We'll use 4 corners method to proving.

I chosed the poins in the maps by coordinates as (56,26), (56,36), (66,26) and (66,36)

I chosed these coordinates as 10 degrees differense because of simplify the calculations.

We'll try to create the shape of area contain these 4 corners, after that we'll calculate the diagonals, we'll read the diagonal on the map and will try to understand the shape of the earth. If the diagonal seem like a diagonal in the plain area or has it a curvature and longer than estimated. This will our proof.

The lenght of: (66;26) to (66,36) = 451,33 kms.

The lenght of: (56;26) to (66,26) = 1112,55 kms.

The lenght of: (56;36) to (56,26) = 621,33 kms

The lenght of: (66;36) to (56;36) = 1111,87 kms.

You can check these numbers by using an online map coordinates.

Now we have a shape like that:

In this shape, We have wonder is that shape has a depth or is it a plain shape? For understand it, we'll calculate the diagonals. (cross)

I pre-accept it as "flat plain shape" and calculating the diagonals as:

Diagonal= 1231,84 kms.

Now;

We'll calculate the curvature for this number, in any of curvature calculating web site:

It is calculated as 1233,33 kms.

And it will be about 1232 kms on the map as flat plain.

Now we are looking to the measurements of diagonal coordinates.

Lenght of: (56,26) to (66;36) = 1232 kms.

Lenght of: (56,36) to (66;26) = 1232 kms.

As we see that there is no curvature and the shape of selected area is completely flat.

Simply, understandable and repeatable by everybody.