Finding top surface area of a mesh by python

Question

I need to find the top surface area (in one plane) and total surface area (by summing all the faces) of a complicated mesh. For that I need to know how to fit a plane to the top of a mesh to find the top surface area. The mesh can be .obj or .stl or .ply

Answer 1

Here as requested is faces whose normal vector has angle from up vector of less than 80 degrees.

import bpy
import bmesh
from math import radians
from mathutils import Vector

obj = bpy.context.active_object
mw = obj.matrix_world
bm = bmesh.new()
bm.from_mesh(obj.data)

up = Vector((0, 0, 1))
# local space    
area = sum(f.calc_area() for f in bm.faces if up.angle(f.normal) < radians(80))
print("local", area)
# global space
area = sum(f.calc_area() for f in bm.faces if up.angle(mw * f.normal) < radians(80))
print("global", area)
# or avoid a lot of matrix mult by converting "up" to local space.
up = mw.inverted() * up
area = sum(f.calc_area() for f in bm.faces if up.angle(f.normal) < radians(80))
print("global", area)

bm.free()