Mine surveying and mapping area is projection area or hillside area.

In engineering survey, we generally use the rectangular coordinate system of Gaussian projection plane under the national reference ellipsoid. The point coordinates measured by measuring instruments are rectangular coordinates of Gaussian projection plane, and the calculated area is the area of Gaussian projection plane, which means that all our topographic maps are based on Gaussian projection plane.

In cadastral survey, we usually choose the meridian in the center of the survey area as the central meridian of Gaussian projection to establish an independent local coordinate system, and choose the average elevation of the survey area as the projection height of Gaussian projection, all in order to reduce the influence of projection deformation on the side length and area, so that the difference between the side length of two points measured in the field and the calculated side length of two points measured is less than 5cm.

Therefore, the area of mine surveying and mapping is based on the projected plane area, not on the surface area or the slope area of the mountain.

If you need to calculate the surface area, you need to know the elevations of all coordinate points, and build irregular fitting surfaces by knowing the coordinates and elevations to calculate the surface area, such as common irregular triangulation and grids to fit surfaces.