The correct answer to this open question is the following.
Unfortunately, you did not mention the source to help you answer the question. You did not include an excerpt, text, reference, context, name of the article, or the name of the author to know exactly what you are referring to.
However, we can assume that the topic is the early profession of George Washington as a surveyor. If that is the case, then we can say that the secondhand topic refers to how George Washington learned to survey land using regular field books.
At the beginning of the 1800s, a surveyor was a decent way to make a living and a socially accepted profession. And that is what George Washington decided to do to earn some money for almost 50 years, surveying the Virginia territory. That is why he had the chance to knew the Native American territory very well and got along with many natives.
