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2.13 Creating Alternative Map Systems in Saga for Pacific-Centered Analyses & Products

  • Exercise Title:  Creating Alternative Map Systems in Saga for Pacific-Centered Analyses & Products

  • Abstract:  In cases where the data analyses and products are mainly centered over the Pacific, it is impractical to work with the -180˚ to +180˚ coordinate system centered in the Atlantic.  In this exercise you'll learn how to set up a base map, project area frames and graticule for a Pacific-centered system. 

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

  • Required Software:

    • Saga (version 2.1.2 or higher)

  • Other Resources: 

  • Authors:   Murray Brown and Lica Krug

  • Version:  30-Jul-2018 (using SAGA version 6.4.0)

SPECIAL NOTE:  Users of this exercise have run into memory problems, due to the huge size of some global polygon shapes. You can tell you are having memory problems if Saga reports "Module caused access violation."  To avoid these problems, there are notes below where you should delete older versions of the shapes as you create new versions (transformed, merged, cut, etc.)    [Thanks to Emily Artack for reporting this.]
1.  Indian & Pacific Oceans MapRun Saga, and select FILE > SHAPE > LOAD SHAPE to load the world borders shapefile.

2.  We need two borders files, the original and a copy that has been moved 360 to the east.

Select TOOLS > SHAPES > TOOLS > TRANSFORM SHAPES

  • For SHAPE select the WORLD BORDERS shape.
  • For TRANSFORMED SHAPES shapes leave it as CREATE.
  • In Options, enter 360 for Translation X.
  • In Anchor Point, enter 0 for X and Y.

Leave everything else unchanged.  Click OK.

 

3.  Now you have two shapes, and one is TRANSFORMED (meaning moved to a different location)

4.  Now we need to combine them.  Select TOOLS > SHAPES > TOOLS > MERGE LAYERS.

Then click on the ellipsis (...) to the right of LAYERS.

5.  Here is the objects selection tool.  You'll see it often in Saga.

6.  Move both layers on the left side ("available")  to the right side ("selected" ) with the >> control.

Then click OK.

7.  Now you can click OK.

8.  This new shape, named MERGED LAYERS appears.  You should delete the original two shapes, to save memory space.

Right-click on the new merged shape and select ADD TO MAP.

 

9.  This is your new map.  It goes from minus 180 on the left to +540 on the right.

 

10.  Take a minute, for the sake of safety, to save the new double world borders shape with the filename world_borders_doubled_minus180deg_540deg.shp
11.  Before we can specify only the middle portion of the map (the Pacific region), we need to make certain that each country is represented as multiple separate polygons, and not as multi-part single polygons.  The reason for this is that strange things happen to polygons with parts inside and outside the data management area.
12.  Select TOOLS > SHAPES > POLYGONS > POLYGON PARTS TO SEPARATE POLYGONS. 
  • For POLYGONS, select MERGED LAYERS
  • Leave LAKES unchecked

Then click OK.

 

13.  This creates a new shape named MERGED LAYERS [PARTS].  You could examine it, but it will appear identical to the map above.

14.  Now we're almost ready to cut the double-globe shape down to the desired Pacific area.  But first we need a normal graticule with lattice lines, and a special "frame" graticule.
15.  Select TOOLS > SHAPE > TOOLS > CREATE GRATICULE.

Enter these settings to create a 10-degree graticule (i.e. one with 10-degree spacing between the lattice lines). 

Notice that the longitude range is from +20 to +380.

Then enter OK to create the new graticule shape.

NOTE:  You can actually set the longitude range for whatever you want.

 

16.  Here the new graticule has been shown with ADD TO MAP.  You can see where the 20-380˚ system will be. 

 

17.  You can save the new graticule shape you just made in the folder PRODUCTS > SAGA > VECTORS with the name graticule_20deg_380deg_10delta.shp
18.  Now run the graticule module again, but with these settings, to create the special frame. The difference is in the WIDTH and HEIGHT division items.

Make sure GRATICULE is set to CREATE.

Then click OK.

19.  This makes a frame that exactly surrounds the graticule, as you see here on the map.  We will use it below.

Make sure to view this map, to insure that all the geographic coordinates are as you expected.  Use your imagination (and the settings) to make the frame and graticule easily visible.

20.  Now save the new "frame" in the folder PRODUCTS > SAGA > VECTORS with this filename: frame_20deg_380deg
21.  Select TOOLS > SHAPES > POLYGONS > POLYGON-LINE INTERSECTION.

This module adds new lines to a polygon, cutting or bisecting the existing polygon borders.

22.  Make these selections:
  • For POLYGONS select the merged polygons (name might be different)
  • For LINE select the new frame graticule you just made

Then click OK.

 

23.  This new polygon shape appears.

24.  Now you have all polygons cut into "inside" parts and "outside" parts.  Next we will cut away the outside parts.
25.  Select TOOLS > SHAPES > TOOLS > COPY SHAPES FROM REGION.

For SHAPES, you must click the ellipsis to go to an object-selection window.

26.  In the object-selection window, move the new polygon you have just made to the right side (left side not shown here).  This means it is selected for the module calculations.

Then click OK.

27.  Now make these further selections:
  • For METHOD, select INTERSECTS
  • For REGION, select SHAPES LAYER EXTENT

A second SHAPE option appears on top.

28.  Now you're asked for the specific shape to define the EXTENT. Select the frame you made above.

Then click OK.

 

29.  This new shape appears, with a very long name indicating the TOOLS you used.

30.  Before you see the new object, select it and make these COLORS settings.

NOTE:  The VALUE RANGE is set automatically, so leave it alone.

31.  Here is the final map.

32.  To make certain that all countries in the map can be colored identically, the separate physical polygons must be "dissolved" back into multi-part polygons.  To do this, select TOOLS > SHAPES>POLYGONS > POLYGON DISSOLVE.
  • For POLYGON, select the new Pacific map
  • For DISSOLVE FIELD(S), select UN which is a unique numerical identifier assigned to each country in the World Borders dataset (It will be set = 4)

Then select OK.

NOTE:  This is the opposite of what we did above with the tool POLYGON PARTS TO SEPARATE POLYGONS.

33.  Now you can save the new shapefile in PRODUCTS > SAGA > VECTORS with the name map_20deg_380deg_worldborders_saga
34.  Dummy Grid for Indian-Pacific Map.  Just as we did for the Liberia project area, dummy grids can be created for the new system.  In the next panel we make only one grid, covering the same map area for consistency.
35.  Select TOOLS > GRID > TOOLS > CREATE GRID SYSTEM.  Then make these choices.
  • Select the LOWER LEFT COORDINATE AND NUMBER OF ROWS AND COLUMNS method
  • Set CELLSIZE = 1
  • Set LEFT = 20
  • Set COLUMNS = 360
  • Set BOTTOM = -90
  • Set ROWS = 180
  • Put a CHECK by USE OFFSET
  • Set X OFFSET = 1/2 CELLSIZE
  • Set Y OFFSET = 1/2 CELLSIZE

Then click OK.

36.  Here is the new grid, zoomed extremely close (to the bottom-right corner) to see how it matches the expected corner coordinates.  The cursor coordinates for that corner show on the bottom line.

Within the limits of the computer chip's display ability, the match is exact.

 

37.  Save the dummy grid in the folder PRODUCTS > SAGA >GRIDS with the name frame_grid_pacific_global_1deg.sgrd
38.  A huge amount of information has been provided in this very long exercise.  That's because there are so many thing to consider and choices to make when you decide on a project map.  There is no single "best map" so you have to study this matter yourself, with a view toward the actual data and products you'll be working with, to optimize the base map.  Don't hesitate to contact the authors if you have questions.