What is representative sampling and how can it be used on two-dimensional areas?

Representative sampling is a term analytical chemists use as a statistical basis for collecting samples from large sample sources that evaluates the whole source fairly.

This is because when sampling from large areas, it would be possible for the person carrying out the analysis to directly influence the results of the analysis to further a hypothesis as a result from where the samples are taken from. So how can it be used on two-dimensional areas?