The ImageJ software, developed by the NIH, is a freeware tool for analyzing photographs. It is a useful tool for our purposes of measuring snout-to-vent length (SVL) of Pacific newts, especially those that are curved. Here are instructions for using ImageJ to calculate SVL:
(1) Download the ImageJ software from the NIH website: https://imagej.nih.gov/ij/download.html. The download includes a detailed (and somewhat overwhelming) user's guide of all the software's features & capabilities. Don't worry - we only need to use a few simple features.
(2) Make a copy of an iNat newt picture and save it on your computer. To do this:
--> Right-click on the picture in iNat and select "Save Image As..."
--> I use the 8-digit iNat ID as the file name so I can later go back and enter the SVL (snout-to-vent-length) value.
(3) Launch the ImageJ software. You'll see the ImageJ toolbar on your desktop.
(4) Select File --> Open... and open the newt picture.
(5) Right-click the line tool on the toolbar and select "Straight Line."

(6) Draw a line on the measuring device in the picture (for example, 1 inch). See the yellow line on the ruler below.

(7) Select Analyze --> Set Scale... The "Set Scale" dialog is displayed.
--> Enter the "Known Distance" and "Unit of Measure."
--> Click OK.

(8) Right-click the line tool again, but this time select "Segmented Line."
--> Double-click on the tip of the newt's snout to start the segmented line.
--> Click to place pivot points along the segmented line.
--> Double-click to stop at the newt's vent. Refer to this drawing:
Diagram for measuring snout-to-vent length (SVL)
(9) Select Analyze --> Measure. The calculated length is displayed in the Results dialog box.

(10) According to the cutoff of 50 mm (2 in) suggested by @tyap and other herpetologists, this newt would be categorized as an adult. In iNat, set the "Life Stage" Annotation to "Adult" and add the Observation Field "SVL" along with the value calculated by ImageJ.
The following charts show Pacific Newt roadkill at Lexington Reservoir and cumulative rainfall amounts (measured in inches) for each of the four migration seasons we've been surveying. We use the Weathercat website based in Santa Cruz for rainfall and temperature info due to its proximity to Lexington Reservoir and its convenient numeric format which is easy to copy into Excel for analysis purposes.
Cumulative rainfall as of Jan 24, 2021 is much lower than the three previous seasons (Figure 2). This correlates with the lower newt mortality we're seeing for these months (Figure 1).

(Updated on Jan 24, 2021.)