Learn Dutch: Uit de plattegrond blijkt dat er daar een vlakke ondergrond is. - It appears from the map that there is a flat surface there.

This sentence requires two types of "there": a presentative one to introduce the undefined subject (een vlakke ondergrond) and a locative one to point to a physical place. Normally, both would use the word er, creating the awkward double sequence dat er er.... To avoid this, Dutch substitutes the locative er with daar.

Why is the preposition uit used with blijkt here?

The verb blijken means "to appear" or "to become evident". When you state the source of that evidence (like a map, a study, or a report), you use the fixed preposition uit. So Uit de plattegrond blijkt translates closely to "From the map it becomes evident".

Why is it een vlakke ondergrond with an -e on the adjective, even though it follows een?

You only drop the -e ending on an adjective after een if the following noun is a neuter (het) word. Because ondergrond is a de word (de ondergrond), the adjective keeps its normal -e ending, resulting in vlakke.

Why does the verb is go at the very end of the sentence?