1.1.3. Minimum Dimensions
The symbolization of graphical objects for screen maps has to be adapted to the peculiarities of screens. Line widths, minimum sizes for point symbols and minimum distances between graphical elements have to be larger than on paper maps in order to be optically well discernible.
|Line width (Brühlmeier 2000)||
The picture on the left shows how lines with different line thicknesses are visualised on a screen. You might be surprised not to see the expected difference of the width of two lines. For example with the Adobe SVG Viewers 3.0 and 6.0, line widths do not increase linearly, but grow irregularly. Lines between 0 and 0.3 pixels are always rendered with the same width and in gray colour. Between the pairs 0.3/0.4 pixel and 0.6/0.7 pixel differences of the apparent line width become recognizable. Other renderers may have completely different characteristics. (Räber et al. 2003)
You might ask yourself how it is possible to visualise a line that is smaller that one pixel which is the smallest element of a screen. If so, have a look at the popup window below.
Click here for more information
Visualising a line that is smaller than one pixel is realised with
anti-aliasing. As you learned in the lesson Computer Graphics, anti-aliasing
smooths a line so that the line looks better on the screen (without
jaggies). When drawing a line that is smaller than one pixel, you are forced
to apply anti-aliasing, but instead of using a dark colour as start colour,
you start with a light colour and end in white. The line then looks thinner.
In reality, it is not, but it seems so. Thus, the human brain is fooled so
that it thinks that the line is thinner.
The following two pictures show the difference of a line larger and one smaller than one pixel.
If you do not believe me, take a look at the picture "Line width" on the left above. When you look closely at the picture you can see, that the lines have always the same width. They just get lighter the "thinner" they are.
|Minimum distance between surfaces (Brühlmeier 2000)||Minimum distance between lines of different widths (Brühlmeier 2000)||
A series of test objects is used to identify the minimum distance
between two surfaces. In order to unequivocally differentiate the two
areas, they have to be separated by at least 1 pixel. A similar test can
be done with lines to find the minimum distance for linear elements. As
you can see in the right figure 1.5 pixel is that minimum distance. But
for very thin lines the distance is 2 pixels, since they are rendered in
grey. (Räber et al. 2003)
Dimension of point signatures
|Minimum dimension of point signatures (Brühlmeier 2000)||
The distance the user keeps to the display device when studying a web map is approximately 80 cm, whereas a paper map is typically held at a distance of only 30 cm. In case of doubts, distances between map elements and minimum sizes of signatures should be chosen larger rather than smaller. (Räber et al. 2003)