Longitude & Latitude without a GPS Device – Explained for Kids

As a followup to my last post about the Herbarium of my late uncle:

Of course the mentioned database and others offer exact dates to the specimen finds. This is what makes it so interesting to me, because if used statistically en masse (the number of records is huge), it gives the possibility to track the travel of a, let’s say, neophyte over time and space. Roughly only, though, but usable, for example to determine when Viola tricolor appeared, where (it is a neophyte from the New World), and also to see when which plant disappeared from where.

Yet, another question appeared in front of my minds eye:

How did my uncle determine latitude & longitude so exactly as given in the database in 1949?
I cannot give a certain answer to this, because I cannot ask him. He certainly could not have had a portable radio, because the working concept of transistors dates to 1953. The use of radios is explained in this wonderful article:

(Hint, it is for kids, but certainly suitable for adults, too)

At first, I suspected that the locations in the Herbarium were based on house numbers of the resp. street, but some of the points are in total landscape. I have checked some of them inside of my current geographical reach, and they seemed plausible.

So I go on thinking, my uncle had a magnetic compass, a good stopwatch, a stick, and knew how to take his measurements properly. He certainly did not carry a device like this: