LEGIBILITY AS THE CONDITION
FOR CARTOGRAPHIC VISUALISATION
Stanislav Frangeš
University of Zagreb, Faculty of Geodesy
Kačićeva 26, 10000 Zagreb, Croatia
sfranges@geof.hr
Stanislav Frangeš was born in Zagreb in 1959. In 1984 he graduated at the Faculty of Geodesy,
University of Zagreb. In 1993 he obtained MSc and in 1998 PhD in geodesy at the Faculty of
Geodesy, University of Zagreb. He hold courses on General, Topographic, Thematic and
Practical Cartography, Map Reproduction and Cartographic Visualisation. He is the assistant
professor and head of the Institute for Cartography at the Faculty of Geodesy. He published
more than 60 papers and 80 maps. He was awarded for the design of photomap Velika Gorica
at the International Cartographic Exhibition in Ottawa in 1999. He is the editor-in-chief of
Geodetski list.
Summary. Map users had the same eye abilities or limitations in the past as they have today and will have in the future,
regardless of the fact whether the cartographic presentation is printed or shown on the screen. The following demands
are made for cartographic presentation: legibility, clearness, accuracy, plainness, and aesthetics. The paper deals
especially with the legibility being the demand made for map visualisation. Some conditions have to be fulfilled for the
purpose of meeting this demand, and these are the respect of minimum sizes of cartographic presentation elements,
adequate load and usage of known features on cartographic presentation.
Keywords: cartographic visualisation, legibility.
1. Introduction
The most important changes in cartography are associated with the development of computer
technology, and regarding the function and usage of cartographic presentations, the accent has been
given to cartographic visualisation.
2. Cartographic visualisation
According to Taylor (1994), modern cartographic visualisation encompasses digital cartography
and computer graphics. Considering the quality, it is a remarkable change of visual presentation in
almost realistic time that leads to better understanding of many spatial objects. Referring to the
quantity, it is the possibility of faster and cheaper production of a wide range of various
cartographic products.
In the context of digital geographic data, visualisation relates to graphic presentation of geometry
and attributes of database object either on screen, paper or some other medium (Guptill and
Morrison 1995).
2.1. Visualisation tools
While about ten year ago the possibilities of map visualisation used to be very modest, all higher
quality software packages today used in cartography offer a variety of possibilities. They have the
tools for manipulating with objects, text, retrieved objects, for special effects, export filters etc. All
programs support also basic elements that are used for creating a more complex drawing. These are
the tools for drawing rectangles (square being a special case), ellipses (circle being a special case),
general curves (straight line as a special case), for creating bitmaps and writing texts. Special tools
enable the work in layers, creation of round edges of a rectangle, the production of pie graphs,
classifying and connecting, and making single objects thicker or thinner, making them straight or
doubled, focusing, zooming, distance and angle measuring, filtering for the purpose of improving
image quality, masking, shading, vectorization, rasterization, etc.
Digital tools are being developed continuously, and hence, it is expected that more convincing
visualisation tools for the presentation design will appear. So far, focusing has been developed that
is used for determining the contrast between objects and environment, defining thereafter the limits
of clear distinctness. The accompanying effects are known as disappearing, fogging, and similar.
Furthermore, zooming has been developed almost perfectly enabling the distinction between
graphic, contextual and intelligent zoom, and also the work with the sections, windows, graphs and
icons (Timpf and Devogele 1997). There are also animated figures, three-dimensional
representations, shading with various models of illumination, e.g. flat shading, the method of
intensity interpolating (Gouraud shading), the method of interpolating normal onto the surface
(Phong shading), the method of ray-tracing and the method of radiosity, blinking and glittering,
various filters, simultaneous representation of various cartographic presentations, colour
transformation and other phenomena (Kraak and Ormeling 1996, Maxwell 2000).
The development of visualisation software requires, especially for cartographic purposes, the
research of real needs and aim that users want to reach. Cartographers should take part in scientific
visualisation, as well as users and creators of tools, leaning in it on scientific and professional
cognition, but also on individual skills (Harrower et al. 2000). The tools of cartographic
visualisation give users the possibility to carry out extensive transformations and changes of data
presentation, e.g. different observation angle, changing various conditions etc. enabling the
comparison of essential facts. It should not be exaggerated when using visualisation tools. One
should, namely, try to achieve visual simplicity and, when it is not necessary, avoid any burdening
of a user.
2.2. Demands posed upon cartographic visualisation
Before the consideration and making of demands has commenced, one should reflect upon our
experience of a cartographic presentation or upon what is necessary to experience a cartographic
presentation. This question touches the essence of cartographic communication itself. According to
Uçar (1979), cartographic presentation is a visually very clear and to a certain extent intuitive model
of space. Cartographic communication is graphical and visual communication.
It should be considered in this case that the sense of sight is created on the basis of received
impulses as completely psychic event experienced individually. In order to have the sense of sight
become the experience of sight, some higher psychic function should be involved as well, as for
example memory, abstraction and similar. Hence, it can be concluded that we actually “watch” with
brain (Knizhnikov 1997).
In “classical” manuals of cartography, as for example are those written by Peterca et al. (1974),
Lovrić (1988), Robinson et al. (1995) or Hake et al. (2002), various demands are stated that the
cartographic visualisation should meet. The following three demands can be therefore demanded:
legibility, plainness, and accuracy. Apart from that, the cartographic visualisation should meet also
the demands that can be posed upon any graphic presentation. The following is of the greatest
importance for the cartographic visualisation: clearness and aesthetics (Fig. 1).
Each of the above stated demands can be carried out on single ingredient parts of the cartographic
presentation, but it is much more adequate to do it through their purposeful combining. It has quite
certainly much greater importance for the formation of the efficient cartographic visualisation, and
thus also for the cartographic presentation appropriate to the user. In the process of carried out
research (Frangeš 1998, 2000a; Frangeš et al. 2000, 2001) the following demands were made for
map visualisation: legibility, clearness, accurasy, plainness, and aesthetic. Their realisation requires
the fulfilment of some conditions (Fig. 1).
C A R T O G R A P H I C


LEGIBILITY
minimal sizes
CLEARNESS
simplicity
graphic density
differentiation of
known features
contrasting quality
layer arrangement
V I S U A L I S A T I O N



ACCURACY
positioning
accuracy
accuracy signs
PLAINNESS
symbolism
AESTHETICS
harmony
traditionality
hierarchic
organisation
beauty
Fig.1. Demands posed upon cartographic visualisation and conditions
that should be fulfilled in order to meet these demands
The order of the above stated demands is not especially important, because some demands are
overlapping in part, and some are mutually opposed or set additional conditions for their successful
realisation.
2.3. Eye abilities and optical illusions
Before any demands are made it should be remembered that human eye is still the basic factor of
limitations in map visualisation. Namely, the abilities and limitation of the eye were the same for
the user of cartographic presentation in the past, as they are today and will be in the future
regardless of whether the presentation is in analogous or digital form on the screen.
Along with eye abilities and limitations one should also take care of optical illusions. These are
subjective comprehension of observers that make size, forms and colours of objects or presentations
different from those that are related with geometrically or physically measurable relations. Optical
illusions occur with all people having normal sight in the similar, but not completely equal way.
They are different if observed statically when the object is accurately fixed in the resting eye, or in
dynamic way of observation when an observer moves his eyes, as well as in observing a
cartographic presentation when various areas are compared at the same time.
The most frequent are geometrical and optical illusions of size, direction and inverted appearance
with the following being the most interesting for map visualisation:
 vertical stripes will be estimated longer than equally long horizontal stripes (Fig. 2a);
 two equally long parallel lines seem to be unequally long when at their ends there is an arrow
placed in various directions (Fig. 2b);
 horizontal lines are parallel, although it does not appear like under the influence of other lines
(Fig. 2c):
 two parallel lines intersected by one line placed obliquely to them and interrupted in the space
between these tow parallel lines cause the parts of intersection line to look moved away one
from another, and not creating a straight line (Fig. 2d);
 a circle surrounded once by smaller, and the other time larger circles appears in the first case
enlarged, i.e. in the other case reduced. The intensity of the illusion depends on the distance
between the circles (Fig. 2e).
There are also physiological illusions among which the following are present on the cartographic
presentation:
 unfilled shapes, e.g. squares or circles appear larger than those filled in (Fig. 2f). The way of
filling the shapes has got an important influence (hatching, lines parallel with the edge, colour),
as well as the width of the outline;
 black shapes on a white ground seem smaller than white shapes on a black ground (irridation).
Black circles on a white paper appear therefore more distant than shown by the results of
measurement (Fig. 2g). The effect is inverted with white circles on a black ground. The
phenomenon is weakened by the vignetting of the ground.
a)
b)
c)
d)
f)
e)
g)
h)
Fig. 1. Optical illusions
Among psychological illusions there is only one picked up in connection with the map
visualisation:
 equal grey tones (dot or line raster) appear different on different grounds, either lighter or darker
(Fig. 2h).
With regard to colours it is also necessary to take care of optical illusions because they are not
limited only to plain presentations. Namely, the colours in the field of cartographic presentation
appear because of mutual influences often completely different than in the legend where they are
arranged one next to the other. The coloured surface encircled by the other, larger coloured surface
looks as some feature on coloured ground that then appears to be continuing under that feature. The
contrasting effect of colours is revealed there. It is also significant that red, brown, blue, green and
other coloured lines appear essentially thinner than black lines of equal width. Optical illusions are
closely connected with the theory of shaping according to which individual symbols on
cartographic presentation are not isolated but understood as units of shaping some unit that should
be determined by the relation between the ground and individual presented symbols (Guyton 1978,
Schoppmeyer 1978, URL1).
3. Legibility
Legibility of cartographic presentations encompasses the recognition and interpretation of map
elements that are visualised by that presentation. Legibility is an unambiguous information transfer.
For the purpose of its realisation on a cartographic presentation some conditions should be fulfilled.
This is first of all the respect for minimum sizes beyond which some feature cannot be recognised
with respect to its shape and spreading quality. These are also the load of cartographic presentation
and the application of known shapes enabling better distinction on a cartographic presentation.
It should also be pointed out that the legibility of the cartographic presentation is remarkably
influenced by the illumination of the environment in which it is observed. Namely, in too dark,
insufficiently illuminated area the legibility is significantly reduced, and too much light has got the
same effect making the recognition difficult, even blinding an observer.
3.1. Minimum sizes
In the process of mapping the spatial object into the plane we obtain the picture of the object. Due
to the size of the object and to the needed reduction in mapping, it can be mapped as a basic
geometric and graphic element: dot, line and surface. In this process one should take care of
minimum sizes in order to distinguish single elements. It is a certain size beyond which some
graphic symbol or its part cannot be recognised on the cartographic presentation Frangeš 2000b).
The research of minimum sizes on the screen has been made by Malić (1998), Nelson (2000),
Brunner (2002) and others. Our own research has shown that starting with the normal distance of
observing the screen, the raster structure will not be noticeable any more with the resolution of 500
dot/inch. The exception is the lettering and thin lines that are a little bit declined from the vertical,
i.e. horizontal line, and the lines that because of its widths lie in the middle between one or two
pixels. If these lines are of shiny colours, then their indentedness is weakened which causes the
problem of bad distinction.
In presentations on the screen the types of lettering with serifs have proved themselves to
completely inappropriate, and aerial and universe have proved themselves to be the best types for
visualisation on the screen Even then some letters are to close to the size of 11 dots, and the usage
of italics makes it a bit better. In using various types of lettering on the screen one should be very
careful, because the choice of fat or italic lettering causes illegibility with some types, and it should
be supplemented by the usage of colours. Levelling up of the lettering along the selected curve that
is used most often for the description of water streams does not look legible on the screen even if
made with 12 dot size aerial.
The signs for the presentation on the screen should also be specially adapted. The usage of round
form signs is recommendable because they are distorted only on high resolution screens. The usage
of rectangular sign can also cause problems, as well as the usage of concentric circles and smaller
squares placed in larger squares. These signs are therefore applied only when they are big enough
and have larger spaces among individual lines, which requires more space, larger density and
replacement of other elements of the cartographic presentation contents.
However, the resolution of 20 and 21 inch monitors that are used mostly in digital cartography runs
up to optimum of 1024 x 768, 128 x 1024, 1600 x 1200 pixel and similar which means that the
space between two neighbouring red, green, i.e. blue phosphor point on the screen is from 0,22 to
0,28 mm. It has a rough effect in reading cartographic presentation on the screen, and the line
widths beyond these sizes cannot be achieved. For a square object on the screen there are at least 3
x 3 such points needed, i.e. 0,84 x 0,84 mm, unlike the minimum sizes that can be o,3 x 0,3 mm on
the paper. Such notions lead to the conclusion that every presentation on the screen must be
presented 2,5 to 3 times larger according to adequate paper so it could be legible (Frangeš 1998).
3.2. Load of cartographic presentation
Regarding the load of cartographic presentation we can, according to Frančula (2001) distinguish
among numeric, graphic and visual load. Numeric load of cartographic presentation is an average
number of the object at square centimetre of that presentation and it serves as a criterion for
comparing the density of contents. Graphic load of cartographic presentation is indicated by the
percentage of drawing quality on the surface of that presentation. The load for human eyes is not
influenced only by the number of objects and drawing quality, but also their map graphics, so the
concept of visual load is also introduced. Due to the optical illusions it is most often distinguished
with the simplest signs from their graphic load.
High graphic density contributes to visible obscuring of the overall cartographic presentation
because of visible obscuring of some surfaces. Bertin (1974) recommends here to have 5-10% of
the surface of that presentation printer occupied by black or some other dark colour.
Graphic load with names occupies a large part of the cartographic presentation as compared to the
rest of its contents, and it sometime goes over 50%. Therefore, the choice and shaping of names on
cartographic presentations are given special attention. Cartographic letters have their specific
properties, unlike the letters used for texts and other purposes. Namely, the letters in some book are
read almost always on a white ground, and cartographic letters are read on a variegated ground
making the cartographic presentation.
The density of names on cartographic presentation depends on the scale of cartographic
presentation, land character and specific characteristics of usage and intention of that presentation.
Cartographic presentations at larger scale give for a certain area much more names than those at
smaller scale. However, the number of names is not reduced proportionally with the reduction of
the surface, so with the scale reduction the name load is increased. Referring to land character the
density of names also shows certain regularities. Thus, on hilly and densely inhabited land there are
usually more names than on poorly inhabited high mountain landscape. It is wrong to expect equal
density of names on the entire surface of cartographic presentation because they are most often not
equally arranged in the field. On the other hand, he usage and intention of cartographic presentation
also affect its load with names, For example, the cartographic presentation intended for information
purposes contains much more names than the presentation of the same area and at the same scale
for school usage. Topographic maps contain more names than some thematic maps, but the thematic
map can contain a larger number of names connected with the theme they present (Frangeš 1998).
Still, the main tasks of a cartographer are to occupy with names as little space on cartographic
presentation as possible making them legible. The description placement is therefore especially
important for the legibility of cartographic presentation.
3.3. Known features
In other to achieve good legibility it necessary to provide the distinction of applied graphic elements
on a cartographic presentation. According to Spiess (1996) the most important general principal of
legibility is to make differences always clearly visible, and not only approximately guessed. In
order to achieve this distinction, graphic variables should be applied in a sensible way. The
distance, i.e. the number of symbols belonging to one family that we reliably recognize on a
cartographic presentation and clearly distinguish them from the others, is also important for
legibility.
There is one more factor influencing the legibility. Namely, known features are read more quickly
and more accurately than some new ones. It is therefore necessary to used more and more known
and accepted symbols on a cartographic presentation that are applied in almost unchanged or very
similar form.
It is generally known that some objects, some colours or their combination in adequate proportion
are more preferable than similar objects having other proportions, colours or their combinations.
We prefer the objects that we like more. Thus, in cartography some symbols and colours, and some
lettering groups and types of lettering for names on cartographic presentation are preferred more
than the others. The preferred elements of a cartographic presentation are read more quickly and
more accurately and their meaning is more easily remembered. The results of the research started by
Lovrić (1983) and continued among the students of the Faculty of Geodesy and Faculty of
Agronomy and among map users of Institute for Cartography at the Faculty of Geodesy show that
picture symbols are preferred, and among geometric signature the rounded ones more than those of
rectangular form (Frangeš 1998). Partial results of the research are presented in the table 1.
Table 1. Partial results of the research carried out to analyse the preferment of picture and geometric
symbols among the students of the Faculty of Geodesy and the Faculty of Agronomy, and among
the map users of the Institute of Cartography
Meaning of a symbol
Ship line
Tree
Travel agency
Oil site
Bar
Picture symbols
Geometric symbols
Percentage of test participants
for picture symbols
Percentage of test participants
for geometric symbols
Percentage of uncomitted test
participants
88
82
90
87
91
11
17
7
4
4
1
1
3
9
5
As known symbols are preferred with respect to legibility, thus the familiarity with language and
geography of the presented area allow the reading even when some letters are difficult to be
recognized.
5. Conclusion
Map user had the same eye abilities or limitation in the past, as they have today and will have in the
future, regardless of the fact whether the cartographic presentation is in analogous form or in digital
on the screen. Minimum sizes depend mostly on human sight, but also on the conditions of
observation that are usually not optimal. Referring to the legibility of cartographic presentation one
should always take care of the load of cartographic presentation. Namely, only the presentation with
adequate load can offer efficient map visualisation of some contents. It should also be accepted that
preferred elements of cartographic presentation can be read more quickly and more accurately and
their meaning is more easily remembered.
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URL 1: Illusions – http://www.anomalies-unlimited.com/Illusions