Marine Data Literacy 2.0

Providing instruction for managing, converting, analyzing and displaying oceanographic station data, marine meteorological data, GIS-compatible marine and coastal data or model simulations, and mapped remote sensing imagery

 

 

 

 

Home > 2. Marine GIS > 2.4 Map Frames

2.4 Creating Map Frames in Saga

1.  Run Saga, and re-open the Liberia shape of populated places (file link above).
2.  Select TOOLS > SHAPES-TOOLS > CREATE GRATICULE.

NOTE:  This is the old tool that we used to make graticules in MLD.  It has been replaced by 2.40It is used here only as an easy way to make frames, the outside edge of graticules.

3.  Use these settings, which specify a map-wide rectangle.
  • The MINIMUM and MAXIMUM values are set to the correct edge coordinates; these were picked at the beginning of all Liberia work in MDL
  • The DISTANCE X and DISTANCE Y settings are set to the total width and total height of the map, i.e. MAXIMUM minus MINIMUM

Be very careful at this step, and make sure Saga says <<GRATICULE [CREATE] in the top line, because it often tries to overwrite an existing object name.

Click OK.

4. This new line shape appears, which is simply an empty rectangle we'll call a "frame."
5.  And this is the combination map of the populated places shape and the new frame.
6.  Frames can be used for many purposes, especially to create a visual edge around a specific area of interest.  They also give you a digital object that can be used for cutting or trimming datasets.  You should make and keep a small library of these shapes for future use.
7.  But we aren't finished with this new frame.  Right-click on it and select SPATIAL REFERENCE.

This is the tool to formally select the coordinate system for the new shape.

  • For the GCS select WGS84
  • Look for EPSG CODE 4326 to make sure it has been accepted
  • If you don't pick a PCS, then it is left unused, even if it still has a past selection showing.
8.  Click to the right of SHAPES to get this selection screen for the file to be used.  Select the populated places shape.

Then click OK and OK again to run the tool.

9.  Save the new file in a good workspace with the name frame_liberia_lines_projected.shp
10.  You should check the folder to make sure you have all these files.  The PRJ and MSHP files contain the desired projection information for the new frame.
11.  And here you can see a map of both the shapes, the populated places points and the new frame.
12.  Execute the MODULE again, but select RECTANGLE for GRATICULE TYPE (in Panel 3, above), resulting in a closed rectangular shape, called a "polygon" in GIS terms.  To prove it is a polygon, use ADD TO MAP to see that it is color-filled. 

Make all the same files as above, but with the files name frame_liberia_poly_projected

13.  Now we're going to use a more complicated situation, a coastline shape that is not in WGS84.  The file is projected and in a system that typically uses very large number values for the coordinates.  Make an entirely new map of the coastline file above.  From its filename you can see a clue to the projection name, UTM 23 S.  This Universal Transverse Mercator projection is one of a large family of projections derived from a set of standard north-south slices of the globe (like orange peel pieces).  UTM north-south coordinates are in meters measured from the equator and east-west coordinates from midpoint verticals in each slice.
14.  You can examine the files folder to see that the coastline has 5 files, including the desired PRJ and MSHP files.
15.  If you use ADD TO MAP to make a new map of this shape, you can immediately see that the projection uses values that are very different from typical latitudes or longitudes.  Some very large values are involved, so the simple arithmetic you used above (Panel 3) to estimate the area "box" would be very inconvenient here.

For this reason, we'll estimate the box in WGS84-type latitude/longitude values (i.e. degrees), make the box as a shape, and then convert the box to the units you see here.

16.  The trick you need to know, is that Saga can draw a WGS84-type graticule over the map.  [This is only possible when the shape has PRJ and MSHP files.]

Locate the graticule function, as you see here.  Any trouble here might be because you don't have the coastline shape selected in the data list.  Then click ADD GRATICULE.

17.  The graticule appears, and mercifully it has the WGS84-type coordinates.
18.  By inspection, you can estimate the coordinates of a good data area from the map.  Keep it simple, and stick with whole degrees, half degrees or possibly quarter degrees (as you see here).  Write these down.
  • Left = -48.75 = Minimum Width
  • Right = -47.75  = Maximum Width
  • Bottom = -26 = Minimum Height
  • Top = -25 = Maximum Height

 

19.  The easiest way to make a box is to use the old CREATE GRATICULE tool.  We only use it here to make the frame, and do not use it now for graticules.

So find it and open it.

20.  Enter the proper values, as you see here.  You must do the simple math for the WIDTH values.

 

 

NOTE:  When the WIDTH and HEIGHT completely cover the total area of interest, then you are are making a FRAME, not a GRATICULE.

21.  The new graticule shape appears.
22.  To check its accuracy, use ADD TO MAP to see it on a new map of its own.
23.  If all is OK, then make sure to rename the shape as you see here.
24.  Now, as you should do with all new shapes, its SPATIAL REFERENCE should be specified.

Right-click on the shape, and select SPATIAL REFERENCE.

25.  In the PICKER tool, select WGS84.  Make sure the EPSG Code changes to 4326.

Then click OK.

26.  Were you successful?  Select the shape and MAP to see if the ADD GRATICULE function is open.
27.  To insure against future problems, you can save the current WGS84 shape in an appropriate folder with the name frame_shape_parana_brazil_geodas_coastlineextractor_shore_wgs84_saga.shp
28.  Now we need to change the coordinate system to UTM 23S.  Select the COORDINATE TRANSFORMATION (SHAPES) tool.

29.  Study the tool to see how many extra functions it has that you won't use now.  Then enter the bold items you see here.  The second one (LOADED SHAPES) will open a separate control in the next Panel.

30.  For the LOADED SHAPES select only the UTM coastline shape, as a template for the new projection of the frame.  Then click OK two times.

31.  This new shape should appear.  The name is probably wrong.
32.  Change the name to reflect the new projection.
33.  Save the new frame with this filename:  frame_shape_parana_brazil_geodas_coastlineextractor_shore_utm23s_saga
34.  And here are the the original coastline file (in UTM 23S) and the frame (same projection).
The line and polygon objects are "utility files" that can be used for enhancing maps or managing data in later exercises.