HORIZON FINDER, Version 1.0

by David Dearborn

Terminology. Elevation means the height of a location above sea level. Altitude means the vertical angle between a viewed point and the horizontal plane through the observer. Elevation, then, is a distance; altitude is an angle. Cf. Astronomy in Prehistoric Britain and Ireland, p. ix.

This Program, written by David Dearborn, scans a greyscale image in Adobe Photoshop format, and returns a digitised and calibrated horizon profile, i.e. a file containing a list of the azimuth, altitude and declination of a sequence of horizon points from one end of the horizon to the other. The program runs on Apple Macintosh platforms only.

The procedure is as follows:

1. Read the image into Photoshop. If it is in colour, convert it to greyscale. Note the Photoshop X co-ordinate of the two points on the horizon that you will use for the calibration. Save the image in Photoshop format.
2. Start up the "HorizonFinder" program and open the image file. Enter, as prompted, the azimuth, altitude and X co-ordinate of your two chosen fiducial points, followed by the latitude of the site.
3. The program will then
   • replot the image (which will look red) with vertical green lines at the fiducials and a blue line along what it identifies as the horizon; and
   • output (in a file called "OutA" [or "OutB" if "OutA" already exists, etc) a digitised and calibrated horizon profile, i.e. the azimuth, altitude and declination of a sequence of horizon points from one end to the other.
4. If you are satisfied with the identification of the horizon, quit the program (hit the Return key and, when prompted for a threshold value, enter a number greater than 255) and retrieve the output file.
5. If you are not satisfied, there are two ways in which to try improving the horizon identification:
   • Vary the "threshold value". Increasing it will generally push the identified "horizon" upwards on the photo; decreasing it will push the identified "horizon" downwards. You are given the option of varying the threshold value when you press the Return key after the image is displayed.
   • Return to Photoshop and try enhancing the contrast between the land and sky, or eliminating points of contrast that are being falsely picked up, such as dark clouds in a bright sky.

Coming soon: a modification to allow the user to input more than 2 fiducial points and use the best least-squares fit to determine the calibration.

Click here to download the program and test image.

Please direct any queries to David Dearborn, dearborn2@llnl.gov.