The method can be made even more efficient with the use of Visual Basic for Applications, the macro language of Excel. It would be tricky, but an Excel macro could be developed to automatically read the dimensions of the masking lines and post them into cells within a template. A strict protocol would have to be followed such as proceeding from left to right and bottom to top in terms of placing the masking lines. The reference object would be covered first regardless of where it appeared in the photo, then the sweep from left to right. The more automatic this approach, the sophisticated the macro would need to be, especially if multiple trees were being modeled through a single photograph. At the beginning, we would need to keep it to a single tree so that the first mask would be the reference object, and all subsequent masks would be on the trunk going from bottom to top. Each limb would be a separate image.
1) Basically, if I understand this right, the idea behind the photo measurement is that the rate of change in perspective (trunk width) should change smoothly in a linear fashion as the target gets farther away. Your formula essentially is calculating the equivalent of the optical scaling factor based upon distance from the lens and apparent width of the reference object, much like is provided with a reticuled monocular.
2) The process for modeling the volume of a tree using photo measurement proceeds in the same way as with the monocular. The distance to each measurement and height above eye/photo level can be measured using the rangefinder and is input into your spreadsheet
3) I am not sure why you would need to maintain the line direction consistently, but if you say so. It really isn't a problem to do it this way.
4) Does the line across the tree need to be exactly horizontal or vertical, or can it be drawn at an angle?
5) So long as the focal length on a zoom lens does not change from image to image on a single tree the same scaling factor should work for multiple images in a set. So you could first take an overview photo to see how the tree is formed. You would need to make a sketch of the tree structure and measuring points to keep track of the position of the measurements. You could zoom in as close as possible so the base of the trunk and the reference scale filled most of the width of the image. Without changing the focal length you could then shoot all of the targeted points. (Ideally you would have multiple images that could be stitched together to form a pan of the entire tree, but if parts were missing it would not really matter for measurement purposes) This would assure that the image being measured for any measurement was as large as possible. This would help alleviate the problem of tiny widths in an image of the entire tree in one photo.
6) How do you determine the length of a branch or trunk segment that isn't vertical in the volume measurement protocol? The angle of the trunk or branch might not be perpendicular to the viewer? (Short of doing an azimuth and plotting the positions of the end points of the segment in 3D.)
7) I would think it would be better to try to model just one tree per photograph, or at least simply treat each different tree as a separate entity on a separate spreadsheet page, rather than trying to do it all on one single spreadsheet page. it would be a nightmare to keep your data in order if multiple trees were on a single sheet, and if it makes the macro harder to write, why bother? it seems a bad idea all around.
"I love science and it pains me to think that so many are terrified of the subject or feel that choosing science means you cannot also choose compassion, or the arts, or be awe by nature. Science is not meant to cure us of mystery, but to reinvent and revigorate it." by Robert M. Sapolsky