Importing Climatological NetCDF Satellite Grids into Marine GIS OTGA – UEM TRAINING COURSE – FUNDAMENTALS OF OCEACOGRAPHIC DATA MANAGEMENT , 23-27 JANUARY 2017, MAPUTO, MOZAMBIQUE Prepared by: Harrison Ong’anda, National Coordinator, Kenya National Oceanographic Data Center (KeNODC) honganda@kmfri.co.ke, hochieng2003@gmail.com

Run Saga. Select TOOLS > IMPORT/EXPORT- GDAL > GDAL: IMPORT RASTER.  For some reason, the files can't be loaded with the expected IMPORT NETCDF tool, which may relate to specific formatting problems.

Within the tool, you'll need to set up these options.  We picked the easiest method for interpolation, but you can explore this later on your own.Click OK to continue.

Make this selection, because we only want the SST data.  Then click OK.

The SST grid appears in the DATA > GRIDS list

You can use ADD TO MAP to see the grid You can use ADD TO MAP  to see the grid. Data value shown, 41342 for a temperature clearly shows that "scaling" of raster temperature values has occurred, so a formula will be needed to remove scaling No color on land indicates a __FillValue has been correctly identified and used X and Y values are too large to be lon/lat, so reprojection will be needed

In the PROPERTIES panel, select DESCRIPTION to see the underlying metadata for the grid.  Read through and try to understand all the lines. The SST_SCALE_FACTOR and the SST_ADD_OFFSET both show that the SST data values have been scaled.  This allowed them to be stored here as "unsigned 2 byte integer" values, to save space. The UNDEFINED COORDINATE SYSTEM refers to the use of raw X and Y axis values, rather than longitudes and latitudes.

Select the above system and SST grid for input. To unscale the data values, select TOOLS > GRID > CALCULUS > GRID CALCULATOR. Select the above system and SST grid for input. For FORMULA enter the expression shown here: (g1*0.00071718497)-2 For NAME, enter something like SST DESCALED so you know exactly what the result is called Then click OK to proceed.

This new grid should appear beside the old SST grid

If you use ADD TO MAP to see the new grid, you will still see the non-geographic axis coordinates.  But wherever the cursor is placed, you should see ordinary temperature values, e.g. 28.872662 in this example.

To continue on and fix the latitude/longitude problem, select TOOLS > PROJECTION > GEOREFERENCING > DEFINE GEOREFERENCE FOR GRIDS Select the SST DESCALED grid for georeferencing. Then click OK

Make these selections to project the grid.  Be careful with each item, because some of them have very similar options. The CELLSIZE has been calculated from total map width (360º) divided by number of columns (8640) When finished, click OK.

Look in the data objects, and you'll see a new grid with the physical descriptors shown here.  Rename it as shown so you don't lose it later.

Before any other steps, you should save this new grid in the folder MOZAMBIQUE > PRODUCTS > SAGA> GRIDS > COLORWEB > SST with the filename sst_11mu_day_jan_global_modisa_colorweb_4km_descaled_geopr oj.sgrd (or 0p05deg instead of 4km if you prefer; they are roughly the same). 

Close saga project for now

Run Saga Load the grid - sst_11mu_day_jan_global_modisa_colorweb_4km_descale d_geoproj.sgrd

Also open the Mozambique-area frame grids you made

Now run TOOLS > GRID > TOOLS > RESAMPLING

You should make these choices: For system and grid (at the top) select the global SST grid. For TARGET GRID SYSTEM, pick GRID OR GRID SYSTEM Then for GRID SYSTEM (which quickly changes to a set of drop-down choices) pick the high-resolution ("0.05") AOI-sized ("400x520") grid This tool will sample your existing grid to calculate what would be the values for each new grid cell, and then will create it in the Saga grid format. Click OK to continue.

The new grid appears in the desired system position The new grid appears in the desired system position.  If you made a small mistake then just delete it, and go back one step.

If everything technical is OK, then change the name of the grid slightly to indicate it is not global now.  In this case "globe" has been changed to “mozambique". You make this change in the top line of the SETTINGS > OPTIONS > GENERAL > NAME panel, and then click APPLY

Here, you can use ADD TO MAP (new, of course) to see the sampled grid and all its settings, including the color LEGEND on the right.  It looks fine, so you can continue.  If anything looks wrong, then try to fix it in the SETTINGS

Now, use SAVE AS to save this newly made, newly renamed grid to the folder PROJECTS > MOZAMBIQUE > PRODUCTS > SAGA > GRID with the filename SST_jan_mozambique_modisa_ocweb_0p05deg_nonull_geoproj.sgr d or similar depending on your naming preferences.

GeoTIFF creation 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.

Select TOOLS> Import/Export - GDAL/OGR > GDAL: Export GeoTIFF For GRID SYSTEM, select the system for the SST grids. Click the ellipsis (...) to the right of >>GRIDS

This grid selection window opens.

Select the SST grid and move it to the "selected" side with the > control. Then click OK.

Back on this window, click the ellipsis to the right of FILE. Navigate to PRODUCTS > SAGA > GEOIMAGES and enter the name Mozambique_wod13_jan_raster, then click OK.

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.

This is how the file is displayed in other GIS software