Representative samples (like gallup polls) are important to family survey research because the results can be to the national population of the sample?

These samples are important because survey results can be generalized to the national population. Generalization is when something is concluded about a larger population from data from a smaller population of results. For example, something like the Gallup poll might ask a pool of people how they feel a president is doing, take those results, and apply it to the entire population of the country.

It should be at the center of any map going vertically: the prime meridian, as it passes through greenwich,  england, is considered 0 degrees longitude