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
  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:

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.