Visualization of Gravity Field Models and their Differences
This is an interactive Java Applet to visualize the Geoid. You have many options
to load other models, change grid resolution, viewpoint, zoom, light source parameters ...
You must enable Java for your browser to use it.
Netscape 6.xx, Mozilla, MS Internet Explorer are recommended browsers.
Older versions of Netscape will work, but much slower, because they do not have installed a
JIT-compiler.
If you don't see it correctly on your browser, please inform us per Email
(include the used browser version and operating system).
Please have some seconds patience for loading the applet. See below the applet
for usage info.
Tip: The most impressive details are visible for highest grid resolutions < 0.5°, when you use the
Anomaly functional instead of Geoid for "egm2008" (perhaps
with a reduced boost factor of 5000).
Usage
Most buttons (or the button-labels) have a help option: if you move the mouse cursor
over it, a small popup will explain its function.
The Earth can be rotated either with mouse cursor dragging (sometimes a bit slow)
or the keyboard actions (see below).
Button panel
- You can select all Models from the table for display with the button top left
Model selection. Then a new frame pops up with options for new server queries:
- Functional: you can toggle between Geoid and
Gravity Anomaly for visualization
- Select Model and Select Subtrahend:
By default the Ellipsoid is used as subtrahend to the model. You may also
inspect the differences between two models, if you select another subtrahend.
Please note, that the colormap scale for undulations is automatically adopted to give
a best view. You may change it however, as described below.
- lm-Range: Range of spherical harmonics degree "l": Set the (lmin,
lmax) pair for model queries.
This means, only the spherical harmonics of the intervall [lmin <= l <= lmax]
will be used for grid computation.
('lmax' corresponds to the spatial resolution of the model). Please note, that
the models have different maximum resolutions (lmax).
- Grid: Sets the grid resolution. The smaller this value, the more details
become visible, but also the slower will be the display. For displaying grids < 0.5°
you should select a model with an appropriate lmax > 360, e.g. egm2008
Depending on your OS-/Browser-/Java-version such very fine resolutions may require your configuration actions
to supply sufficient Java heap space, see Troubleshooting below.
See Performance tips for speed considerations.
- Then press the button Show it! to actually invoke the query.
- The Button Headers on the fly toggles the display mode of model headers at the right side.
If 'on' they will be shown 'on the fly' without clicking,
- zoom [+], [-] With these two button you can zoom and unzoom your region of interest.
Before you do this, move it into the center of view, as described below.
- boost Sets the 'exaggeration factor' i.e. 'hboost = 10000' means
that 1m of geoid undulation is displayed as 10km high mountain.
- grid The same as above Grid.
- smooth Toggles between two display modes. If it is 'off' you will see
the discretizing triangles of the grid, else the colors inside the triangles are
interpolated.
- rotate With this toggle button you can let the Earth rotate around its polar-axis.
- Light Pops up a new window to set some light control variables (position,
focus parameter 'gamma').
- new Frame Pops up a new window with the same Applet. This has the additional
feature, to be resizable (with your usual window-manager handles). With this window you can also
compare two models or views, or make screen shots from it. Both frames are controlled independly,
but the memory usage adds up, of course.
Keyboard actions
Please note, that the Applet must have the focus for keyboard actions!
- The arrow keys (left, right, up, down) can be used to move the earth.
- Pressing the up- and down- arrow-keys while holding "Ctrl", will change the colormap
scale-factor (always by a factor of 2).
This is especially useful, if you inspect model differences which
have usually small amplitudes.
- The +,- or <, > keys can be used for zooming/unzooming.
(Some browsers seem to ignore these keys sometimes, however the +/- keys on the numeric
keypad should always work)
- The p-key toggles display of plate-boundaries.
- There are some other keys (s,o,c,h,b) to select other background images.
Performance tips
If you want to experiment with certain display variables (as
light, model, lmax, zoom, viewpoint, ...)
it is probably the best way, to first use a small grid-resolution (2° or 4°).
Also 'rotating' and interactive motions will work more fluently with lower resolutions.
After finding out the best display, you can set the resolution to the desired value.
Doubling the grid size will produce a nearly four times increased update time (and
memory requirements).
To use very fine resolutions < 0.5°, you should close all other browser pages,
to have the maximum available Java heap space
(e.g. a grid resolution of 0.2° needs about 540 Mb memory).
Troubleshooting
If you experience some odd behaviour of your applet (no reaction to buttons, no image updating, ...)
you can inspect what is going wrong, if you have a look at the Java Console of your
browser. For Netscape 6.xx you open it with the buttons Tasks -> Tools -> Java Console.
There the main actions and errors are logged and you have the possibility to control the
Java Virtual Machine (JVM).
One common error, which freezes the applet is the "OutOfMemoryError". It occurs
if the pre-allocated heap memory for your JVM is exhausted, usually already in the
step "generating sphere points". This can happen for grids < 0.5°, because
some older browsers only allocate 64Mb for Java.
(You can see the error message in the Java Console window of your browser.)
You can circumvent this failure e.g for Windows-PCs, if you change the default value in the browser preferences under
"Java" -> "runtime environment" ->
"runtime parameter -Xmx500m " to allocate eg. 500Mb heap space.
If you see only a blue sphere in the window but no geoid, then there is a network/server problem
(eg. your local firewall blocks the connection).
Please inform us about this, we try to help.
Technical notes
The Applet is loaded from the server host "icgem.gfz-potsdam.de" with "Potato.jar".
It requires a running potato-server listening on TCP-port 1891.
After start of the applet, it queries from the server the list of available models,
which build up the model choice of the
applet. Then it serves model queries in form of grid values.
The applet needs quite a lot of memory, depending on grid-resolution. In the default resolution
of 1° x 1° it occupies approximately 16 Mb,
resolution and about 60 Mb for 0.5° x 0.5°.