how to Find the total surface area of a 3D mesh plot

I have a 2D matrix containing the height information of a 3D object. I would like to know how to calculate the total surface area of this mesh plot.

XY calibration: 139nm/pixel and height in nm.

I would like to calculate, total area of intersection in a decrement of 0.5um from maximum height to lowest height in a selected area. because the data is huge, so i am selecting small part of the data(partial data is attched). consider 100*150 matrix or pixels. i am taking the average value of all height in selected area and calculating the difference between individual original height and average height and then plotting the same in 3D graph(shown below)(x and y are 1 to n in pixel and z in height nm )

At starting point i.e., maximum height, the area of intersection is zero and then decrese the height by 0.5um , then there are many small area of section, adding them together to find total area at the point of intersection. Continue the same and record the area until it reached lowest value. At lowest height, the area of intersection is the complete area of the selected portion from the data.

Please let if you have any idea on this topic, help me with this.

Thanks in Advance,