Marine Data Literacy 2.0

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Home > 2. Marine GIS > 2.28 WGS84->UTM

2.28 Reprojecting Grid Data in Saga: WGS84 to UTM

  • Exercise Title:  Reprojecting Grid Data in Saga: WGS84 to UTM

  • Abstract:  This exercise demonstrates how to convert an existing lat/lon grid to the UTM coordinate system.  Initially the grid must be formally identified and labeled as WGS84 (i.e. regular lat/lon) with one tool, and then a second tool is used to make the conversion to UTM.

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

    • N/A

  • Required Software:

  • Other Resources: 

  • Author:  Murray Brown

  • Version:  4-27-2016

1.  This exercise is almost identical to the reverse exercise for vectors, 2.27 Reprojecting Vector Data in Saga:  UTM to WGS84.  The re-projection process here, however, results in an entirely new gridfile, completely within the new projection.  In the vector re-projection (2.27), the shapefile was left unchanged, but the auxiliary files with it (*.PRJ and sometimes also the *.MSHP) signal to the GIS program that it should be treated differently.
2.  Download the zipped rlief data grid, above, to PRODUCTS > SAGA > GRIDS.  Notice that it has no PRJ or MSHP files.
3.  You can plot the grid on an ordinary map, as you see here with the World Borders shapefile, and it looks and behaves like an ordinary WGS84 projection, the standard default behavior for most GIS's.

But the grid is not formally projected, and that must be done before we can change the projection to anything else.

4.  To set the projection to WGS84, select MODULES > PROJ4 >SET COORDINATE REFERENCE SYSTEM.

For GRIDS, select the relief grid we just plotted above.

5.  We only need to select the item, for Saga to take care of everything else.
  • Saga automatically changes the PROJ4 PARAMETERS to the text string you see on the top line.
  • The EPSG CODE should also change automatically
6.  To make this formal WGS84 setting permanent, you right-click on the grid object, and select SAVE AS.  Then save it to the original location with the same name.
7.  Here, you can see the new PRJ file. 

OPEN it in any editor to see what it contains.

9.  For grids, to re-project, change the PROJECTED COORDINATE SYSTEM to whichever system you want.  Here the author selected UTM 22S.  [This makes no sense for a west African dataset, but he has a class full of Brazilians who need this projection for their coastal data.]  Saga takes care of the math and it will be fine.

When you select UTM 22S, then the PROJ4 PARAMETERS and the EPSG CODE changes automatically.

Click OK.

10.  Now Saga shows you that the process of changing from WGS84 grid cells to UTM grid cells has required some complicated geometric changes.  If you have reason to change anything here, then do so, but the author recommends a quick OK.
11.  Now a completely new grid object appears in Saga (the upper one).  The internal math has created a matrix of cells that are very similar to the original matrix. 
12.  And here is the new grid, mapped in Saga.  Note the location values below, for a random location on the map.  These are obviously not geographic (i.e. degrees), so our re-projection is complete and successful.
13.  You can save your new grid in PRODUCTS > SAGA > GRIDS with a name like relief_liberia_gebco08_saga_1min_utm22s.sgrd