If measurements are taken at different points in an inconsistently thick wall, the volume calculations will not be accurate. Plus, materials have varying densities. If a wall includes multiple materials, the volume measurement needs to consider the density of each material to get accurate calculations. Material compaction can also affect volume measurements, especially when it comes to soil or aggregate used in retaining walls.
Photogrammetry can solve these issues, allowing you to account for various materials, shapes, and densities as you measure your walls.