techniques these charts are shown to be mosaics of partial charts that are considerably more accurate than has been assumed. Their accuracy exceeds medieval mapping capabilities. These sub-charts show a remarkably good agreement with the Mercator map projection. It is demonstrated that this map projection can only have been an intentional feature of the charts’ construction. Through geodetic analysis the author eliminates the possibility that the charts are original products of a medieval Mediterranean nautical culture, which until now they have been widely believed to be.
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Biographical Note

Roel Nicolai, PhD, MSc (1953) is Principal Geodesist with Shell. This book is a revision of his PhD thesis and his first publication in the area of the history of science and cartography.

Table of contents

Preface xi
Acknowledgements xiv
List of Illustrations xv
1. Introduction 1

1.1 The Origin of Portolan Charts: Research Challenges 1
1.2 Objective and Scope of this Book 4
1.3 Four ‘Pillars’ 5

2. Key Characteristics of Portolan Charts 11
2.1 Preamble 11
2.2 Distinguishing Characteristics 12
2.3 Disputed, Unclear and Unsatisfactorily Explained Aspects of Portolan Charts 19

3. Existing Hypotheses on the Origin and Construction Method of Portolan Charts 44
3.1 Scholarly Views on the Origins Debate 44
3.2 Ideas on Antique Origins 46
3.3 Hypotheses on Medieval Origins 52
3.4 Hypotheses on Portolan Chart Construction 59
3.5 Plane Charts versus Plane Charting 63
3.6 The Rotation Angle 76
3.7 Methodological Considerations 82

4. Cartometric Analysis; Methodology and Existing Research 86
4.1 Introduction 86
4.2 Quantitative Analysis Methods – A Conceptual Classification 92
4.3 Existing Cartometric Studies 103
4.4 Chart Selection 116
4.5 Cartometric Analysis Approach 119

5 Cartometric Analysis of Five Charts 133
5.1 Carte Pisane 133
5.2 Anonymous Genoese Chart (Ricc 3827) 138
5.3 The Ristow-Skelton No. 3 Chart (RS-3) 143
5.4 The Dulcert 1339 Chart 147
5.5 The Roselli 1466 Chart 148
5.6 Analysis of All Results 152
5.7 How Difficult is It to Make an Accurate Map? 195
5.8 Conclusions 197

6 The Relationship between Portolans and Portolan Charts 201
6.1 Introduction 201
6.2 Existing Research 202
6.3 A New Analysis of Lo Compasso de Navegare 216
6.4 Summary of the Analysis 276
6.5 Conclusions 283

7. Constraints on Shipping and Navigation in the Medieval Mediterranean 285
7.1 Introduction 285
7.2 The Physical Conditions of the Mediterranean 286
7.3 Medieval Ships 298
7.4 An Accuracy Model for Medieval Mediterranean Navigation 311
7.5 Conclusions 322

8. The Map Projection, Artificial or Intentional? 324
8.1 Introduction 324
8.2 A Priori Geodetic Objections 327
8.3 Existing Research 331
8.4 Conceptual Workflow and Test Criteria 335
8.5 Design Principles for a ‘Medieval’ Geodetic Network 337
8.6 Introductory Information on Geodetic Network Analysis 341
8.7 Network Definition and Analysis 348
8.8 Network Adjustment #1 (Accuracy) 351
8.9 Network Adjustment #2 (Shape) 358
8.10 Summary of the Geodetic Analysis 367
8.11 Accuracy Enhancement by Averaging in the Context of the History of Science 369
8.12 Conclusions from This Chapter 376

9. Navigational Practices in the Twelfth and Thirteenth Centuries 377
9.1 Drawing up the Interim Balance 377
9.2 Navigational Tools: Charts & Dividers, Needle & Star 379
9.3 The Toleta de Marteloio 382
9.4 The Mathematical Seaman 388
9.5 Early Time Measurement at Sea 391
9.6 Distance Measurement at Sea 392
9.7 Bearing Measurement – The Mariner’s Compass 397
9.8 Conclusions 413

10 An Arabic-Islamic Origin of Portolan Charts? 414
10.1 Introduction 414
10.2 Ibn Fadl’allah’s Map – The ‘Mamun Geography’ 415
10.3 Progress in Astro-geodetic Position Determination 417
10.4 Sezgin’s Hypothesized Construction Method of Portolan Charts 420
10.5 Further Arguments Presented by Sezgin for an Arabic-Islamic Origin of Portolan Charts 423

11. Conclusions 429
11.1 Recap of Conclusions by Chapter 429
11.2 The Four ‘Pillars’ of the Medieval Origin Hypothesis 433
11.3 Key Conclusions from this Study 437

12 Synthesis 439
12.1 Introduction 439
12.2 The Appearance of Portolan Charts in Medieval Italy 439
12.3 Remaining Issues 442
12.4 Recommendations for Further Research 450

Appendix A: The Historical Setting of the Emergence of Portolan Charts 453
A.1 The Medieval Mediterranean – Relevant Historical Aspects 453
A.2 The Awakening of Europe 454
A.3 The Italian Maritime Republics and Aragon 455

Appendix B: Calculation of the Length of a Rhumb Line 467
B.1 An Arbitrary Line on a Sphere 467
B.2 The Line in a Specified Direction on a Sphere 468
B.3 Mercator Sailing – How to Plot Sailed Distance on a
Chart? 469

Appendix C: The Plane Charting Examples from Chapter 2 471
C.1 Dimensions of the Earth 471
C.2 The Routes from Livorno to Dellys 471
C.3 Four More Arbitrary Routes 479

Appendix D: Accuracy Model for Medieval Navigation 483
D.1 Introduction 483
D.2 Along-course Accuracy 484
D.3 Cross-course Accuracy 490

Appendix E: Cartometric Analysis Details 495
E.1 Coordinates of Identical Points 495
E.2 Preprocessing – Wind Rose Analysis 498
E.3 Main Cartometric Analysis 499

Appendix F: Reliability of the CALS7k.2 Archaeomagnetic Model 504
Appendix G: Is the Map Projection Accidental? A Statistical Analysis 508

G.1 Preamble 508
G.2 Statistical Basis 508
G.3 Test Results 510
G.4 Discussion 515
G.5 Conclusion 516

Appendix H: The Preface of the Liber de existencia riveriarum 517
Bibliography 520
Index 538

Readership

Map historians, medievalists and all those interested in the history of science.

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