Marine Data Literacy 2.0

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Home > 7. Bathy/Topo Data > Tiled Hi-Res Relief

7.5 Merging/Viewing Tiled High-Resolution Relief Grids in 2-D and 3-D: CGIAR 3-sec SRTM

  • Exercise Title:  Merging/Viewing Tiled High-Resolution Relief Grids in 2-D and 3-D: Consultative Group on International Agricultural Research (CGIAR) 3-sec Shuttle Relief Topography Mission (SRTM)

  • Abstract:  In this exercise you'll obtain the 3-sec Shuttle Radar Topography Mission (SRTM) data for the Liberia area, from an online service that provides "tiles" of the data (a very typical method for high-resolution, global datasets).  In Saga, the tiles will be "mosaicked" and trimmed by a special module, down to the Area-of-Interest rectangle, using a trial-and-error method to find the minimum cell-size that Saga can handle.  The final raster product is viewed in Saga's regular map mode and in 3-D mode in Saga.

  • Preliminary Reading (in OceanTeacher, unless otherwise indicated):

  • Required Software:

  • Other Resources: 

  • Author:  J.B.L. Gusmao-Junior (Joao) and Murray Brown

  • Version:  8-4-2014

1.  For several years the authors have puzzled over the 3-D function in Saga, because every time we tried to use it, a horrible image appeared.  So eventually we gave up.  Recently, in the workshop in Pontal do Sul, Joao Bosco solved the problem.  It turns out it was caused by improper (vertical) exaggeration of the grid.  His demo images of the solution are shown below, followed by a general method you can use for any relief grid (topography and/or bathymetry).

2.  This pair of images shows Joao's area of interest, along coastal Brazil near Paranagua.  You can see a topography grid on the left, and a bathymetry grid on the right.  Both grids show elevations with typical color palettes.  You can see the 3D control button along the top of the viewing space.

3.  Joao applied the 3-D utility, which opens this screen.  The first 2 items are obvious, and he selected the bathymetry grid here.  But the many other items are not actually critical at this point, except one important item:  EXAGGERATION. 
  • If the units of measurement are identical for the XY values and the Z values, then the exaggeration could start at 1.0 to yield a reasonable figure.  You could make slight adjustments to improve your image, as you wish.
  • If the units of measurements are different, then the exaggeration is the factor you need to multiply the Z values to reduce (or increase) them to the same order of magnitude as the XY values.
  • Typically XY values are in degrees (roughly 100s of km) and the Z values are in meters.  So a typical exaggeration value for a "flat-looking" image of relief on the order of 100 m would be something like this:
    • 100 m (km/1000 m) (1 deg/100 km) = 0.001
  • In the figure below, Joao used a slightly larger value to get a better appearance
  • Only the GRID SYSTEM, ELEVATION and EXAGGERATION are absolutely needed to begin 3-D work.  You can play with all the other items later on your own time. Begin with a factor of 0.001 and adjust it up and down to get the best image.

4.  And here is the result of the above settings for the bathymetry grid.
5.  And Joao, having solved the riddle of 3-D has also found that simply placing another grid over the first grid, as you see here with the topography, works well.  Saga takes care of the details, and successfully trims away no-data areas.
6.  So now, let's try the method for our test area, Liberia.
7.  Open the CGIAR-CSI website to access the most reliable server for 3-sec SRTM data.  [NASA's surrogate for SRTM distribution, the USGS, is not completely reliable.]

Take some time to read over the interesting data available from this agriculture-related resource.

Then select DATA in the top row.

8.  Scroll down through the datasets to find this item, and click on it.
9.  This page opens, with an extremely long and badly organized text.
10.  Scroll down through a massive description page, to find this official download interface hidden in the text below a map(?) that fails to appear on the authors' computers:
11. This graphical interface opens.

12.  Make these choices:
  • Enter AOI coordinates for Liberia
13.  Then click on this control
14.  The search returns these six 5 X 5 "tiles".  You can see where they are in the map keys on the right.

15.  You can download the GZipped versions of the ArcInfo ASCII files for each grid (not the GeoTIFF).  Save them in DATA > BASEMAP > RELIEF > CGIAR.  Then unzip each one in place.
16.  Use TOOLS > IMPORT/EXPORT-GRIDS > IMPORT ESRI GRID to load all 6 grids into Saga.
17.  You should always take a preliminary look at downloaded grids to see where they are.  Here are all 6 grids in separate colors for easier viewing.

The sixth grid is the tiny blue dot below the Liberia frame.

18.  Grid 36-12 contains only the unnamed cape along the Ghana coast.  It is outside our area of interest.
19.  Grid 34-14 contains Ascension Island, also outside our area.
20.  We need to combine these grids, and then cut them down to the area of interest.  Fortunately one module does both.


21.  Click the ellipsis (...) to the right of INPUT GRIDS.  Then select the desired grids and move them to the right side.  You can ignore the other grids.

Then click OK.

22.  Now make these choices on the main MOSAICKING module page.  Then click OK.
23.  This page appears.  Don't panic, because it's easy.
24.  Make these choices:
  • FIRST 4 LINES, just enter the Liberia area coordinates.  Saga will make small changes to match up with the actual grid structures.
  • CELLSIZE - Begin with 1 degree to see what you get
  • Saga automatically changes the last two rows, so you don't need to do anything

Then click OK.

25.  You get this 1-degree grid.  Obviously you can do better, so let's move on.
26.  Here's what you get with a cellsize of 0.1 degree.  Looks good, but let's try even smaller.
27.  Here's what you get with a cellsize of 0.01 degree.  Let's try even smaller.
28.  But unfortunately a cellsize of 0.001 degree fails.  Here you see 0.005 degree.  That's as small as the authors can go with Saga (at least on our 32-bit version; maybe 64-bit can go smaller?).
29.  You can zoom into the upper right corner to see exactly how Saga has made the new grid, and you'll see that it is cell-centered, and that the cells are removed one-line from all sides.  This is perfectly OK, and entirely invisible for normal viewing.
30.  Now, to use the 3-D utility, find the 3D control along the top border of the map area and click it.
31.  This screen opens, which you saw above. 
  • For GRID SYSTEM, select the new 0.005-degree grid system
  • For ELEVATION, select the new 0.005-degree grid
  • For EXAGGERATION, begin with 0.0005

Then click OK.

32.  The authors leave it up to you to play with the COLOR settings (Hint:  use PRESET > TOPOGRAPHY) to get the image you like.  Take your time so Saga teaches you how these controls work and what they mean for your final image.