106
Chapter 5 Working With Digital Spatial Data and GIS
for an example of several DLGs in GIS). The DLGs are created by digitizing—
a USGS map is used as the source and the map features (like roads or rivers)
are turned into digital format using GIS. See Hands-on Application 5.3: USGS
Digital Line Graphs for more about how to obtain DLGs.
There’s also a second way of viewing the world, in which not everything
has a fixed boundary or is an object. For instance, things such as temperature,
atmospheric pressure, and elevation vary across the Earth and are not best
represented in GIS as a set of objects. Instead, they can be thought of as a
surface that’s made up of a near-infinite set of points. This way of viewing the
world is called the continuous field view. This view implies that real-world
phenomena continuously vary, and rather than a set of objects, a surface filled
with values is used to represent things. For instance, elevation gradually varies across a landscape from high elevations to low elevations filled with hills,
valleys, and flatlands, and at every location, we can measure the height of the
land. Similarly, if you were standing at a ranger station in a park and preparing a rescue plan for stranded hikers, you would want to know the distance
from the station to every location in the park. Thus, you could have two layers
in your GIS—one showing the elevation at every place and another showing
the distance to the park at each location.
How Can You Represent the Real World
as Continuous Fields?
When representing these kinds of continuous fields in GIS, the three vector objects (and thus, the vector data model) are often not the best way of
representing data. Instead, a different conceptualization called the raster
data model is usually used. In the raster data model, data is represented
Digital Line Graphs are made freely available by the
USGS. A good online utility for examining available
DLGs is EarthExplorer, a Website designed to facilitate access to a wide range of geospatial data (not
just DLGs). To get started, open your Web browser
and go to http://edcsns17.cr.usgs.gov/NewEarthExplorer/. Using EarthExplorer is a four-step process. First, select the Search Criteria tab and type
the name of a location of an address or place name
(such as Reno, Nevada). Next, click on Data Sets
and place checkmarks in the DLG 1:100k and DLG
Large Scale boxes under the Digital Line Graphs
option. Next, click Additional Criteria—this is where
you could further refine your search, if desired. Last,
click Results to see what EarthExplorer turned up.
From the available options in the results, you can
see the footprint that the DLG would cover. If desired, DLGs can be downloaded to use in GIS (they
may need to be imported into a specific format to
use, and the USGS requires a user to register and
log in with them to download data). Check your local
area to see what types of DLGs are available.
Hands-on Application 5.3
USGS Digital Line Graphs
continuous field view
the conceptualization
of the world that all
items vary across the
Earth’s surface as
constant fields, and
values are available at
all locations along the
field.
raster data model a
conceptualization of
representing spatial
data with a series of
equally spaced and
sized grid cells.
Chapter 5 Working With Digital Spatial Data and GIS
for an example of several DLGs in GIS). The DLGs are created by digitizing—
a USGS map is used as the source and the map features (like roads or rivers)
are turned into digital format using GIS. See Hands-on Application 5.3: USGS
Digital Line Graphs for more about how to obtain DLGs.
There’s also a second way of viewing the world, in which not everything
has a fixed boundary or is an object. For instance, things such as temperature,
atmospheric pressure, and elevation vary across the Earth and are not best
represented in GIS as a set of objects. Instead, they can be thought of as a
surface that’s made up of a near-infinite set of points. This way of viewing the
world is called the continuous field view. This view implies that real-world
phenomena continuously vary, and rather than a set of objects, a surface filled
with values is used to represent things. For instance, elevation gradually varies across a landscape from high elevations to low elevations filled with hills,
valleys, and flatlands, and at every location, we can measure the height of the
land. Similarly, if you were standing at a ranger station in a park and preparing a rescue plan for stranded hikers, you would want to know the distance
from the station to every location in the park. Thus, you could have two layers
in your GIS—one showing the elevation at every place and another showing
the distance to the park at each location.
How Can You Represent the Real World
as Continuous Fields?
When representing these kinds of continuous fields in GIS, the three vector objects (and thus, the vector data model) are often not the best way of
representing data. Instead, a different conceptualization called the raster
data model is usually used. In the raster data model, data is represented
Digital Line Graphs are made freely available by the
USGS. A good online utility for examining available
DLGs is EarthExplorer, a Website designed to facilitate access to a wide range of geospatial data (not
just DLGs). To get started, open your Web browser
and go to http://edcsns17.cr.usgs.gov/NewEarthExplorer/. Using EarthExplorer is a four-step process. First, select the Search Criteria tab and type
the name of a location of an address or place name
(such as Reno, Nevada). Next, click on Data Sets
and place checkmarks in the DLG 1:100k and DLG
Large Scale boxes under the Digital Line Graphs
option. Next, click Additional Criteria—this is where
you could further refine your search, if desired. Last,
click Results to see what EarthExplorer turned up.
From the available options in the results, you can
see the footprint that the DLG would cover. If desired, DLGs can be downloaded to use in GIS (they
may need to be imported into a specific format to
use, and the USGS requires a user to register and
log in with them to download data). Check your local
area to see what types of DLGs are available.
Hands-on Application 5.3
USGS Digital Line Graphs
continuous field view
the conceptualization
of the world that all
items vary across the
Earth’s surface as
constant fields, and
values are available at
all locations along the
field.
raster data model a
conceptualization of
representing spatial
data with a series of
equally spaced and
sized grid cells.