Found in the Options popup menu, the Projection Size dialogue sets the internal scale of the projection. This internal scale is a property of the projection and determines the size of the graticule. It differs from the scale of the map (e.g. 1:20,000,000), which is applied when the projected data is exported to a graphics file format.
The scale factor influences the area and distance distortion properties of the projection: an inappropriately large factor or an overly small factor both increase area distortion. Note that changing the scale factor does not alter angular distortion.
Arthur Robinson chose a factor of 0.8487 when he designed his projection. For the particular case of Robinson's projection, this factor results in a graticule with an area that is very close to the area of the terrestrial sphere.
Flex Projector offers three methods for choosing the scale factor:
- Adjust the scale factor manually with the slider.
- Press the Minimize Area Distortion button to let Flex Projector find a scale factor that minimizes areal distortion for the projection. This is the recommended method. Note that this computation may take a few seconds on slow computers.
- Press the Adjust to Area of Earth button to have Flex Projector compute a factor that results in a graticule that has the same area as the Earth. This method is similar to the one that Robinson used.
- Press the Minimize Area Distortion at Point button and enter the longitude and latitude of a point where areal distortion will be eliminated.
It is recommended to adjust the projection size at the end of the design process when all other settings (length of meridians, bending, etc) are fixed, because the results of the Minimize Areal Distortion button and the Adjust to Area of Earth button depend on the other settings. Changing the internal scale does not change the shape of the projection, only the size of the graticule is adjusted.