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Marine Data Literacy 2.0

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Home > 2. Marine GIS > 2.27 UTM->WGS84

2.27 Reprojecting Vector Data in Saga: UTM to WGS84

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

  • Abstract:  To "re-project" a mapping file in Saga, we will first assign the correct, current projection to it (in the first step) and then we will direct Saga to display it in a desired second projection (in the second step).  The actual coordinates for the points will remain unchanged, but the auxiliary files (*.MSHP and *.PRJ) contain the re-projection parameters.  Most true GIS programs will recognize and apply the reprojection via the use of these files.

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

    • N/A

  • Required Software:

  • Other Resources: 

  • Author:  Murray Brown and Mihael Souza

  • Version:  3-8-2015
1.  Select FILE > TABLE > LOAD to load into Saga the table of depth measurements.  You can see from the displayed values that they cannot be latitude or longitude.
2.  To convert to latitude and longitude, i.e. the WGS84 system, you need to do some preliminary work:
  • Convert the table data to a proper GIS shape
  • Specify the current projection of the shape (because Saga won't actually "know" that yet)
3.  Select TOOLS > SHAPES-POINTS > CONVERT TABLE TO POINTS.  Make these choices and click OK to convert the table to a point shape.
4.  Here you can see the points displayed on a Saga map.  The scale figure below shows that very unexpected units are available.
3.  To specify the current projection, select TOOLS > PROJECTION-PROJ 4 > SET COORDINATE REFERENCE SYSTEM.

When the module appears, it may have settings from its previous use, which you can ignore.
4.  We know the Projected Coordinate System, so drop down the menu to its right and select SAD69/UTM ZONE 22S.
  • When you make any specific choice in this tool, the other choices change automatically
  • The top line, PROJ4 PARAMETERS will change (as you see here).
  • And the EPSG CODE will change (as you see here).

Then, for SHAPES, select the existing point shape we made above.

Now, click OK.
5.  The module has improved the shape with the addition of projection information, but you must capture this in the files.  Select the shape, and use SAVE AS to save it.
6.  Keep the same location and table name, so now you have these 5 shape files (DBF, MSHP, PRJ, SHP, SHX).   The projection information is in the PRJ file.
7.  The attributes table for the shape contains the same data we saw in the original text table.
8.  Now to change the projection select TOOLS > PROJECTION-PROJ 4 > COORDINATE TRANSFORMATION (SHAPES)
  • For GEOGRAPHIC COORDINATE SYSTEM, select WGS 84
  • The top line, PROJ4 PARAMETERS will change (as you see here).
  • And the EPSG CODE will change (as you see here).

Then, for SHAPES, select the point shape to be reprojected.

Now, click OK.
9.  This will create a new point shape with exactly the same name as the first one.
10.  To avoid identification problems, you should rename the new shape to reflect the reprojection, as you see here.  The original projection name can be retained for further clarity.
11.  If you examine the properties table for the point shape, it will still have the same (UTM) lon and lat values.
12.  But if you map the new point shape it will plot with expected longitude and latitude values for the WGS system, as you see here.  The land map is part of World Borders, and we know that it is WGS84.

Saga has "aliased" the bathymetry shape to behave in GIS as completely reprojected.
13.  Inspection of the resulting 5 files, indicates that the MSHP and the PRJ files together are needed to plot these data correctly in WGS84 coordinates. 
  • Tests with Google Earth and QGIS show that the new coordinates are respected, and the shapes plot in their expected locations.
  • Tests with Integrated Data Viewer (IDV) failed, because it does not use these auxiliary files.
14.  Navigate to an appropriate folder location and save the new shape as  bathymetry_utm_sad69_22S_parana_brazil_reproj_wgs84.shp