apply trapezoid rule across first axis and then apply it again across the second axis

Trapezoid rule across the first axis and the second

Trapezoid rule across the first and second axis

double integral

Trapezoid rule across first and second axis

use trapezoid rule for the first and second axis

Could split each square of the above surface into two triangles using the corners as triangle vertices, and calculate the volume of the area under those triangles

Trapezoid rule along the two axes.

Trapezoid rule across both x and y axis

Apply trapezoid rule twice.

trapezoid rule twice along two axis.

Apply the trapezoid rule on x and y

another integral

A double integral

double integral

Integral

Estimate the value at the center (or average of corners, or something like that) and multiply by value, iterate.

Double integral

Trapezoid rule along the two axes

double integral

Double integral