What are some of the factors that could contribute to an area being sparsely populated?

Which modern technology has not made a great difference in the inuit way of life?

This is a 2017 topographic map of puerto rico in the united states, and i'm sorry that people there have suffered such a disaster. i want to ask, what do the yellow dots and areas represent on this graph? and the blue area and the little blue circle, and the little blue sign that looks like a signal tower, what do they stand for?

Which of the following are commonly shown as a topographic map