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Exercise Title:
Converting Grids and Motion Vectors to Google Earth Format, using Panoply
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Abstract: In this
exercise you'll learn how to create regional visual products for use in
Google Earth. The global source files include a pair of NetCDF analysis of sea surface height and
geostrophic currents. The
beauty of using Panoply for these extractions is that you can select
precise geographic regions with an easy interface. Also, the
creation of the current arrows from the U and V component grids is
extraordinarily easy in Panoply.
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Please note that the global
files below are large, so you might want to download them a day early to
save waiting time.
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Theoretically, HDF files could be included
here, but most online global product sources do not provide HDF files
fully compatible with Panoply.
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Preliminary Reading (in
OceanTeacher, unless otherwise indicated):
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Required Software:
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Other Resources:
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Author: Murray
Brown
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Version: June 2011
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IMPORTANT NEW INFORMATION: Recently
the Saga GIS software has been shown to deal quite adequately with most
NetCDF (NC) data grids (and possibly also GRIB files). This should be
viewed as a useful addition to the rapid, direct NC analysis and display
capabilities of Panoply (see below). If you are also interested in
this route for your data, then please check
Viewing and Managing
Well-Formed NetCDF Grids in Saga. |
| 1. Download the
above 2 KML files to the folder PRODUCTS > SAGA > VECTORS. Download
the NetCDF data files to DATA > OCEAN. |
| 2. Unzip the 2 gz
files, in place, to reveal their NetCDF contents |
| 3. Run Panoply,
and open the file
nrt_global_merged_madt_h_20110526_20110526_20110526.nc.
Notice that all the data objects within the NC file are listed. But
Panoply can only visualize objects that have the annotation [lon][lat] in
their TYPE column. In this case only one object complies, and that is
the sea surface height, or H. |
| 4. Select the sea surface
height (H) and click on CREATE PLOT to see this graphic in a new window,
entitled H. |
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| 5. Click near the bottom of
the graphic to reveal a set of 5 control tabs that allow you to modify the
graphic. |
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| 5. Select the MAP tab, and
make these choices to make the graticule invisible. |
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| 4. Select the SCALE tab to
make these choices to make the H data invisible. |
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| 5. Now you have a nice,
generic white globe, with vaguely correct continental boundaries.
Notice that the arctic is shown as solid land, major lakes and inland seas
are not shown (including the mediterranean!). But this will serve our
purposes right now. On your own time, you can search for a better global
figure, if necessary for your own projects. |
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| 6. Select FILE >
EXPORT KMZ and navigate to the folder PRODUCTS >
PANOPLY to save the graphic with the name globe_white_low_res_countries.kmz |
| 7. In Windows Explorer, find
the new KMZ file and double-click on it to open it in Google Earth. |
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| 8. Select FILE > OPEN to add
the world borders KML version, from above. |
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| 9. Also add the global
10-degree graticule. |
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| 10. If you want to change the
color of the graticule, then you have to it carefully. First, open the
graticule folder in the list of temporary places. You'll find inside a
single vector object named GRATICULE. Right-click on this object, and
select PROPERTIES. NOTE: We aren't sure, due to the skimpy
documentation, but apparently when you're editing PROPERTIES it is best to
do it at the data object level and not at the folder level. |
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| 11. This edit window opens.
Click on STYLE, COLOR. |
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| 12. This window states that
the GRATICULE object has sub-objects (i.e. the individual lines), so you
must decide if they will also be edited here. Click SHARE STYLE to confirm
that changes here will also apply to sub-objects. |
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| 13. We are not sure that it
is required, but we recommend that you change the OPACITY of the object to
50% here. Then click on the white box beside COLOR. |
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| 14. In this color-selection
page, click on BLACK and then select OK. |
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| 15. The graticule lines are
now black. |
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| 16. Select FILE >
SAVE IMAGE AS and then save this basic globe with the name
globe_countries_graticule_basic.kmz |
| 17. Return to the SCALE tab
at the bottom of the page, and change the COLOR TABLE to the PANOPLY GCT
palette. |
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| 18. Now you should see
approximately the same global figure as before. |
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| 19. Click near the bottom
of the graphic to activate the tabs menu, then select the MAP tab. |
20. Within the "projections"
is actually the function that allows you to subset the data.
- Select PROJECTION = EQUIRECTANGULAR (REGIONAL)
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21. And here's how you select
the region:
- For CENTER ON LON, enter the longitude exactly midway in your area of
interest; use the -180 to + 180 convention
- For CENTER ON LAT, enter the latitude exactly midway in your area of
interest; use the -90 to +90 convention
- For WIDTH use the exact longitudinal width of your area of interest
- For HEIGHT use the exact latitudinal height of your area of interest
- Don't click FIX PROPORTIONS, or Panoply might make automatic changes
in your values
You might have to repeat one of these settings to get Panoly to complete
the calculations and actually display the desired area. |
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| 22. And here is the sea
surface height, H, for Liberia. |
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| 23. Select FILE > EXPORT KMZ
and then save this image with the name
Grid_0001 in nrt_global_merged_madt_h_2011052_aviso.kmz |
| 24. In Google Earth, use FILE
> OPEN to show the new KMZ image in place. Unfortunately the Liberia area
isn't very dynamic, in terms of H, but you can see the obvious utility of
this method for other areas. |
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| 25. In addition
to the display of a single grid, as seen above, Panoply can work with 2
grids simultaneously. This is particularly important to us, because
one of the 2-grid functions is the display of motion vectors from U and V
component grids. We'll learn how to do this in the next few panels. |
| 26. You can close the NetCDF
H grid, and open the NetCDF UV grid. Select and double-click on the U
component grid to open it. |
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| 27. Here is the U grid.
You can see the typical strong east-west bands near the equator. |
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| 28. Now on the SOURCES page,
select the V grid. |
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| 29. Right-click on the CREATE
PLOT icon, and select COMBINE IN XXXX function (referring to the previously
loaded U portion of the grid). |
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| 30. Here you can see that the
title below the graphic refers to both the U and V grids, indicating
successful loading of both grids. |
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| 31. Click near the bottom of
the graphic to open the settings tabs, and select ARRAY(S). Then open
the menu of PLOT MAPS to select VECTOR MAGNITUDE. |
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| 32. The vector arrows
immediately appear, but typically they are too small or too large. |
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| 33. Open the CONTOURS & VECTORS tab
and adjust the SCALE LENGTH. Smaller values make larger arrows; larger
values make smaller arrows. |
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| 34. Here is the result from
setting the SCALE LENGTH to 25. You might have to fiddle with another
setting to get this change to become visible. |
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| 35. Now, just as you did in
Panel 21 above, display only the Liberia area of interest. Notice that the
arrows are much close together, indicating that automatic thinning of the
arrows was in effect in Panel 34 above. |
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| 36. Now, remove the color
palette, as you did in Panel 4 above. |
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| 37. Select FILE >
EXPORT KMZ and navigate to PRODUCTS > PANOPLY to save the vectors as
Grid_000_in_nrt_global_merged_madt_uv_201105_aviso.kmz |
| 38. In Google Earth, use FILE
> OPEN to visualize this new KMZ file. It is an image, so you cannot
see the scalar H grid below it. |
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| 39. To "see through" the
vectors image, right-click on the vectors object and select PROPERTIES.
Then set the TRANSPARENCY to about 50%, as you see here. |
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| 40. Although this is not an
ideal solution, as the use of a real vector file for the arrows would
provided, it is acceptable. |
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| 41. So the above
display options are available and useful for NetCDF grids. GRID files
could also be displayed, using exactly the same methods. |
