Marine GIS > 2.40 Adding Graticules
Adding Graticules to Saga Maps
|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.
|2. Run the latest version of
|3. Use FILE > SHAPES > LOAD
to add the PLACES shape.
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
|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.
- 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
- EPSG Code -
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
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:
|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
|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
|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
|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