I was reading a news article about the Universal Atlas of Fernão Vaz Dourado (16th century Portuguese cartographer) - here is the atlas for those who aren't familiar with it - and found myself inquiring about the creation and use of these kind of charts.

I know that all those lines have something to do with the use of compass. Besides that, I wouldn't know what to do with a portolan chart. There must be a way to measure distance traveled in order to determine one's position, but I don't see how would that be determined. By time traveled there's the need to know the speed of the ship and, thinking about celestial references, although there's the astrolabe to determine the latitude, I don't recall there being an instrument to determine longitude. So, what kind of instruments/techniques were required to use a chart like these?

Additional questions:

• How common were these kind of charts inboard ships during the Late Medieval and Renaissance periods?
• How much would a portolan chart cost around that time? How would the price/rarity vary between one limited to Europe vs a "World" Atlas like the one drawn by Fernão Vaz Dourado?
• The proper use of the chart was common knowledge among sailors or restricted to navigators?

And I think that's all my curiosity can muster for now. Thank you in advance for your answers.