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mathematical defined, three-dimensional high-realistic
landscape presentation together with all nature and artificial
objects and phenomenon. These kinds of presentations allow
users to observe the presentation exactly on the same way like
observe reality (Bandrova,1998).
Designing of a 3D map depends on the expected use and users’
needs. For easier understanding, cartographic model contents
can be divided into three groups: main content, secondary
content and additional content. Main content consists of objects
and phenomena that have the essential importance for the users.
The first of all there is relief presentation, which is the
fundamental part of every 3D model. Relief is determined as a
space surface, defined as a triangular mesh (TIN) or rectangular
mesh (DEM). There are also other objects of main importance.
They should be presented accented, clearly visible and
distinctive. A lot of authors suggest, that they should appear as
similar to the real ones. But here we can come into conflict. If
we want that 3D model presentation have communication
function of 2D maps we have to include symbolization system.
Some objects are too similar in reality and they do not “jump”
from the surroundings, as they should according to their
importance to the users. The other reason is the need of using
different levels of detail in different observation distance in
perspective view. There are also technical reasons: dynamic
presentations can be faster and visibility of a symbol in limited
resolution (screen or paper print from printer) is better. Users
are familiar with contemporary 2D maps and symbols on 3D
maps should follow those on 2D maps. They shouldn’t be too
geometric because such signs don’t attract users’ attention
(Rojc,1986). In the group of secondary content we can find
objects and phenomena that give the complete impression of
landscape, but they don’t have any significant importance for
users. Land cover, hydrology, less important buildings and other
structures, administrative boundaries could be recognized as
secondary content. They should be presented by quite simplified
and unaccented symbols that lead to less complex and faster
visualization. Reality can be very effectively replaced by
fractals, textures and images, e.g. as the presentation of forest or
rock area. With increased observation distance presentation of
secondary content can be efficiently presented as a color surface
covering DTM. The last group, called additional content
consists of data, which are not regular part of presentations.
Additional content is presented exclusively on users demand. In
fact it is an addition explanation or illustration of particular
objects of main content. It can be just a record of horizontal and
vertical position of an object or a complete and huge detailed
description of it. Several types are possible: written text,
speaking, or even both which leads a much more efficient
memorizing process; hyperlink, picture, movie, specific voice.
The advantage of content organized this way is that even very
huge amounts of additional content data have no influence on
visualization speed.
Every object in reality is a 3D body. Some of them have one or
two predominant dimensions and this influence on
cartographers chose how to present them on the map. In 2D
maps design we use point, line and area symbols. Using six
Bertin’s variables (color, shape, texture, direction, size and
brightness) various sets of different map symbols can be made
(Bertin, 1974). In 3D presentations basic graphical elements
(point, line, area — polygon) are supplemented with volumetric
3D object. All landscape objects and phenomena are placed on
the terrain model, presented with appropriate symbol.
Geometric point 3D symbols are suitable for presenting mostly
man-made point objects, like buildings, churches, monuments,
199
etc (figure la). Such symbol consists of simple geometric
bodies. Its size and shape is fixed for selected object type and
selected level of detail. Some point-like objects, especially
natural-made, like trees, bushes, waterfalls etc. can be better
presented with typical realistic (real-like) point 3D symbols
(figure 1b).
d)
Figure 2. a) Geometric and b) realistic point 3D symbol, c) line
3D symbol and d) surface in a 3D space.
Objects with one predominant dimension and mostly fixed other
dimensions, like roads, railways, power lines, oil pipelines,
smaller watercourses, fences etc. are usually presented as a line
3D symbols (figure 1c). Area 3D symbols can be used for
objects where only one dimension (often height) is mostly fixed
(figure 1d). Such objects can be forest, orchard, or even blanket
of snow. Area 3D symbols can be constructed as a filled solid
body or, most common, as a mesh of points with different
density. The last options are volumetric 3D symbols, where
every single object is presented with its exact real dimensions
along all three coordinate axes. Typical examples are large and
important (mostly man-made) objects and also water bodies,
like lakes, sea and large rivers.
Using of Bertin's variables in 3D presentations is slightly
different like in 2D maps. In perspective views close objects are
clearly visible and bigger like distant ones. Therefore we cannot
use the variation of Bertin's variations only for distinguish
different object types. Every object type should have different
appearances according to the distance between observation
point and an object. Since we cannot perform linear transition
we have to decide to limited number of discrete steps — different
map symbols for the same object type; similar as map symbols
for 2D maps at different scales. Usually we call them levels of
detail (LOD). Figure 2 shows map symbol for a church in four
levels of detail. Symbols for close objects are quite realistic
looked, bright, made with irregular shapes and detailed. With
increased distance symbols became simplified, darker, less
detailed and geometric (Petrovic, 2001).
Figure 3. 3D cartographic symbol for church in four LOD.
