On a hemispherical photograph, the horizon is displayed as a circle, with the
zenith at its centre. Before analysing the picture, the position of the horizon
has to be defined. By default, Hemisfer assumes that the zenith is at the
centre of the picture and that the diameter of the horizon circle is the equal to
the small size of the picture. From these values, the user can change the centre x
and y coordinates and the radius of the circle, either entering numbers directly
or by clicking on up and down arrows 
.
The position of the horizon is displayed as a green line
on the picture. If the
lens is not hemispherical, then the horizon is not visible.
In this case the limit of the field of view is displayed instead, as a cyan line
. The zoom function
can be used to make this positioning easier.
Photographs taken with the same camera and the same lens should have the same position of the horizon. If a picture is loaded with exactly the same size as the previous one, then the same horizon will be supposed. To save disk space, it is also possible to crop the pictures (to cut the borders) before loading them into Hemisfer. Large numbers of pictures can be cropped in a single batch by some graphic softwares, for example Irfan View (shareware, free for private, non-commercial use).
The next steps after positioning the horizon are usually to set the
north and the slope on the
picture. Once this is done, it is possible to save these settings in a
parameter file (file menu or 
button).