The extend reference is used to assign the profile "extend" direction to the midline legs.
The azimuth button displays either a long arrow inside a circle, oriented to the
current extend reference direction, or a short arrow directed left or right (for fixed "extend").
When the profile "extend" is compute using the "extend reference" it is set to right if it differs from the reference by less than 90 degrees. Otherwise it is set to left.
When the Fixed extend reference setting is set, the button has only two states, either LEFT or RIGHT.
Tapping the button toggles between them. The new shots extend is set according to the state
of the button.
This dialog has
When a new shot is downloaded TopoDroid sets its "extend" automatically. This holds also for shots entered manually, without specifying the "extend" in the shot dialog. By default the "extend" is either LEFT, RIGHT, or VERT (vertical), depending on the angle the shot makes with a reference azimuth. The "Extend Reference" dialog allows to change the reference azimuth, by rotating it clockwise or counterclockwise. Alternatively, the reference azimuth can be set using the Android compass sensors.
It is also possible to fix the "extend" for coming shots to either LEFT or RIGHT, by selecting the appropriate buttons in the dialog.
The "extend" of splay shots is determined according to that of the leg(s) at the splay station and the azimuth between the splay and the leg. The circle of azimuths is divided in four regions: two "cones" around the directions at 90 degrees with the leg, and the remaining sectors, one around the direction of the leg, the other around the opposite. The Splay vertical threshold setting defines the width of the two cones.
When the angle between splay and leg is closer than this value to 90 degrees the splay gets a "vertical extend". Otherwise it gets the same extend as the leg, or the opposite, according to the sector the angle falls in.