Photo measuring

General discussions of measurement techniques and the results of testing of techniques and equipment.

Moderators: edfrank, dbhguru

User avatar
dbhguru
Posts: 4462
Joined: Mon Mar 08, 2010 9:34 pm

Re: Photo measuring

Post by dbhguru » Wed Jan 17, 2018 3:43 pm

Ents,

Please find attached another test of the photo measuring method. The reference object is a 12-inch segment of a yardstick on the other side of my basement room. The target is a whole yardstick. The reference object is oriented close to 90 degrees to the line of sight. The target is oblique to the line of sight, closer than the reference object, and at the left side of the image. As you can see, the measurement error is only 0.514 inches or 1.43%. I'll take it.

If anyone wants to use this workbook for their own test, first unprotect the photo worksheet. Delete the photo, beige rectangular area, and the blue and yellow line objects. Lastly, delete the contents of the green cells. You're ready to do a new test. Just follow the instructions. If you need more instruction, let me know.

Bob
Attachments
Oblique Photo Measurements 13 .xlsx
(3.53 MiB) Downloaded 24 times
Robert T. Leverett
Co-founder, Native Native Tree Society
Co-founder and President
Friends of Mohawk Trail State Forest
Co-founder, National Cadre

User avatar
dbhguru
Posts: 4462
Joined: Mon Mar 08, 2010 9:34 pm

Re: Photo measuring

Post by dbhguru » Sat Jan 27, 2018 3:07 pm

Ents,

With warmer weather, I did a test outside this afternoon of the photo measurement method. I'll spare everyone another Excel workbook, and just report that the measurement errors on the targets were as follows in inches.

0.847658294
0.583353056
0.347799113
0.585533019
0.172977198
1.197645905

These results are consistent with past tests. Errors rarely exceed an inch. In this test, the average error over the six trials was 0.6 inches. When you think about it, that is darn good. The method continues to exceed the accuracies I initially expected. The method appears to have a future.

Bob
Robert T. Leverett
Co-founder, Native Native Tree Society
Co-founder and President
Friends of Mohawk Trail State Forest
Co-founder, National Cadre

User avatar
Don
Posts: 1560
Joined: Tue Mar 09, 2010 12:42 am

Re: Photo measuring

Post by Don » Mon Jan 29, 2018 11:14 pm

Those in the Cadre that are following Bob's "photogrammetry" exercise this far, should check in at the Cadre Forum.

Stuff's happening there, that I'd think you'd want to weigh in on...
Don Bertolette - President/Moderator, WNTS BBS
Restoration Forester (Retired)
Science Center
Grand Canyon National Park

BJCP Apprentice Beer Judge

View my Alaska Big Tree List Webpage at:
http://www.akbigtreelist.org

User avatar
dbhguru
Posts: 4462
Joined: Mon Mar 08, 2010 9:34 pm

Re: Photo measuring

Post by dbhguru » Wed Feb 28, 2018 3:58 pm

Ents,

Elijah's recent image of the Dutch elm in upstate NY and my subsequent photo-measuring of its diameter and height illustrate that we can get well with the ball park if we have location information and reference object size. If the reference object is at the same distance from the camera as the target, then the rest is simple, or can be. We can do a lot with a little.

BTW, you don't have to use Excel to get the photo sizes of the reference and target objects. Software like ImageJ works just a well, and of course there is always Photoshop. Here is a simple calculation to estimate the target size.

Estimated Target Size = (actual reference object size/photo reference object size) x photo target object size.

This simple formula assumes that the line of sight intercepts the approximate middle of both the reference and target objects widths (or lengths) at 90 degrees. If this is not close, all bets are off. In addition, if the target length is long such as the height of a tree, then distance becomes a factor. The upper part of the tree is farther away than the distance to the base and to the reference object. Also, object orientation must be taken into account if not at 90 degrees to improve accuracy. This is done in the photo-measuring worksheets that have been previously presented.

Ideally, we make it as easy on ourselves as we can, but that means different things in different situations. It is hard to state hard and fast rules. Presently, there is a collaboration developing between an associate professor and his post doc at Virginia Tech and yours truly to submit a paper on photo measuring. Don and I will also likely submit an appendix to the current American Forests tree-measuring guidelines on photo-measuring. We'll keep members of the BBS informed.

Bob
Robert T. Leverett
Co-founder, Native Native Tree Society
Co-founder and President
Friends of Mohawk Trail State Forest
Co-founder, National Cadre

Post Reply

Return to “Measurement and Dendromorphometry”