Using QGIS Plugins
In the QGIS Tutorial, we used the OpenStreetMap XYZ Tile Layer as a base map. In that
tutorial, we focused on making a map of California; however, you may have noticed,
if
you navigated to other countries on the map, that the labels for those countries used
the languages spoken in those places. The labels on the Japanese portion of the map,
for
example, are in Japanese.
Unfortunately, since that layer is owned by OpenStreetMaps, there isn’t a way for
us to
alter that. As such, for this assignment we’ll need to install a QGIS Plugin to access
alternate base maps.
To install a plugin, navigate to the “Plugins” section of the top ribbon, then click
“Manage and Install Plugins” in the resulting dropdown menu.
You’ll then be presented with the Plugins window. The plugin that we want to install
is
called “QuickMapServices,” so click on “All” in the menu on the left-hand side of
the
window, and then search for “QuickMapServices” in the search bar. Click on the result
in
the menu below the search bar, and then click on “Install Plugin” on the bottom
right-hand corner of the window.
After the Plugin has installed and you have closed the Plugins window, you should
then
notice that there are extra icons in the top menu bar.
Click on the tiled Earth icon with a plus sign on the bottom right to show the menu
of
all available services.
Do you see a “Google” option on the list? If you do not, click on “Settings” on the
menu,
and select the “More Services” tab. Ignore the caution message and click on the “Get
contributed pack”, and Google will then become available together with a long list
of
other GIS services.
Hover over the “Google” option, and then click on the “Google Road” option. This is
a XYZ
Tile layer provided by Google, and all of its labels all over the world are in
English.
You should then notice that there is a layer called “Google Road” in the Layer list
on
the lower left-hand side of the main QGIS window.
Adding the Pokemon Delimited Text Layer
Next, you’ll want to add the Pokemon TSV that you created with XQuery as a delimited
text
layer to your QGIS project. To do that, follow the steps outlined for adding a delimited
text layer in the QGIS introductory tutorial.
If you created a TSV file, you’ll need to change a couple of options when you’re
importing the file. Make sure that you select “Custom Delimiters” and make sure that
the
box next to “Tab” is checked. Furthermore, note that the top row of the TSV file you
created does not have the names of the columns; if your TSV looks like the example
picture above, then the first row should just contain relevant data. As such, you’ll
need to ensure that the box next to “First record has field names” is deselected.
Finally, also if your TSV looks like the example above, the X Field will be “field_4”
and the Y Field will be “field_3”. If you remember, the Y coordinates are the latitude
coordinates, while the X coordinates are the longitude coordinates.
Your QGIS map, zoomed out, should now look something like the following image.
Notice that not all of the locations are in Japan: one is in Hawaii. This is because
some
of the Pokemon XML files in that collection reference locations that are supposed
to be
in Hawaii (and appear to be from the Pokemon Alola world rather than from Kanto);
it
does not appear that the original creators of the Pokemon Map Project restricted
themselves to the Kanto region. However, the map that we want to create will focus
on
Japan.
Note: the picture does include a point in France. That is because it was made working
with an older version of the collection. It is not correct, and you should not have
a
point in France.
Zoom in on the points clustered in Japan. Your map should look something like the
image
below.
Changing Point Colors According to Type
First, look again at the data in the TSV that makes up the delimited text layer. To
do
that, right click on the Pokemon delimited text layer, and then select “Open Attribute
Table” in the resulting menu.
You can see that there are 21 separate rows for 21 separate locations, but there are
only
13 distinct types in this list. We’re going to change the color of the points for
each
of these types using “Case” statements.
Close the attribute table window and right click on the Pokemon delimited text layer
again, this time selecting “Properties” from the resulting menu.
The “Layer Properties” window should then appear. Select “Symbology” from the menu
on the
left-hand side of the window, and then click on the “Simple Marker” symbol from
underneath “Marker. These are the options that determine what the points currently
on
the map look like. You can see that the color that the points are on the map corresponds
to the color in the box next to “Fill color.” If we wanted to change the color for
all
of the points on the map, then we’d change the color in that box. However, we want
to
change the colors of all of the points on the map according to the Pokemon types with
which they are associated, and so we need to create some expressions. To do that,
click
on the gray square on the far right hand side of the window, next to the “Fill Color”
box, and underlined in red in the image below.
Under the “Expression” section of the menu, click on “Edit…” to edit an expression
that
governs how the colors of the points should be adjudicated.
Case Statements in QGIS
A case statement is like an “If, then” statement, only it’s more convenient to write
when
you have numerous conditions that you want to test for.
With an If, then statement, you want to test to see if a condition returns true. If
it
does return true, then you want something to happen.
An example, in pseudocode: If 3>0, then print “3 is positive”.
Here, the condition that we want to test is if 3 is greater than 0. It is greater
than
zero, and so, if this were a statement in a programming language like Python or Java,
the statement “3 is positive” would be printed.
If -4 > 0, then print “-4 is positive” would not print out “-4 is positive”, because
-4 is not greater than zero.
A case statement in QGIS works in a similar manner.
It’s set up like this:
CASE WHEN condition THEN result WHEN condition THEN result . . . END
With this, WHEN a condition is true THEN a result shall occur.
You use CASE and END to bracket all of the accompanying WHEN statements so that QGIS
is
able to recognize them for what they are: multiple cases that could be true.
Using Case Statements to Change Point Colors
After clicking “Edit…”, the Expression String Builder window should open. It should
look
something like the image below.
In the blank white space on the left hand side of the window is where you will be
writing
the case statement. The middle column, though the white background on my image makes
it
very hard to see, contains the different functions that one can use to build an
expression. Finally, the column on the right contains explanations for each
function.
Essentially, we want to use a case statement like this:
CASE WHEN type = ELECTRIC then color is yellow END
In other words, the case when the Pokemon type is equal to ‘ELECTRIC’ should result
in
the color of the point being yellow.
What syntax do we need to use to make this happen?
First, we should remember that the first column, “field_1”, is what stores the Pokemon
type names. To reference a column, you use double quotes; to represent a string value
in
a column, you use single quotes. As such,
WHEN “field_1” = ‘ELECTRIC’ then color is yellow
would be the first part of the WHEN statement that we eventually want to produce.
Second, to change the color of a point, we use the function rgb(_, _, _).
This function uses the rgb value of a color, or the amount of red, green, and blue
that
any given color is made of. The first argument of the rgb function contains
a number from 0-255, which indicates the amount of red that the resulting color should
have, the second argument also takes a number from 0-255, which indicates the amount
of
green that the resulting color should have, and the third argument takes a number
from
0-255 that indicates the amount of blue that the resulting color should have.
If you go to HTML Color Picker, you can use the color wheel to choose a color and then find
out the different ways in which that color can be represented. You can find out the
rgb
values for a color using this tool, indicated by the value underlined in red in the
picture below.
Now we know what the full WHEN statement should look like:
WHEN “field_1” = ‘ELECTRIC’ then rgb(255,255,0)
Use the HTML Color Picker to select colors for each of the different types, and then
write a CASE statement to change the point color when the value of “field_1” are all
of
the different types. You can either type the expression into the window or click on
the
different functions in the middle list to compose the expression.
When you’re finished, click “OK.”
The main QGIS map should now look something like this, depending on the different
colors
that you chose:
You can see that now there are different colors for different Pokemon types. If this
were
being developed for a project website, we'd want to prepare a label and a legend to
associate colors with Pokemon types, but that part is optional for this homework
assignment.
Finishing Touches
As explained in the introductory QGIS tutorial, use Print Layout to create a nice
picture
of your map, adding perhaps a label or even an explanation for the different type
colors.
