A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Don Seltzer
Date: 2016 Jan 29, 09:39 -0500
"NIH Image is a public domain image processing and analysis program for the Macintosh. It was developed at the Research Services Branch (RSB) of the National Institute of Mental Health (NIMH), part of the National Institutes of Health (NIH). It has been superseded by ImageJ, a Java program inspired by NIH Image that runs on the Macintosh, Linux and Windows.
Image can acquire, display, edit, enhance, analyze and animate images. It reads and writes TIFF, PICT, PICS and MacPaint files, providing compatibility with many other applications, including programs for scanning, processing, editing, publishing and analyzing images. It supports many standard image processing functions, including contrast enhancement, density profiling, smoothing, sharpening, edge detection, median filtering, and spatial convolution with user defined kernels.
Image can be used to measure area, mean, centroid, perimeter, etc. of user defined regions
of interest. It also performs automated particle analysis and provides tools for
measuring path lengths and angles. Spatial calibration is supported to provide real
world area and length measurements. Density calibration can be done against radiation or
optical density standards using user specified units. Results can be printed, exported
to text files, or copied to the Clipboard."
Bill and Greg,
Out of curiosity, I tried using Adobe Photoshop to measure the pixels. It has movable horizontal and vertical guides that I placed on the upper, lower, right and left limbs. I then split the differences between those guides and put two new guides at the center of the square they formed (one vertical, one horizontal giving me a set of crosshairs at the center). Next, I used the convenient ruler tool to measure the pixels from the crosshairs to the horizon. The result after all that: 2,402.5 pixels (apparently PS can acheive sub-pixel accuracy!). Using Greg's corrections, I get an Ho of 22°18.0'; an intercept of 0.9' away. In other words: roughly the same result. ;)