When using the term scientist think specifics - Botanist, Physicist, Chemist. Don't use an umbrella term. It makes you look, well, foolish.
Since when are there footballs that are 1km long let alone several? You must be one of the biggest fools we've had on this site.
Just stop for one second and think about this. Please.
You have one football and one pencil.
Now, place the pencil vertically. The height from the nib to the base (where the pencil touches the football) is, say, 10 cms? Okay, with me so far.
Now, move the pencil to another part of the football and measure again. Still 10cms.
The football is the same shape as a globe. So when you went 20km and measured the height using your ruler (or pencil) you did the same thing as above with the football.
You can't detect curvature by measuring a point on a spherical surface. It's so obvious I won't go into the details but just think about why this is for a second will you. The surface of a globe appears the same whichever part of the globe you are on.
Do you understand this?