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Bikeway
Continuous Variation
Convergence of Information Principle
Discretize
Flood Hydrograph
Generations
Greenway
Local Knowledge
Parkette
Smoothing Filter
Surrogate
In the
City of Vancouver, a Bikeway is a street or pathway that has been re-
designed or "retrofitted"
specifically for cyclists. Often we see automobile traffic
diverted, or small parkettes
created. The investment in pedestrian facilities is not
as large as it would be on
a greenway, which is less common.
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The real world exhibits the property of continuous variation (related to
"infinite
variation", sometimes used
interchangeably), which is that continuous values tend
to "fade" into each other,
producing what is conceptually a smooth and ever-
changing surface. Temperature,
for example, exhibits continuous variation, as its
values change minutely with
every millimetre and every millisecond. Humans have
an innate appreciation of
this, but computers need everything in digital (ideally
boolean/binary) form in order
to process it. In order to compute things, we must
first discretize
them, or sample them so that discrete values can be taken and
mapped.
It is
the goal of Geography to know the world, but our studies have shown this
to be impossible - the Earth
is too complex and wonderful to ever be completely
understood, let alone mapped.
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Convergence of Information Principle
Simply
put, the more sources of information that you have, the more sound your
analysis
will be, ceteris paribus. Agreement from a large number of sources is a
lot
more
convincing that relying on information from one source, whch may be in
some
way
deficient. If your doctor tells you that you need an operation that you
may not
be
comfortable with, you may seek a "second opinion". By doing so, you are
using
the
convergence of information principle.
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The act of "simplifying" or taking samples from continuous
data in order that it be
better processed. A crude,
non-geographic example could be the transition from
vinyl to compact disc. Records
reproduce music through the continuous, variable
grooves on the record's surface.
These exhibit continuous variation. A compact disc,
onthe other hand, is scanned
by a laser, which reads something as either magnetized
or demagnetized, either 1
or 0; there are no shades of meaning in this discretization.
Note that
while compact discs may be cheaper, faster, more efficient, and can
hold much more data, the
quality (i.e. intensity) of the sound is lost - this is the quality
of depth that is provided
by continuously varying data.
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A
graph showing time on the x axis, and stormwater flow on the y axis. Rainfall
is
often included. These graphs
are used to show the temporal distribution of water flow
in streams. Urban flood hydrographs
are more "peaked" due to the speed with which
water runs off impermeable
surfaces like pavement and rooftops. (see text)
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"Generations"
of error are a way of thinking about the propagation and "magnification"
of error in datasets. Each
time the format or structure of a dataset os changed, the process
often inadvertently introduces
new values through processes like interpolation or
generalization; processes
that are acceptable only at sufficiently small scales. Tracing the
"lineage"
of a dataset is a good way to determine the number of "generations" of
error
that have been introduced.
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In the City of Vancouver, a Greenway is a pathway (or, in some cases,
a series of
sidewalks)
with an enhanced pedestrian environment. The Ridegway, for example, is
a greenway
which cuts east-west across the city and includes public art, benches,
special
landscaping, special sidewalk paving, and so on. All greenways are also
considered
bikeways.
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The "lineage" of a dataset refers to its "ancestry", or from where and
by what process
it
was derived. For example, the lineage of many of our current DEMs can be
traced
back
to the digitizing of paper maps. This os something to keep in mind when
we are
speking
of error propagation and counting the "generations"
of error in a dataset.
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Local
knowledge is the additional information about a study area that is brought
to an analysis through the
person or group carrying out the analysis. Even this definition
is likely too restrictive,
given the wide range of forms this type of information and general
familiarity can come in.
It may derive from personal experience, travel, co-incidence,
or any number of other sources.
It is often used to supplement analysis and provide a
"gut feeling" indication
that is often invaluable.
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A Parkette (also called a "Pocket Park") is a small neighbourhood park
that may
created
out of a disused (or deliberately sealed) street right-of-way. Parkettes
are also
sometimes
created out of building lots in residential neighbourhoods.
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A smoothing, or high-pass, filter, is a spatial filter used to "average"
out pixel values in
a raster
image. a "roving window" scans each pixel and applies a new value to it
based
on
the values of its neighbours. These "windows" are generally square, and
of odd-
numbered
dimensions. (eg. 3 x 3, 5 x 5, the most common is 3 x 3.)
New values are determined by the application of a matrix - in this case,
a 3 x 3 matrix
consisting
of nine values of 1/9. Each pixel value is multiplied by 1/9, and the centre
cell
is
given a new value equal to the sum of all of these products.
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Surrogate (as in Surrogate Data)
Surrogate
data is used in cases where the originally desired data is either unavailable
or
would
be too costly to collect. An example might be the case of a team of field
biologists
who,
in seeking to map the distribution of a particular insect or bird, might
instead map
the
types of habitats the insect or bird is known to live in. This introduces
some uncertainty,
however,
in some cases it is the only feasible option.
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