Geography Reference
In-Depth Information
3)
The resulting vector is normalized.
4)
The brightness of the triangle is calculated according to the angle of
the resulting normal vector to the light direction in the 3D scene. The
light source is usually taken with parallel rays.
5)
The computed brightness is used to obtain luminance gradation of the
corresponding color determined by the above-sealevel elevation of the
point and the user-specified color scale.
6)
The 2D projection is drawn according to the parameters of the picture
plane.
Similar algorithms known in the literature on 3D computer graphics are
Gouraud Rendering
(or
Gouraud shading
). To construct maps of different
scales, from global to local maps, an appropriate data array with an optimum
spatial resolution is automatically selected, with the quantity of triangular
elements in each grid cell fitting the selected map scale [31]. Currently there
are some global databanks representing the surface topography to different
resolutions [32], such as the best known GTOPO-30 and SRTM-90 ones, with
30 and 3 arc-second resolutions, respectively. The GTOPO-30 and SRTM-90
databanks are open-file digital elevation models developed by the U.S.
Geological Survey (USGS). The mapping program uses higher-resolution (90-
m) SRTM-90 data at the local level, when zooming to specific areas of interest
(in the territory of Russia). Then the vector and point layers, and explanatory
texts, are superposed onto a raster image. The vector technology is applied to
the level-by-level screen visualization of shorelines, rivers, national frontiers,
fractures and faults of different geometries (thrust, reverse, strike-slip, normal,
and oblique-slip faults). This technology aims at reducing the amount of saved
data by storing the coordinates of linear objects as vectors. The thickness of
lines in the image remains constant when objects are zoomed.
The point information is stored in simple text files and consists of such
layers as geophysical observation points and locations of volcanoes and
settlements, and can be easily populated with any other point data.
Full description of layers in the geographic database is given in Table 1.
The system is open to adding vector, raster, and point digital geographic data,
which can be converted into the system format.
Additional options developed for the geographical subsystem include:
a)
different ways of visualizing (with or without animation) earthquakes
in geographic maps and cross sections (Figure 8);
