How to use terrain to calculate irregular areas like google maps does?

Hello
How can I use terrain to calculate irregular areas like google maps, creating points that calculate the area.

How could I do it?
Thank you

Once you have a list of the points that define the space you want to measure, you can use either of two methods to find the area of an irregular polygon…

Reduce to triangles or calculate directly from coordinates.

Hello…
I can not get the list of points by picking … I only get one … how can I get several points?