Marine Data Literacy 2.0

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Home >2. Marine GIS > 2.40 Adding Graticules

2.40 Adding Graticules to Saga Maps

  • Abstract:  The old graticules exercise has been completely deprecated in favor of this new version, using a new Saga method.  The importance of getting or making a proper PRJ (projections) file for each shape is explained in a pair of examples of geographic data files.

  • Preliminary Reading

    • N/A

  • Required Software:

  • Other Resources: 

  • Author:  Murray Brown

  • Version:  04-21-2016

1.  Create a suitable empty folder for this exercise.  Then download and upzip both of the Resource files listed above, as you see here.

Both files are GIS shapes.  The upper one has an extra PRJ file but the lower one doesn't.  You'll learn more about PRJ files below.

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2.  Run the latest version of Saga.
3.  Use FILE > SHAPES > LOAD to add the PLACES shape.
4.  BRIGHT IDEA:  If you associate the SHP file extension with Saga, then you can open any shape directly by simply double-clicking on it in the the files list.
5.  In Saga, the the data list, right-click on the PLACES data object and use ADD TO MAP to display this initial map.

You can see that as you move the cursor over the map, the correct latitude and longitude values show along the bottom margin.

But these are simply image coordinates and not yet geographic coordinates.  Saga, at this point, does not "know" that these data are an earth map, because no PROJECTION file is present to signify that. 

It is a MAP, but it is not yet an EARTH MAP.

6.  At the top of the Saga window, right-click on the MAP section to see what Saga "knows" about this shape.  Two important functions are grayed out, meaning not available.  These are:
  • ADD GRATICULE
  • ADD BASE MAP

In this exercise we'll use the first one.  The second one is covered in 2.41 Adding Global Imagery to Maps in Saga.

 

7.  NOTE:  Saga determines if GRATICULE and BASE MAP can be activated by checking to see if a PRJ file is present.  The most current version of Saga can be confused by this check, if a different shape was just managed or drawn.  This tiny problem is hereby referred to the authors for assistance.  But, in general, if the shape has a PRJ then you can tell that by the physical appearance of the ADD GRATICULE tool name.  If the tool is not grayed out, but the PRJ file is actually missing, then Saga will jump to the other file (with a PRJ file) when you click ADD GRATICULE.
8.  To supply a lacking PRJ, then right-click on the shape object and select SPATIAL REFERENCE.
9.  You're about to use possibly the most confusing tool interface in Saga.  Here you must supply one (or more) of several different items to define the coordinate system(s) of the shape.  Some options include:
  • User Defined - You supply a long list of parameters
  • Loaded Grid - You pick an existing grid with a PRJ and direct Saga to use it also for your grid
  • Loaded Shape - Similar, but for shapes
  • Well Known Text File - A special markup language described at https://en.wikipedia.org/wiki/Well-known_text
  • EPSG Code - EPSG Geodetic Parameter Dataset code value; unique numerical code from huge registry covering all reference systems.  If known you can enter it here, and you're done.
  • Geographic Coordinate System (GCS) - Comprehensive list of systems to choose from; a choice here is automatically reflected by the EPSG Code, above.  See note below about going to PCS and trying to return to GCS.  All maps have a GCS.
  • Projected Coordinate System (PCS) - Comprehensive list of systems to choose from; a choice here is automatically reflected by the EPSG Code, above.  Some maps actually have no projection, which makes this item difficult to understand.  NOTE:  If you leave PCS and go back to the GCS choices, then you cannot immediately re-choose the same system; you must choose any other system (to "clear" the item), then make your desired choice again..

The original data may have a different sort of projection information file than the PRJ files shown here.  If they are ASCII, then read them to see if they contain usable physical system information.

After nearly a decade of using this interface, the author is still half-ignorant of exactly how it works.  Take it slowly and try some different options to see what's going on, and to make sure you're confident in your understanding.  The best possible case is for the original authors of the data to state explicitly the GCS and PCS they used, but that is rarely the case.  Examine their publications or contact them directly to make sure you apply the correct information.

10.  And here is the dreaded CRS picker.  In the absence of evidence that the dataset is projected, then the easiest selection is GCS = WGS 84, to yield EPSG CODE = 4326, as you see here.  Heaven help you if it's more difficult.  Make these choices and click OK.  The PCS choice here is random and not important.

WGS 84 is simply LATITUDE = Y and LONGITUDE = X, using global physical benchmarks selected in 1984.  In the old days this simple "projection" was called the equirectangular projection (also called the equidistant cylindrical projection or geographic projection) .  It would make life much easier for GIS folks, if Saga would include these terms -- even if not absolutely correct -- in the PCS list.  Even if it's just a dummy entry, at least it would make the GCS/PCS pair more understandable.

11.  As soon as you click OK, the necessary information is added to the data object, so that Saga is now completely cognizant of (and can use for mapping) the shape's geographic system and/or projection.
12.  Creating the PRJ information is necessary, but not sufficient to do the job.  You just also save the complete new suite of files.  Use SAVE AS
13.  Navigate to a suitable location, such as the original folder, then enter populated_places_liberia_gns_projected.shp and click ENTER.
14.  You should check to make sure you see the new PRJ file, as shown here.
15.  For your information, here's the contents of the new file:

GEOGCS["GCS",DATUM["WGS84",SPHEROID["WGS84",6378137.0,298.2572236],TOWGS84[0,0,0,0,0,0,0]],PRIMEM["Greenwich",0],UNIT["degree",0.01745329251994328]]

16.  The new version of the file can be loaded anytime, with immediate ability to add a graticule.  The version that is already loaded however, must be redrawn again, with ADD TO MAP, to force Saga to recognize the availability of the PRJ information.

17.  Here we have drawn the map again, and the ADD GRATICULE tool is now available (i.e. no longer in gray letters).

18.  Click the ADD GRATICULE control, and this is what you see.  It is the default graticule, which we can adjust to suit our tastes as shown below.

19.  The GRATICULE object is now listed as a separate shape under the list of MAPS.
20.  To change or adjust it, right click on it and select PROPERTIES.

There is a mistake here that is possibly in the Saga code.  Notice that the FIXED INTERVAL is set to 5 degrees, but you can see 1-degree lines in the image above.

21.  Change the interval to 1, and click APPLY to get the image to agree with the settings.  Now Saga will always show the right intervals.
22.  But, where is this graticule in space, and how big is it?  Here we've decreased the size of the map with the ZOOM tool to see that it is global.  The default graticule is not bounded like a frame.  If you want to make pretty maps, then manually select the area you want.  Then SAVE the map, as shown next.
23.  Here's how you save the map for later use.  First ZOOM back to the view you want.  Then right-click on the desired map object in the MAPS list, then select SAVE AS IMAGE.
24.  Navigate to an appropriate location and enter populated_places_liberia_gns_projected.png or similar (several formats are available).  Then hit ENTER.
25.  Now you have the same image saving properties as usual, including the KML file which makes it so very easy to open the map in Google Map.

Study these choices, make your selections, then hit OK.

26.  Here's the constellation of files you would create from the above.
27.  We cannot find a method to save the actual Saga map, i.e. a SAVE AS for the shape and the graticule together.  A question about this has been submitted to the authors.
28.  Now let's look at a different shape, one that already has a PRJ file.

29.  Load the other shape supplied with this exercise, a coastal map from Parana, Brazil.

30.  If you're curious, here's the contents of the PRJ file.  It's obviously more complicated.  The GCS seems to be WGS84/UTM ZONE 23S, and the projection seems to be TRANSVERSE MERCATOR, so this map will be really different.

PROJCS["WGS 84 / UTM zone 23S",GEOGCS["WGS 84",DATUM["WGS_1984",SPHEROID["WGS 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG","6326"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.01745329251994328,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4326"]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",-45],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",500000],PARAMETER["false_northing",10000000],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AUTHORITY["EPSG","32723"]]

31.  Use the ADD GRATICULE function, which should be dark font.
32.  And here is the map.  You can see very obvious differences from the plain vanilla rectilinear grid from Liberia.  The grid lines are not parallel to the map margins at all, so the map margins are not true North-South.
33.  And if you zoom out really far, you can see that the north-south lines actually seem to converge below.  So this is a very different kind of map, and you can see how we really do need the PRJ information to get good graticules.
34.  MDL has not done a good job of dealing with PRJ information prior to this time, for which the author apologizes.  The availability of this relatively new MAPS > GRATICULE tool (not to be confused with the old one at TOOLS > SHAPES > TOOLS > CREATE GRATICULE) demands better explanations and a more complete exercise.  For this attitude adjustment, I am very thankful.  If you run into the inevitable problems, please notify me and let me try to help.  Murray Brown