[Jan 2020]
This page describes a method to use LibreOffice Draw as an aid to measuring the real size of objects in a photograph. A summary of the method is as follows:
The use of a photographic reference scale is recommended. These are available commercially, or you can print your own (also, see here for another printable version and tips on use). Purchase or make these, and practice taking photographs with them; take photos of objects of known dimensions and then make photo measurements to verify your technique.
Set up LibreOffice Draw as follows:
I suggest setting the line and font colors to light gray as these provide good contrast for general use, but you may need to change these for some photos, depending on the background colors.
When using the dimensioning tool, I have found this technique effective : Use the page zoom option to make the object to be measured as large as possible in the frame. Set an initial dimension slightly smaller than the wanted dimension. Hold down the Altkey and adjust each end of the dimension line to the exact desired position (you may want to zoom in even more for the final adjustment).
To set up a page for measurement:
Use the LibreOffice Calc spreadsheet (see resources below) to calculate actual dimensions.
Photographic reference scale:
Note : I laser printed this scale onto card stock, but I found this was a bit too flexible and had a tendency to curl. I suggest printing out the scales and gluing them to a rigid backing such as hardboard, plastic sheet, or similar. In any case, the finished product should be measured with an accurate ruler or digital caliper to verify that it is accurate.