Learn Polish: Nawigacja wskazuje tu skrót, tymczasem droga nie ma asfaltu. - The navigation shows a shortcut here, whereas the road has no asphalt.

While tymczasem often means "meanwhile" (two things happening at the same time), it is frequently used as a discourse marker for strong contrast, meaning "whereas" or "yet." It highlights the frustrating mismatch between the GPS's promise and the actual state of the road.

Could we use the other contrast connector, za to, in this sentence?

It would sound unnatural here. za to implies a trade-off or a balancing compensation, like "on the bright side." Since an unpaved road doesn't compensate for a shortcut, but rather ruins the idea of one, tymczasem is the right choice for this direct contradiction.

Why does the sentence use asfaltu instead of the dictionary form asfalt?

This is a classic example of the genitive of negation. The verb mieć normally takes a direct object in the accusative case. But when a verb that takes the accusative is negated, as in nie ma, the object must switch to the genitive case, which is asfaltu.

Why use wskazuje instead of a simpler word like pokazuje?

Both are perfectly fine, but wskazywać (to indicate or point out) is highly natural for navigation systems. It gives a slightly stronger sense of directing you toward a specific path, whereas pokazywać just means "to show" in a general sense.