Accurate Topography from a Hillshade Image

… “hillshade”?

I might be wrong but that image is the shaded view or a mesh.
To work properly you would need to have a grey-scale image where each grey level always means the same Z altitude.
That image have the same color near the sea level as the color “uphill” … rebuilding a shape from such image is really complex and will give approximate results.
(the light/shadow is a white to black gradient depending on the normals of the mesh, but it is not enough to properly define a shape. A normal map would have more information, but still tricky.
See this really unconclusive attempt from normal map Surface from planes (using planes z-axis as face normal at that point) ||| Planes still on XY-Plane and need to be moved on z-axis acrding to angle - #12 by maje90 )

Imho, find a proper image first. A heightmap image.