There were of course considerable errors, the roll of the ship could impede the flow of sand in the glass and bad weather could make sun or star sights impossible for days on end. The measurement of time was assisted by the use of pocket sundials. These gave the local time by the sun at the particular latitude they were set for. Nine pocket sundials were found aboard the Mary Rose. The sun dials' built in compass needle enabled them to be orientated to magnetic north. Really accurate measurement of longitude had to wait for the development of a reliable ship born clock, the Harrison Chronometer of the 1770s finally allowed navigators to do so. The chart had existed since the middle of the thirteenth century. These were called "portolans" and probably originated in Venice or Genoa. This sheep or goatskin chart gave coastal outlines with the positions of ports marked on them. Compass roses featured all over these charts to enable courses to be plotted. By the 16th century, these charts were in common use by the Portuguese.

The existence of dividers on the Mary Rose may indicate that the ship carried charts. The pilots of the period also had a "portolano", a form of sailing directions that went with such charts. This described coasts and ports, anchorages, rocks, etc. The successor to the portolano, the routier or "rutter," was becoming popular in the 16th century.

These followed the format of present day sailing directions, giving tidal information and views of the coast as seen from seaward. In 1541, "The New Rutter of the Seas for North Partes" was published for circumnavigation of the British Isles.

By the 16th century the compass was becoming an essential aid for the navigator. The compasses on the Mary Rose were mounted on gimbals in a bowl to compensate for the movement of the ship. The compass bowls had glass tops and were set in wooden boxes for protection.

The pilot also needed a lodestone, a piece of magnetic oxide of iron. (The Pole Star was often referred to as "The Lodestar"). This stone was used to stroke, and thereby magnetise, the compass needle. The compass needle points not to true north, but to magnetic north, a wandering point in the Canadian Arctic.

The angular difference between true north and magnetic north on any part of the earth's surface is known as "variation". By 1530, the Portugese had the means of measuring and tabulating variation. Another error of the compass is caused iron fitting and items within the ship. This causes a deflection of the needle known as "deviation".