Grid Products with Saga
Abstract: In this
exercise you'll learn how to make grid products might be needed for common
scientific purposes, including further study at your desk, submission of
reports, online products and typical hardcopy publication requirements. No attempt is made here to optimize the color palettes, so only a simple
rainbow palette is used here. The companion GIS exercise following this one addresses more complex palette issues.
Products for Additional Work
Products for Journals &
Products for Geographic Information Systems
(GIS) - Images or Data
Products for Google Earth
Products for Web Mapping Systems (WMS)
Preliminary Reading (in
OceanTeacher, unless otherwise indicated):
Other Resources (Saga grids):
Discussed in this Exercise:
|Purpose of Products
|Work in Saga
||All grids and vectors
||Expect to return to major datasets forever,
and take measures to do this efficiently with project compilations
|Journals & Dissertations
||Get width from editor/advisor; never more
than 600 pixels for emails
|Geographic Information Systems - Image
||Image raster; not data
|Geographic Information Systems - Data
(ESRI ASCII grid) or FLT (ESRI binary grid)
(for FLT version)
||Data raster; displays as image
||Zip both to *.ZIP, then change extension to
|Web Mapping Systems (WMS) -
and other Open Geospatial Consortium Web Services (OWS)
||GeoTIFF raster (*.TIF)
containing any desired numeric grid
||Appears to be "noise" when viewed by humans
|2. This exercise builds
directly on the Masking Methods products. Just reopen the SPRJ project
file you made there. We will clean out any unnecessary files, so you
have only what you need for analyses and publications. Preliminary
working files are not needed now, providing they have been saved with good
|3. WORK IN SAGA. Here are
the "core" files from that exercise. You can take a few minutes to
check over your project, and delete any objects not included here.
You probably have short versions of
these names, for example jfm_0_masked. That's OK. Take a
few minutes to rename the grids as you see here. Your own files
probably won't be in exactly this same order. All the information you
need is in this panel and the next 2, if you have trouble.
|4. Here are the maps from the project,
arranged with the 0-5 m data on top, and the JFM data on the left.
wonder why they look so different, then you really do need to check the
for Depth- or Height-Limited Grids in Saga
5. Just to help you get oriented, here again are the
names of the grids, arranged in the same pattern as above.
Why take all this trouble with these files? You are a
professional marine scientist, and your life work involves building and
maintaining quality databases and data products. A primary tool will
always be GIS, so you must embrace organizing, cleaning, identifying and
properly saving data in logical ways. Master these steps and you'll go
|6. At the top of the left panel, select
MAPS > TREE to see this list of the maps you've made already in this
project. If you have extra maps, carefully delete them so they match
up with this short list.
|7. Now that your project has
been cut down to the objects you really need, then you can select FILE >
PROJECT and save it again as
This cleanup process should become a habit...it will really help you in the
|8. Now select MAPS > THUMBNAILS at the
top of the left panel. You'll see small versions of all current maps.
It should be the same group as the TREE list.*-
Right-click on a
map, for instance the first one, and select SAVE AS IMAGE.
NOTE: You must do this from
MAP > THUMBNAILS!!
|9. Navigate to PRODUCTS >
SAGA > GEOIMAGES and:
- For FILE NAME, enter
- For SAVE AS TYPE, select PNG
Then click SAVE.
|10. Now you'll see these
- Set the WIDTH and HEIGHT of the saved image. Suit yourself or comply with a
rules. Currently we
recommend 400 X 300.
- Set FRAME WIDTH to zero (but make some experiments later to see if you
like that setting)
- Check SAVE GEOREFERENCE (WORLD FILE)
- This gives you the auxiliary file for image location in many GIS
- Check SAVE KML FILE- Gives you the auxiliary file for imag
location in Google Earth
- Check LEGEND > SAVE - Saves an image of the colored value
- Set ZOOM (the legend size) to 1 (but make some experiments later to
see if you like that setting)
Now click OK to make the files.
|11. If you check
the GEOIMAGES folder, you'll find these new files.
- Shown below in next panel
|12. JOURNALS AND
DISSERTATIONS. Here is the PNG file for
the JFM dataset,
displayed in the graphics editor IrfanView, to verify that it duplicates the Saga
Don't forget that most journal editors or university professors
have favorite image sizes/limits. Apply these above in Panel 10
INFORMATION SYSTEMS - IMAGE OPTION. Here is the content of
the PGW file. It is just a simple ASCII world file that sets the X- and Y-cell
dimensions (1st and 4th lines, respectively) and bottom-left location (5th
and 6th lines) of the image.
The 2nd and 3rd lines have to do with images
that are rotated, one way or the other, which we are not doing here.
All world files
have exactly the same format, so they can be used with any image made from
the original PNG, as long as the physical dimensions have not been changed.
NOTE: The Y-cell dimension is negative, because the first corner in
the array is the northwest corner, even though the grid location is
specified by the southwest corner. This is a cute eccentricity of the
ESRI ASCII grid format.
|14. Now to check the validity of the
GIS-ready image (PNG), let's load it into Saga. The software will
automatically look for a PGW file, and use it if found.
|15. In Saga select MODULES > IMPORT IMAGE.
|16. Make these choices:
- For the IMAGE FILE, find and select
the new PNG file you just made.
- For OPTIONS, select ENFORCE TRUE COLOR.
Then click OK.
|17. You should be able to find and open this new
object with ADD TO MAP > NEW.
Notice that a georeferenced image has the
correct longitudes and latitudes (along the bottom margin of the window),
but the "Z" values are Red, Green and Blue values, not temperatures.
This is the nature of georeferenced images imported into GIS:
No usable parameter values, just
|18. GEOGRAPHIC INFORMATION SYSTEMS -
DATA OPTION. When you have a GIS, then this is probably the most
common method, because it maintains the data values. The ESRI ASCII format is extremely widely used in many
systems, and it is easy to create or edit.
Select TOOLS > IMPORT/EXPORT
GRIDS > EXPORT ESRI ARC/INFO GRID.
|19. Make these choices:
- GRID SYSTEM - The grid system for all your 1-degree products
- GRID - Select one of the grids to export, for example the JFM 0-m
As soon as you select this grid to export, then you will be asked (in the
panel below) where to export and what name to use.
|20. Navigate to the folder PRODUCTS >
SAGA > GRIDS and make these choices:
- For FILE NAME, enter the name of the grid. Saga will add an
- For SAVE AS TYPE, you should select ESRI ARC/INFO ASCII GRIDS
NOTE: The alternative ESRI format is FLT binary grids, which
require HDR files that explain their internal structure. An HDR file
is exactly the same as the first 6 lines in an ESRI ASC file. Only
consider FLT files for extremely large rasters.
Click SAVE to continue.
|21. Now make these choices:
- FORMAT - Select ASCII
- GEO-REFERENCE - Select CENTER. This means that the grid
cells in our products will be specified by their center coordinates.
If our grids have whole-number outer boundaries, this means the cells
are centered 1/2 cell inwards from the edges. This is most common
with satellite data, but not with terrestrial data such as the datasets
commonly used by Saga experts, who seem to prefer CORNER.
- ASCII PRECISION - 4 seems to be ok here for the number of
decimal values; analyze your own data for the best choice
- ASCII DECIMAL SEPARATOR - POINT, although COMMA probably also
Then click OK.
|22. The ASCII file is created, and it can
be viewed in any good editor.
Whenever you make or request ASC files, always examine them to make sure you
are getting what you expected.
|23. Now, let's check the validity of the
exported grid by importing it. Select TOOLS > IMPORT/EXPORT GRIDS
> IMPORT ESRI ARC/INFO GRID, as you see here. Make these choices,
and click OK.
|24. And here is the grid. We don't
have time to work with the colors, but you can see the bad data stripe down
|25. So that covers the
majority of the common data product uses. Now we'll look at more
exotic uses, Google Earth maps and dynamic Web Map Servers (and their
|26. GOOGLE EARTH. Here is the ASCI
text in the KML file you made above in Panel 10. It's
very similar in function to a world file, but it contains the name of the
image it "points to" in the HREF line marked in yellow.
similarities to and differences from the world file. There are no cell
dimensions, which the software calculates from the outer dimensions, marked
<?xml version="1.0" encoding="UTF-8"?>
<name>Maps exported from SAGA</name>
<description>System for Automated Geoscientific Analyses -
System</td><td>Geographic Coordinate System:
[+proj=longlat +a=6378137.000000 +b=6356752.314245 +no_defs]</td></tr></table></description>
|27. Here is the KML file,
opened in Google Earth. Many people fall in love with KML displays,
especially the sexy spinning globe. But, you should not use Google
- Your image covers much of a whole ocean, or
- It displays something that clearly makes more sense when viewed in
Obviously this image does not meet those criteria, and your
teacher doesn't recommend this particular display.
The KML file and the PNG file can be zipped together with WinZip to create a
*.ZIP file. [You can use WinRAR, but the format must be ".ZIP".]
Then manually change the filename extension from ZIP to KMZ and you can open
the combined files in Google Earth also. Many workers prefer this
practice, because it insures that the two (or more!) files don't become
|29. And finally, here is the grid legend, in PNG format .
It can be used with the PNG image (which lacks actual parameter values) or
with the KML image (see
Legends for Grids in Google Earth)
|30. Now we're
going to create a GeoTIFF product that is strictly numerical, and not for
visualizing. Numerical grids are often also called rasters.
These grids are needed by web mapping systems (WMS) that can
subset the data grids, and a host of related systems that are promoted by
the Open Geospatial Consortium.
- Google Earth itself uses only the PNG + KML method.
- ESRI software and the Mapserver 4 Windows can use either PNG + PGW
(see above) or
GeoTIFF (raster) + TFW (see next).
MODULES > Import/Export - GDAL/OGR > GDAL: Export Raster to GeoTIFF
|32. For GRID SYSTEM, select
the system for the SST grids.
|33. Click the ellipsis (...)
to the right of >>GRIDS
|34. This grid selection
|35. Select the JFM grid and move it to the "selected" side with the > control.
Then click OK.
|36. Back on this window,
click the ellipsis to the right of FILE.
|37. Navigate to
PRODUCTS > SAGA > GEOIMAGES and enter the name temp_grid_from_osd_jfm_0m_5m_liberia_wod_odv_saga_masked,
then click SAVE
|38. We don't need to get
into CREATE OPTIONS at this time, so leave it blank, and click OK.
|39. Check to see that the new TIF file,
temp_grid_from_osd_jfm_0m_5m_liberia_wod_odv_saga_masked.tif, has been created.
|40. You can try to open the TIF file with any graphic editor, and you'll see something like this.
It proves the file is not intended for visualizing in the normal way.
NOTE: This image of the file comes from IrfanView. Other graphics
editors can show markedly different scenes, including black-and-white
|41. We need a world file for
the new TIF file, so open the ASCII world file for the PNG image, with the
NOTE: We are simply "borrowing" the PGW file, because this is the
easiest way to make the TFW file.
|42. Save the world
file in the GEOIMAGES folder with this new filename: temp_grid_from_osd_jfm_0m_5m_liberia_wod_odv_saga_masked.tfw. That's all you need
to do. The TIF and TFW files can only be used in special software, and
not ordinary GIS.
|43. Now go back to the
beginning and make all these products for at least ONE of the other