Geography Reference
In-Depth Information
After the aftershock removal procedure, the sample contains many related
events making up swarms. The swarms are removed in the same way as
aftershocks, except for the magnitude relation between the main and
subordinate events: in the case of swarms, the events following the one that
triggered the swarm process may have any magnitude, either larger or smaller
than the trigger event. Another point of difference is that the duration of the
seismic process for swarm sequences is specified by the user interactively
rather than being calculated (as in [29]) from the number of events as in the
case of aftershocks, because the time distribution of events in a swarm is
different from that in an aftershock sequences.
The geographic subsystem
is an important part of any GIS, because
proper visualization of geophysical data is a necessary prerequisite for correct
interpretation. Methods of digital mapping and appropriate GIS-technologies
allow developing a system for visualizing seismological data on the
cartographic basis to meet the user's requirements for plotting earthquake
catalogs, as well as the related information.
The GIS-EEDB subsystem of cartographical support uses
shaded-relief
raster images
for creating digital geographic maps. The 3D-effect is provided
by successive triangulations and by calculating the brightness of triangles.
First, the whole selected area is triangulated, with splitting each
rectangular grid cell at least into two triangles (maximum 16 triangles). The
number of triangles depends on the user-specified enlargement of the picture
size relative to the original data array. The program computes the
brightness
of
each
triangle
based on its orientation relative to the
light
source. In particular,
if the original array and the created image are of the same size, the light plane
is assumed to pass through the top left, top right and bottom left vertices of
each cell. If the illumination direction is different, the cells are split in a
different way as well. The parameters of illumination and the color scale are
specified by the user, and various shades of brightness are then obtained
therefrom.
The basic steps of the shading algorithm, designed by An. Marchuk, are as
follows:
1)
A surface is divided into a certain number of triangles calculated from
the ratio of the desired picture size and the dimension of the array
where the heights of all vertices with respect to a reference level are
known for each triangle.
2)
At each triangle, the vector product is found for vectors, parties to this
triangle.
