Specifications

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is add the requisite repository to your
favorite package manager. If you must
install from source, there are plenty of
on-line guides explaining the process.
See Figure 1 for a look at QGIS’s GUI.
GIS is a complex application requir-
ing knowledge about data formats,
how a GIS functions and general car-
tography. Let’s rip through a quick,
need-to-know primer on GIS.
A GIS Needs Geospatial Data
As mentioned previously, using a GIS is
essentially mapping on a computer. To
do this mapping, you need to find data
related to geography, typically called
geospatial data. This geospatial data
that we will introduce into QGIS con-
sists of two elements, namely spatial
features and attribute data. Examples of
spatial features might include streets,
rivers or land cover—any feature you
might find on a map. Meanwhile,
attribute data describes the characteris-
tics of the spatial features and is stored
in a database within the GIS. For exam-
ple, most of those streets have names
and lengths; the land-cover types have
names and areas associated with them.
In the case of land cover, a GIS might
store attribute-related categories, such
as high-density urban, low-density
urban, cropland, forest and so on,
which you then could query easily.
Peel Back the Layers
Your paper road map would think you
were completely mad if you command-
ed it to “just show me the rivers and
mountains, please” or “flip the county
boundaries on and off”. On the other
hand, because a GIS portrays data in
similar groupings of geographic ele-
ments, called layers, your computer will
execute your command and not label
you loopy. Some examples of layers
are countries, cities, rivers and
oceans. A GIS allows you to control
which layers are displayed on your
screen at any time.
Layers can consist of two types,
namely features and surfaces. In our
above list, the layers with countries,
cities, rivers and specific buildings
are feature-based; oceans are one
single, continuous expanse and, thus,
are a surface.
How a GIS Formats Data:
Vector vs. Raster
The hefty challenge for a GIS is to
portray our lovely yet complex world
accurately yet rapidly—and without
the need for a cluster! There are two
tricks, or methods, a GIS uses to cre-
ate a digital representation of Earth’s
features on your desktop.
The first method is using vector
data (the type used later in this arti-
cle). As complicated as the world can
be, a GIS can represent any geograph-
ical object using three geometric ele-
ments—namely points, lines and poly-
gons. Small stuff like community cen-
ters and traffic lights can be portrayed
as points. Features such as rivers and
pipelines are really just glorified lines,
so they can be shown as such. Finally,
nearly everything else, such as a state
park, though it might be oddly
shaped, is finite and contained in
boundaries, making it a polygon at
the end of the day. Broadly speaking,
the vector format is analogous to
traditional maps, where the world is
abstracted with symbology, and preci-
sion is very important.
The second method is raster data.
Raster data is used to portray Earth’s
characteristics that have no shape visu-
ally, including measurements like ocean
depth, forest-cover type, elevation and
annual rainfall. Some image types you
will encounter include GeoTIFFs, Erdas
Imagine Images, GRASS AIGs and USGS
Digital Elevation Models. Some common
examples of raster-based imagery are
satellite images and aerial photos. In
these two types of raster imagery, the
value of each cell is a measurement
of light that is reflected off the
Earth’s surface. Particular ranges of
these values can signify specific land-
cover or vegetation types.