this is what I was looking for
I think it was pretty easy to understand what you were looking for, maybe others were being deliberately obtuse. Anyhow, the synopsis that
@Richard R gave you is the basic information and doesn't get into it too far, so here's a little more information.
The process of getting a 3D model from 2D pictures is called photogrammetry or structure from motion. These are old techniques that are being used with new vehicles. You can recreate 3D scenes on the ground, too, so it's not just used for UAVs. When you take a picture of a certain point from multiple angles, you can start to understand the relationship between all of the other points as you move. This is done similarly to how your 2 eyes are able to judge depth and see in "3D". But with this technique, we're looking at lots of points from lots of pictures, enabling the software to generate a 3D "point cloud" of the area that you've flown. Yes, this is better done by using a fancy computer, but a slow one can do it if you give it enough time. With a moderate amount of RAM and with a recent GPU and CPU, you can do all of this at home. I started on an old machine and it wasn't fast, but it did produce results.
Ok, so you've got overlapping pictures from the drone, the program (Pix4D or Agisoft Photoscan are 2 of the most popular titles) has generated a point cloud, and every point in the scene has been assigned an X,Y,Z coordinate. With this data, you can select that area that you want for volume calculations and the program will create 3D "posts" or "voxels" for each area that you're interested in for volume. Think of it like hundreds of thousands of 4"x4" posts that are cut to length to recreate the pile that you're measuring. All you have to do is sum up the volume of the posts to get a reasonable recreation of the volume of the pile. In the "old days", getting a volume of a pile meant getting the area of the base and getting several transect through the pile and trying to generate a crude model of the pile. Now you're doing the same thing, but with hundreds of thousands of points, if not millions.
You can create extremely accurate models for planning, volumetric calcs, surveying, and other geospatial products with a Phantom 4, you just need a little knowledge and the right software. If you want to get started, I'd do a Pix4D or Photoscan trial and learn the steps. I don't like the cloud based software solutions because I can't control the data as well and they usually don't have the settings to produce results as accurate as I need. They're great for the hobbyist, but if you're looking at using some of this technology for your engineering firm, you'll want a full software package.
