GIS Analysis Functions
Vicki Drake
Earth Science Department
Santa Monica College
GIS Analysis Functions
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GIS is distinguished from other Information Systems due to its spatial analysis functions
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Spatial analysis functions are used to answer questions about the real world using GIS
databases as a model of the real world
GIS Analysis Functions
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In order to make the most use of the information provided, the right questions must be
asked
GIS Analysis Functions
Questions Categories:
What are the data? What are the patterns in the data? What changes are possible in the
data?
Functions Categories:
Storage and retrieval of data – Constrained query – modeling reality
Answers Categories:
Presentation of current data – Finding new pattern in data – Predicting new information
from data
GIS Analysis Functions
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By defining the most important answers needed, appropriate questions and analysis
methods can be created
Knowing how to combine functions in a GIS system can help produce an intelligent use of
the available data
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An unfortunate side effect of the computer generation, is the ability to produce worthless
information (and very poor maps) at a rapid rate!
GIS Analysis Functions
• Spatial analysis techniques are an attempt to create an understandable image of reality.
• In Basic Spatial Analysis, data may be analyzed at various levels.
– Sorting data in attribute tables for presentation
– Arithmetic, Boolean and statistical operations may be performed on attribute tables
– New data may be compiled based on original and derived attributes or based on
geographical relationships
– Within each level, the operations used may be logical, arithmetic, geometric, statistical
or a combination of any of these four types
GIS Analysis Functions – Basic Spatial Analysis
• Logic Operations – logical searches in databases normally use set algebra or Boolean
algebra
• Set Algebra – three operators: equal to, greater than, less than and combinations
– The practical applications of these include:
• Identifying attribute minima or maxima
• Selection or isolation of particular values selected for a ranking in a thematic layer
• Boolean Algebra: uses the AND, OR, NOR, and NOT operators to test for “True” or “False”
– Venn diagrams – shaded area represent true statements
GIS Analysis Functions
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General Arithmetic Functions – performed on both attribute and geometric data
Operators include: addition, subtraction, multiplication, division, exponential, square root,
and trigonometric functions
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Purposes for operations may include:
– Reclassification of land use, or soil types
– Conversion of distances along roads
– Performing numerical operations on either quantitative or qualitative attribute data
GIS Analysis Functions
© Vicki Drake
SMC 2000-2001
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Attribute data may also be processed to discern new patterns in the data
In the Classification Process, attributes are grouped according to limits set by user
– For example: three classes may be set up for the attribute “year”: A = before 1990; B
= 1991-1995; C = 1996-2000.
– Each object with a “year” attribute is assigned a new year-class attribute of A, B or C
GIS Analysis Functions
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Reclassification involves changing attribute values without altering geometries – only
arithmetic and some statistical operations are used to assign new attribute values.
– Reclassification of polygons and combining polygons
GIS Analysis Functions
• Integrated processing of geometry and attributes is a means by which a GIS can request,
retrieve and display all information on each object type represented.
• Arithmetic, logical and statistical operations may be performed on the new attribute table.
• A new thematic map may be compiled using the geometry and attributes
GIS Analysis Functions
• Geographic Information must be organized within a GIS for optimal convenience and
efficiency of use
• Paper maps organize geographic information as sets of themes – roads, streams, land
cover types, etc.
• Paper maps are constrained by size as to how much geographic area is covered –
sometimes several maps in a series are used for large geographic areas
• Separate maps are necessary to show information at different levels of detail and scales
• Paper maps act as both the “storage” medium and the “presentation” medium of the
geographic data
GIS Analysis Functions
• Within a GIS, storage and presentation of geographic data are separate – the level of detail
of information stored and/or presented is limited to the resources of the technology
(hardware and software capabilities)
• When presenting data – the scale, level of detail of information, and symbols used to
represent reality can be selected at the time of production (remember: presentation is
possible at a lesser detailed level than was stored, but not at a more detailed level!)
• Large geographic areas may be subdivided into smaller units for storage (as separate data
files) and the GIS will ensure that adjacent units match along their borders
• Different types of thematic information (map layers in a paper map) are treated as data
layers in a GIS
GIS Analysis Functions
A data layer is a set of logically related geographic features and attributes
GIS Analysis Functions
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The major analysis functions of a GIS can be categorized into four major groups (from GIS:
A Management Perspective, by Aronoff)
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(1) Maintenance and Analysis of Spatial Data
(2) Maintenance and Analysis of Attribute Data
(3) Integrated Analysis of Spatial and Attribute Data
(4) Output formatting
GIS Analysis Functions
• Maintenance and Analysis of Spatial data
– These functions are used to transform, edit and assess the accuracy of spatial data files
– The capacity to transform source data into a data form used within the GIS system and
being able to edit the data files is a basic requirement
– Sub-categories include: Format Transformation, Geometric Transformation, Map
Projection Transformation, Conflation, Edge Matching, Graphic Element editing, and Line
Coordinate thinning
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SMC 2000-2001
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GIS Analysis Functions – Spatial Data
• Format Transformation – The process by which data files are transformed into the data
structure and file format used by a GIS
• Raster data files to raster-based GIS needs little to no transformation
• Vector-based systems - ”building” topology from coordinate data is a necessity
• Transformation process may be time-consuming and costly if collected data not in a form
suited to GIS – I.e. CAD drawings are not topologically structured, and if used in a GIS are
difficult to transform
GIS Analysis Functions – Spatial Data
• Geometric Transformation – the process by which one data layer is adjusted to another
data for overlay operations
• Adjustment operation is called “Registration” as the different data layers are registered to
one common coordinate system – or registered to a “standard” data layer
– Relative Position – registering multiple layers by identifying common features for
registration (roads, streams, etc.). aka – Rubber Sheeting (stretching of one data
layer to another) (Propagation errors a possibility, if mis-registering!)
– Absolute Position – registering each data layer, first, individually to same geographic
coordinate system – then registering to each other (No propagation errors)
GIS Analysis Functions – Spatial Data
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Transformations between Geometric Projections - a mathematical transformation of a
spherical surface (the 3-dimensional globe) to a flat surface (the 2-dimensional map)
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Latitude and longitude coordinate system developed to uniquely reference points on the
Earth’s surface
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Map projections have inherent distortions in representations of area, shape, distance and
direction
GIS Analysis Functions – Spatial Data
• Conflation – the process of reconciling positions of corresponding features in different data
layers
– An important process in studying changes over time
• Conflation functions used to minimize “sliver” polygon creation by creating a template or
standard to which all other layers will be compared and reconciled
• Conflation can be a manual or computerized procedure
– Used to fix position of shoreline of reservoir (fluctuates over time); baseline position of
meandering rivers, etc.
GIS Analysis Functions – Spatial Data
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Edge Matching – adjusting the position of features that extend across map boundaries.
In a GIS, large coverages may be entered from separate map sheets and then organized to
present data as a continuous geographic coverage.
Data input errors may not allow map boundaries to align perfectly – GIS software makes
the necessary adjustment
GIS Analysis Functions – Spatial Data
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Editing Functions – These may include adding, deleting or changing the geographic position
of features
– Sliver polygons – created by polygon overlays and digitizing errors
• Operator supervision may be needed to correctly identify and remove these
– Line-snapping – corrects undershoots by connecting lines to nodes
GIS Analysis Functions – Spatial Data
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Line coordinate Thinning – reduces the quantity of coordinate data stored in a GIS
– Subsequent reduction in data volume may increase processing speed of GIS
– Degree of thinning must be controlled by GIS operator –
GIS Analysis Functions – Buffering operations
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Buffering is an important spatial analysis function, constraining space around selected land
features.
Buffering combines spatial data techniques and cartographic modeling.
© Vicki Drake
SMC 2000-2001
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Points, lines and polygons can be buffered, as well as raster pixels or sets of pixels
GIS Analysis Functions
• Buffering Example: A national historic and archeological site needs to be accessible to the
public, while protecting sensitive habitats and valuable features
• Setting constraints will allow the GIS to find the best way to protect sensitive areas, while
allowing public access. For example:
– 1 meter buffer for stream banks
– 2 meter buffer for cliff edges
– 50 meter buffer for active archeological sites, 10 meter buffer for inactive sites
– Paths must be connected with crossing sensitive habitat areas
GIS Analysis Functions
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This example demonstrates how a variable buffer can be performed on a selected feature
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Another buffering example: A logging company with restrictions as to the type and
quantity of trees they can harvest.
Descriptive spatial analysis can be performed on the buffered and ancillary data:
– Distance
– Area
– Intersection
GIS Analysis Functions
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GIS is used to determine the best location, within the required parameters.
GIS Analysis Functions
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Restrictions:
– No harvesting within 10 km of Shrine
– No harvesting within 1 km of sea, lake or river
– Logging sites must be within 5 km of existing roads for accessibility – no new roads can
be built
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SMC 2000-2001
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© Vicki Drake
SMC 2000-2001
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GIS Analysis Functions
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The logging sites must be within the ‘blue’ polygon buffer (5 km), and outside of the ‘black’
buffers (10 km of shrine, 1 km of water bodies, rivers)
How accomplish? Use GeoProcessing Wizard in ArcView to “Clip” one polygon inside
another, or use ARCINFO “erase” to extract one polygon from inside another
The shrine, water bodies and river buffer zones must be ‘extracted’ from the possible
logging sites
GIS Analysis Functions
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So far, ARCINFO has deselected water bodies, rivers, and the shrine.
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What about the elevation and slope issues? Logging sites should be below the snow line,
and should not be on a slope >5 degrees
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ARCINFO has also buffered the roads and given the general area where logging sites might
be possible.
Let’s see how to work with these issues
© Vicki Drake
SMC 2000-2001
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© Vicki Drake
SMC 2000-2001
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The light green represents Pine, while the light blue represents the Oak forest. The
light Green are the harvestable trees. Overlaying the polygons of ‘exclusion’ onto the Forest
layer, will finalize the decision of which trees may be harvested
The final decision: Trees in the light green section may be harvested. All other areas have
been eliminated based on either slope, vegetation, or buffers.
© Vicki Drake
SMC 2000-2001
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