Portolan charts were navigational maps based on compass directions usually utilising estimated distances observed by the pilots at sea. The information they contained was very valuable, especially at the outset of the age of discovery with the charts themselves often being treated like State secrets.  Invariably drawn on sheep skin the word 'portolan' is derived from the Italian adjective 'portolano' which means "related to ports or harbours."  This is especially relevant as the (Rhumb) lines which criss-cross the chart represent the thirty-two directions (or headings) of the mariner's compass from a given point.  The lines themselves are used to point the way to ports and harbours drawn and named on the chart. In true Portalan style, all of the major sand-banks around the coasts are marked.

 The major draw-back with Portolans was that they failed to take into account the curvature of the earth and as a result, they were of little use when crossing the open ocean, as navigational tools. This charts covers a huge area but considering the multitude of land falls of the Caribbean it would still have been relatively effective.  The multitude of ports, harbours and town marked along the coastline of the mainland from as far north as Virginia, down around the Florida peninsular into the Gulf of Mexico and the northern coast of South America is testament to the incredible amount of 'previous knowledge' that had been brought to the making of this chart. It is not just the settlements which impress but also knowledge of inland river ways such as the Mississippi.  The labelling of the major ports and settlements along the mainland American coastline inverts itself so that it may easily be read by the person standing on that particular side of the map-table or northern edge of the chart.

Two major forts and noted on the chart, one on the Island of New Orleans and the other at Apalache(e) near present day Port Leon.  It is possibly the original wooden stockade built here by the Spanish in 1680.

It is well worth noting the stylistic longitude and latitude scales which are interwoven into the chart.  The longitude measures are remarkably accurate although the latitude key is clearly referenced from a point other than London.  The chart also includes a key at the top of the chart which names each of the Florida Keys.

Of further note is the colour on this chart which is especially vibrant considering the age of the manuscript.  The use of colour is intriguing however as it does not seem to be used to delineate ownership of sovereignty but rather to be applied randomly.  Perhaps the different colours refer to especially difficult parts of the coastlines, relatively safe or navigable stretches or even unmapped stretches.  The only obvious consistency is that all shoals or sandbanks are coloured green.  The use of standardized hydrographical symbols for rocks or reefs and sandbanks shows the extent to which such information was universal even at this time.