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Geomap: Research in Progress
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This is research in progress to develop graphic interface
that might serve the community studying processes and models in three dimensions
in a comparable way to the role that the GMT software (Wessel and Smith, 1991)
played in representation of two dimensional maps and other geophysical
parameters. At a recent NSF-sponsored Earthscope workshop in Snowbird, Utah,
there were several statements made with respect to the need for a software
appropriate for representation of models, their cross-sections and other
projections. It is recognized that particularly for 3-D studies, proper
visualization of models is essential. We are trying to respond to this need, in
part using the experience accumulated during the 25 years of seismic tomography
at Harvard (Dziewonski, 1975; Dziewonski et al., 1977), even though it did not
make much of an impact at first, since the color graphics did not exist yet. The
first tomographic papers using color maps and cross-sections (Dziewonski, 1985;
Woodhouse and Dziewonski, 1984) made their impact felt. In further studies we
have used 3-D cut-ins (Dziewonski and Woodhouse, 1997), and 3-D iso-surface
representations (Su et al., 1994). This proposal request support for trying to
bring together in an interactive package the existing and new tools for
visualization of 3-D models. Since most of our work has been focused on global
problems, the illustrations presented here will exemplify this aspect of our
graphics. However, incorporating changes that would be needed to include also
the regional mapping capabilities should not be difficult. We believe that its
funding is within the objectives of support for the pre-Earth scope activities.
Active participants in this project are Adam M. Dziewonski, Göran Ekström, and Wei-jia Su.
The concept of the Geomap
software package is a user friendly plotting program specifically designed for
use in the geoscience community. The design of this program is focused on the
ease of use, interactive execution, `what-you-see-is-what-you-get' principle,
and requirement of the least amount of learning. The user menu, tab choices,
check boxes and pop-up dialog boxes are intended to be intuitive, and will keep
reading of the user's manual to a minimum. The current program is written in C++
on the PC platform. The user interface part of the code is written in Microsoft
Visual C++, which provides a very good development environment. The major
portion of the code, however, is machine independent. With some effort, it
surely can be ported to the Unix or Linux system. The memory and disk space
usage are kept to minimum. An early version of this program has been used by
geoscientists at Harvard University for more than a decade. It allows generation
of many types of maps and graphs. We would like to develop this program further
and make it available to geoscience community at large. The intended main
functions of the program are:
1. Global mapping:
- plot grids, coastlines, plate boundaries
- plot symbols of different shape, size, color
- plot texts
- plot vectors
- plot symbols of different shape, size, color
- plot models (2D contours, cross-sections, cut-ins or
3-D iso-surfaces
- plot image data (topography, geoid, etc.)
2.
Regional mapping :
- plot grids, coastlines, plate boundaries
- plot symbols of different shape, size, color
- plot texts
- plot vectors
- plot symbols of different shape, size, color
- plot models (2D contours, cross-sections, cut-ins or 3-D iso-surfaces
- plot image data (topography, geoid, etc.)
3.
Projections:
- Mercator
- Aitoff
- Hemispheric projection
- Equidistant cylindrical
- Additional projections can be implemented as needed
4. Earth
models:
- 3-D spherical harmonic plus Chebyshev, Legendre polynomials or B-splines
for radial functions
- B-spline 3-D models
- spherical harmonic maps
- Block models
- Grid models; Geomap can plot any pre-calculated grid model; this allows
user plot model obtained using an arbitrary parameterization.
- Global models can be plotted within the stated latitude and longitude
limits
5.
Symbols:
- Fixed size symbols
- Variable size symbols
- Automatic scaling of symbols
- Capability to plot symbols on a cross-section
- Vectors
In
addition, the program has these features:
- 3-D cut-ins allow the user to view an Earth model
in a 3-D perspective on several (usually three) planes.
The advantage of this kind of visualization is that no distortion
of any sort is applied. Several parts of the Earth's mantle velocity (or other properties)
can be seen at the same time.
- Cross-sections can be made along the entire great circle
or within the specified limits, using either spherical
or Cartesian geometry.
- 3-D iso-surface plots go beyond the traditional visualization
through maps or slices. A more direct view of the Earth's interior is made available through three-dimensional visualization.
This method is becoming more and more popular with the development of different commercial or non-commercial
software packages. An algorithm of ray-casting has been used to shade the iso-surfaces. Perspective projection has been
used so that the Earth model appears more natural. The program is able to plot the earth model in
in either spherical or Cartesian coordinates.
- Interactive resizing of plots. Multiple maps can be plotted on one page. Each can be individually resized and moved even after
all the elements and models are already plotted.
- Interactive definition of color tables.
The color table can be interactively defined and saved for later use.
The user has option to use predefined color tables.
- Interactive incorporation of text; define text font, size, color,
click-drag to move text, click to delete etc.
- m Interactively change line width, color, line type, fill color
for any plot element and replot in seconds.
- Print to postscript.
We shall present here several examples of the application of Geomap to
plotting Earth models.
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Figure 1 is a screen-dump of the interactive display; post-script
files have significantly better resolution. Here, shear velocity
maps from model S12 (Su et al., 1994) and Grant (1997)
are compared at a depth of 100 km. Notice the menu choices on the right;
many of the available choices are hidden, but can be displayed and implemented
by pointing at the tab.
- To view larger picture click here
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Figure 2 (also a screen dump) shows a semi-circular cross-section through
model S12 along a great circle indicated by the blue line in the map inset.
The cross-section is defined by coordinates of two points: (0, 0)
and (0, 179), in this case. Notice implementation of the text option.
- To view larger picture click here
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Figure 3 is a
screen dump of a two 3-D cut-ins for the Caltech/Oxford model (Ritsema et al.,
1999).
- To view larger picture click here
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Figure 4 is an example of 3-D iso-surface imaging. This is a spherical
harmonic expansion of model s12 up to degree 6. The top
plot shows the slow anomaly at -0.4% value clearly
indicating the two mega-plumes. The bottom plot is for the -0.4% iso-surface
and, with some effort, the fast ring around the Pacific can be seen.
The angle of view, both horizontal and vertical can be changed
as well as the direction of illumination. The depth range is
from the Moho to CMB. Any depth range as well as the latitude and
longitude range van be selected. Both positive and
negative surfaces can be plotted in the same panel, but the
option shown allows the user to distinguish features
associated with the negative and positive velocity anomalies
more easily.
- To view larger picture click here
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Figure 5 is the spherical version of the iso-surface map. In this case
the spherical harmonic expansion is limited to degree 4 and both
negative and positive iso-surfaces are shown. The top 800 km
has been removed in order not to obscure the deep mantle features.
- To view larger picture click here
Last modified: Mon Jan 4
2002 19:05:55 EST