How are rivers defined in Google maps?

Question

Navigation in space

Hello, dear forum users!
There was a need to find out exactly how rivers are defined in maps (Google, Yandex, 2GIS, etc.)? When the map is displayed in outline mode, the rivers are filled with blue. The boundaries of the rivers are marked very precisely. Moreover, the routes are laid taking into account the rivers. You will not be able to reach the goal if you place the marker even a little in the blue zone. This means that in memory (or somewhere) there is data about the boundaries.
I am engaged in the issue of laying the optimal route along the rivers. In order not to reinvent the wheel, I would like to first understand what has already been done.

Answer 1

In geoinformatics, there are many methods for classifying the earth's surface from satellite images. For example, semi-automatic classification. An example of how this is done: https://youtu.be/a8ulRtzLjgk
General principle: a satellite image (a raster image consisting of channels taken in different spectral ranges) is analyzed in a certain way to find areas where some lines of the spectrum are absorbed, others - on the contrary, in a certain certain proportion. Further, when these areas are found, their boundaries are approximated by a vector contour, which is used further to generate a map.

Answer 2

objects are outlined from a satellite image.
manually more accurate, but more expensive.
there are automatic systems, but they can be wrong. for example, a dark blue roof might be mistaken for a lake.
the output is vector data that allows you to determine the water or not water at the specified point.
for routing, they take roads and form a graph of travel through the area, taking into account many parameters.
From the open stream data, I remember someone making maps for harmin with routing along the rivers.
those. made a selection of rivers from the database and formed a routing graph from river lines.