Image Browsing/Harvesting in ncWMS Servers: GODIVA2
Abstract: In this
exercise you'll be introduced to ncWMS, the NetCDF "extension" to Web
Mapping Services (WMS). Among other enhancements, ncWMS provides
automatic color palette mapping to the grid value range, eliminating a
tedious part of WMS development. The demonstration resources
visited here only deliver images, which should be cured in subsequent
Preliminary Reading (in
OceanTeacher, unless otherwise indicated):
ncWMS Overview - OSGeo documentation
- "ncWMS is a
Web Map Service for geospatial data that
are stored in
NetCDF files. The intention is to create
a WMS that requires minimal configuration: the source data files should
already contain most of the necessary metadata." From the
University of Reading website, the center for ncWMS development.
For example, the service maps stock color tables against the parameter
value ranges, a feature not available in standard WMS.
GODIVA2 Demonstration Page - Main demo site, University of
Coastal Ocean Observing Regional Association. This regional-scale
organization and system is one of the best global examples of
cooperation and synthesis. The scope of products is startling,
compared to the state of the art only 10 years ago.
|1. Although we will focus on
the SECOORA clone, here is the main page for the GODIVA2 mother site.
Note the typical menu of product groups on the left, and a map area with
|2. Here's a closer look
at the data menu, with one item opened to show typical parameters.
Notice that the item AUTO-ZOOM ON SELECT is checked.
|3. Here we've selected SEA
WATER TEMPERATURE from the list above, and a map is drawn. The effect
of the AUTO-ZOOM control is obvious, because GODIVA2 has zoomed into the
actual data area, the Irish Sea.
|4. This gives us
a basic idea of how GODIVA2 works, so we'll move on to the SECOORA site, a
regional organization of ocean observing systems in the southeastern USA.
It includes the areas called the South Atlantic Bight and the eastern Gulf
of Mexico (SABGOM).
|5. Open the link to SECOORA
and this is the data menu.
Items marked with (ERROR) have been polled by GODIVA2 and there is
no response at this time, so you cannot obtain these data. We could
look at the SABGOM products, but the global material is more interesting to
|6. Here we've opened the menu
items for NOAA/NCEP GFS MODEL, just to see what's here.
This model was also used in
MDL Exercise 9.13.
|7. Here we're looking at the
SABGOM data products. This is a premier example of multi-agency
cooperation, and state-of-the-art modeling. Please take the time to
come back here and investigate these products.
|8. Here we've opened the menu
items for NOAA WAVE WATCH III, to see what's here. We're going to
concentrate on this group for the rest of this exercise.
This model was also used in
MDL Exercise 9.3.
|9. Here you should just click
on SEA SURFACE WAVE SIGNIFICANT HEIGHT. After a short wait, a map
similar to this should appear; try to find the exact same date. But
make sure to find a date with at least one large dark area (meaning high
|10. Here's a closer look at
the items above the map.
- HEIGHT - Elevation or depth of the visible data product; only zero
for waves because they are at the sea surface
- FIRST FRAME/LAST FRAME - Dates and times to define an animation.
If you begin to use these, then other items appear, related to the
animation. You can try this, but be warned that it seems to be
- FIT LAYER - Same as AUTO-ZOOM
- MORE INFORMATION - Takes you to a complete THREDDS description page
for the data; try this one to see what it includes. ncWMS is set
up to run on THREDDS servers.
|11. Here's a look at the
navigation controls above the map. You can't
draw a zoom box with GODIVA2, so be ready to use just these controls.
The globe icon returns you to global scale. The hand icon turns on the
usual PAN function of your cursor. [The little 4-dot icon beside PAN
is explained below.]
|12. Here is the color scale.
GODIVA2 has attempted to select the proper scale.
- You can manually insert values for the uppermost and lowermost value
limits, or set them for, as an example, whole numbers
- LINEAR - Change between linear color scale and logarithmic scale.
Log scale is better for mainly zero or near-zero values tapering off
toward a small number of higher values, e.g. chlorophyll or sediment
- AUTO - Forces GODIVA2 to examine the data actually being viewed
(possibly not the global values) and reset the minimum and maximum
accordingly. Usually works, but you might have to adjust either of
the limit values to eliminate any black areas (i.e. values outside the
- LOCK - Locks or unlocks the color scale
|13. But where does the color
scale come from? Click on the color scale you see in the previous
panel, and these choices appear. The publisher has selected the basic
palettes, but GODIVA2 matches them to the value ranges. This dynamic
matching is not available in standard WMS.
NOTE: An apparent attempt to meld this capability into WMS is the
publication of multiple different data products, each with its own static
palette. You can see this in Panel 12 of
MDL Exercise 8.9,
which appears to be a selectable palette, but is actually
a set of 10 different data products.
|14. And if you want to change
the base map, click on the + to the left of the
color table, and these possibilities appear.
- Try them with a given data layer to see what you like
- The POLAR items change the projection
- The OVERLAYS item turns off the data layer so you see only the map
|15. So move in on the largest
storm feature on the map, indicated by the high waves. This will give
you a chance to navigate without an actual zoom box.
When you have this
map, you'll see perhaps that the highest values are not colored, but are
black. This indicates they are just beyond the top value on the
|16. You can manually increase
the top value, a little at a time, as you see here. Not very much is
|17. Just keep checking the
map to see what value just removes the black area. In this case the
increase was only 0.14 m. This results in the wave height map.
|18. A little tool we ignored
above, but can look at now, is the icon with the four connected dots.
This lets you select a pathway across the image where you can obtain wave
|19. Click on the little icon
to activate the function.
- Then click somewhere near a major data feature, such as the coast lf
Greenland, northwest of the big wave area.
- Drag your cursor over the area of interest. You can make only
one long track, or multiple "legs" with cursor clicks.
- When you come to the end you want, double-click to stop the
|20. This image of the wave
height values encountered along the track appears. The image can be
saved from this small window with a right-click; this image was saved in
that way. This is a wonderfully fast way to get useful products
directly from the data.
NOTE: The SECOORA folks are advised to
change the graph line to bold black so it is more easily visible.
|21. What else can we do with
the data map? Find these functions just below the map:
- TEST IMAGE - Full globe image without any labels or text
- GOOGLE EARTH - See next panel
- SCREENSHOT - Very clean map image with only necessary labels; best
- OPACITY - 3 choices; useful if metocean features are over land
- PERMALINK - URL that opens this exact same map in any good browser
- EMAIL - Automatically creates an email that sends the PERMALINK to
|22. Here, for
example, is the PERMALINK. It is, in effect, the formula to make this
Click on it and see what happens.
|23. And here we've click on
the Google Earth link to see how it looks. You can also save/send the
KML link if you want, just like the PERMALINK above.
Finally, this author had believed that the ncWMS functionality would also
include data saving capabilities, which would obviously be possible due to
the use of THREDDS as the host server. This is not apparent here, yet,
but it is hoped it will be included later.