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:
- 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.
- 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.
- 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.
- 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.
- If you are not satisfied, there are two ways in which to try improving the horizon
- 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, firstname.lastname@example.org.