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UNIT 3 – MODULE 6: Data
Analysis
TERMINOLOGY
• There are several terms that are important to
know when discussing data analysis:
– Entity – an individual point, line or area in a GIS
database.
– Attribute – data about an entity.
– Feature – an object in the real world to be
encoded into a GIS database.
– Data Layer – a data set for the area of interest in
a GIS.
– Image – a data layer in a raster GIS.
– Cell – an individual pixel in a raster image.
MEASUREMENTS IN GIS
• A standard component to a GIS is the ability to
calculate lengths, perimeters & areas.
• All GIS measurements are an approximation.
• There are three raster GIS measurements
available for calculating distance:
– Pythagorean distance
– Manhattan distance
– Proximity
• For raster data, perimeter & area calculations
can be impacted by: 1) cell size, 2) origin, and
3) grid orientation.
MEASUREMENTS IN GIS
(Continued)
• For a vector GIS, distances are measured via
the Pythagorean theorem. To calculate
perimeters & areas, geometry is used.
Credit: NASA
QUERIES
• A way to retrieve specific data within a GIS
database.
• Allows you to ask questions by selecting a
specific criteria. A GIS database then
highlights the data that fits your query.
• Queries are useful throughout the GIS project,
particularly for checking data quality.
• There are two types of queries:
– Spatial
– Aspatial
ASPATIAL QUERIES
• Questions about a features’ attributes.
• Example #1: “How many luxury homes are
there?” This is an aspatial query because
location is absent from the question.
• Example #2: “Where are the luxury homes?”
This is a spatial query because location
(where) is being asked.
• Spatial and aspatial queries can be asked in
conjunction with one-another. Example:
“Where are the luxury homes that have more
than five bedrooms?”
RECLASSIFICATION
• As previously discussed, a raster GIS involves
a collection of cells that have values assigned.
• Values can be reclassified. This can result in a
new image if working with a raster land use
image. It could also be used to assign new
values to different land uses based on
ecological importance. It all depends on what
you’re trying to accomplish/show.
• Reclassification allows for a greater analysis
beyond the initial analysis.
EXAMPLE
• Top-left image shows
four different land
covers.
• Top-right image shows
two different land
covers after
reclassification.
• Bottom image shows
reclassification of
thematic values (leads
to a new image).
Credit: www.giscommons.org
Credit: www.enggarticles.com
BUFFERING
• A zone around an
object or map
feature.
• Measured in distance,
but can also be
measured in time.
• Very useful for
proximity analysis.
Credit: www.arcgis.com
*
VECTOR & RASTER BUFFER
Credit: www.geo.hunter.cuny.edu
MAP OVERLAY
• Allows the GIS user to
analyze two or more
different data sources.
• Has many applications.
Example: which hotels are
located 500 meters or less
of the main road? Can
apply a buffer zone, then
overlay hotel data.
• Two types of map overlay:
vector & raster.
Credit: Penn State University
VECTOR OVERLAY
• Also called a polygon
overlay.
• Relies heavily on geometry
& topology.
• There are three types of
vector overlay:
 Point-In-Polygon
 Line-In-Polygon
 Polygon-On-Polygon
Credit: Penn State University
TYPES OF VECTOR OVERLAY
Credit: ESRI
RASTER OVERLAY
• Also called a grid overlay.
• Everything represented by cells:
 Point – Single Cell
 Line – String of Cells
 Area – Group of Cells
• With raster overlay, layers can be
added, subtracted, multiplied or
divided.
• Two issues need to be considered
during this process: resolution &
measurements scales.
Credit: Penn State University
NETWORK ANALYSIS
• Network – a set of interconnected lines
making up a set of features through which
resources can flow.
• Examples: rivers, road, pipelines & cables.
• Different network analysis methods available
to solve problems:
– Shortest Path Method
– Traveling Salesperson
– Location-Allocation Modeling
– Route Tracing
Credit: www.irenepro.com
ROUTE TRACING
• The ability to trace flows of goods, people,
services & information through a network.
• One of several methods for conducting
network analysis.
• Can be useful for finding residents impacted
by a broken cable, or customers served by a
specific sewer main.
Credit: ESRI