Basle (or Basel) is a city in northwest Switzerland on the river Rhine.
It is situated where Switzerland, Germany and France meet; some of Basle's suburbs are actually in Germany and France.
In 1880, it had a population of just over 60,000. Today, it has a population of over 500,000.
It is about 179 miles (288 km) from Frankfurt, so Heidi's journey would have been rather a long one. She would then have had to travel a further 99 miles (160 km) the following day to reach Maienfeld.