UNIT 14 - VECTOR GIS CAPABILITIES
Compiled with assistance from Holly J. Dickinson,
State University of New York at Buffalo
A. INTRODUCTION
B. SIMPLE
DISPLAY AND QUERY
Display
Standard Query
Language (SQL)
Boolean operators
SQL extensions for
spatial queries
C. RECLASSIFY,
DISSOLVE AND MERGE
Steps
Forestry example
City zoning example
D. TOPOLOGICAL
OVERLAY
Point in polygon
Line on polygon
Polygon on polygon
("Polygon overlay")
Example
Spurious polygons
E. BUFFERING
REFERENCES
EXAM AND
DISCUSSION QUESTIONS
NOTES
This unit would be
illustrated well with a series of overlays of a real area which demonstrates
the problems discussed here using simple examples.
UNIT 14 - VECTOR GIS CAPABILITIES
Compiled with assistance from Holly J. Dickinson,
State University of New York at Buffalo
A. INTRODUCTION
- Analysis functions with vector GIS are not
quite the same as with raster GIS
- More operations
deal with objects
- Measures such as
area have to be calculated from coordinates of objects, instead of counting
cells
- Some operations are more accurate
- Estimates of
area based on polygons more accurate than counts of pixels
- Estimates of
perimeter of polygon more accurate than counting pixel boundaries on the edge
of a zone
- Some operations are slower
- E.g. overlaying
layers, finding buffers
- Some operations are faster
- E.g. finding
path through road network
B. SIMPLE DISPLAY AND QUERY
Display
- Using points and "arcs" can display
the locations of all objects stored
- Attributes and entity types can be displayed by
varying colors, line patterns and point symbols
ARCMAP - Vector display
- May only want to display a subset of the data
- E.g. want to
display areas of urban land use with some base map data
- Select all political
boundaries and highways, but only areas that had urban land uses
- How would the user do this?
- E.g. one of the
layers in a database is a "map" of land use, called USE
- Area objects on
this layer have several attributes
- One attribute,
called CLASS, identifies the area's land use
- For urban land
use, it has the value "U"
- Need to extract
boundaries for all areas that have CLASS="U"
Standard Query Language (SQL)
- Different systems use different ways of
formulating queries
- Standard Query Language (SQL) is used by many
systems
- SQL phrase structure:
SELECT <attribute name(s)> FROM
<table> WHERE <condition statement>
- E.g. SELECT FROM
LANDUSE WHERE CLASS="U"
- This selects
only the objects for display - no attributes are retrieved by the query
- SQL examples using a list of student names:
- SELECT name FROM
list (selects all names)
- SELECT name FROM
list WHERE grade = "A" (selects names of students receiving an
"A")
- SELECT name FROM
list WHERE cumgrade > 3.0 (selects names of students with a cumulative gpa
greater than 3.0)
- RELATIONAL:
>, <, =, >=, <=
- ARITHMETIC: =,
-, *, / ( FOR USE ONLY ON NUMERIC FIELDS
i.e. RATIO / INTERVAL)
- BOOLEAN: AND,
OR, NOT, XOR
Boolean operators
- used to combine conditions
- E.g. WHERE cumulativegrade
> 3.0 AND grade = "A" (selects students satisfying both conditions
only)
- Boolean operators can have a spatial meaning in
GIS as well
- E.g. when two
maps are overlaid, areas (polygons) that are superimposed have the
"and" condition
- A spatial representation is used to illustrate
Boolean operators in the study of logic, through the use of diagrams called
Venn diagrams
- Thus GIS area
overlay is a geographical instance of a Venn diagram
- "XOR"
is the "exclusive or" - A xor B means A or B but not both
SQL extensions for spatial queries
- Some systems allow specifically spatial queries
to be handled under SQL
e.g. WITHIN
operator
- SELECT
<objects> WITHIN <specific area>
- The criteria for these spatial searches may
include searching within the radius of a point, within a bounding rectangle, or
within an irregular polygon
C. RECLASSIFY, DISSOLVE AND MERGE
- Reclassify, dissolve and merge operations are
used frequently in working with area objects
- these are used
to aggregate areas based on attributes
- consider a soils map:
- we wish to
produce a map of major soil types from a layer that has polygons based on much
more finely defined classification scheme
Steps
1. Reclassify areas by a single attribute or some
combination
- E.g. reclassify
soil areas by soil type only
2. Dissolve boundaries between areas of same type
- By delete the
arc between two polygons if the relevant attributes are the same in both polygons
3. Merge polygons into large objects
- Recode the
sequence of line segments that connect to form the boundary (i.e. rebuild
topology)
- Assign new ID
#'s to each new object
Forestry example
- consider a forestry GIS where the forest is
divided into "stands", average size 10 ha:
- Each stand
carries a list of attributes, including tree species and average tree age
- Attributes apply
homogeneously to area of each stand
- Boundary occurs
between stands whenever at least one attribute changes
- Problem: identify all harvestable areas of
white spruce
- Assign new
attribute “harvestable” to each stand
- Value =
"y" if white spruce AND age > 50 years
- Value =
"n" otherwise
- After assigning
new attribute, all others can be dropped
- Now wish to identify harvestable areas, each
may be merger of several individual stands
- dissolve
boundaries between polygons with same value of "cuttable" attribute
- merge polygons
into larger objects
City zoning example
- Need to know how many individual landuse zones
have been created in the city and how these are distributed geographically
- Each land parcel in the city has a zoning
attribute attached to it
- Dissolve boundaries between parcels if the
zoning is the same
- Result can be a map showing large areas of
similar zoning classes
D. TOPOLOGICAL OVERLAY
- suppose individual layers have planar
enforcement (required in many systems, not all)
- When two layers are combined
("overlayed", "superimposed") the result must have planar
enforcement as well
- New intersection
must be calculated and created wherever two lines cross
- A line across an
area object creates two new area objects
- Topological overlay is the general name for
overlay followed by planar enforcement
- Relationships are updated for the new, combined
map
- Result may be information about relationships
(new attributes) for the old (input) maps rather than the creation of new
objects
- E.g. overlay map
of school districts on census tracts
- Result is map
showing every school district/census tract combination
- For each
combination, the database contains an area object
- however, concern
may be with obtaining the number of overlapping census tracts as a new
attribute of each school district rather than with new objects themselves
Point in polygon
- Overlay point objects on areas, compute
"is contained in" relationship
- Result is a new attribute for each point
- E.g. combine
wells and planning districts, find district containing each well
Line on polygon
- Overlay line objects on area objects, compute
"is contained in" relationship
- Lines are broken at each area object boundary
- Number of output
lines is greater than number of input lines
- Containing area is new attribute of each output
line
- E.g. combine streams and counties, find county
containing each stream segment
Polygon on polygon ("Polygon overlay")
- Overlay two layers of area objects
- Boundaries are broken at each intersection
- Number of output areas likely greater than the
total number of input areas
- E.g. input watershed boundaries, county
boundaries, output map of watershed/county combinations
- After overlay we can recreate either of the
input layers by dissolving and merging based on the attributes contributed by
the input layer
Example
- Wish to use find those areas that are the best
land for timber harvesting
- After overlay, each original layer contributes
attributes to the combined layer
- We get the final map by selecting the desired
attributes of the combined layer
- SELECT FROM
OVERLAY WHERE Species = "Jack pine" AND Soil = "C"
Spurious polygons
- During polygon overlay, many new and smaller
polygons are created, some of which may not represent true spatial variations
Overhead - Sliver or spurious polygons (3 pages)
- physically
overlay pages 1 and 2, page 3 shows resulting spurious polygons
- The small, invalid polygons are called spurious
or sliver polygons and can be a major problem in polygon overlay
- spurious polygons arise when two lines are
overlaid which are actually slightly different versions of the same line
- if the same line
occurs on two input maps, the digitized versions may be slightly different
- in many cases
the lines on the source maps have been compiled from different sources, but are
nevertheless the same line on the ground
- E.g. a road may
be part of a county boundary, also the boundary between two fields or two soil
types or two vegetation types
- The problem cannot be removed by more careful
digitizing - more points simply leads to more slivers
- Some GISs allow the user to set a tolerance
value for deleting spurious polygons during overlay operations
diagram
- If the tolerance
is set too high, some legitimate polygons may be deleted
- If set too low,
some erroneous polygons will remain
- Deletion rules
might also be based on shape, as spurious polygons tend to be long and thin
E. BUFFERING
- A buffer can be constructed around a point,
line or area
- buffering
creates a new area, enclosing the buffered object
Overhead - Buffering
- Applications in transportation, forestry,
resource management
- Protected zone
around lakes and streams
- Zone of noise
pollution around highways
- Service zone
around bus route (e.g. 300 m walking distance)
- Groundwater
pollution zone around waste site
- Options available for raster, such as a
"friction" layer, do not exist for vector
- buffering is much more difficult in vector from
the point of view of the programmer
- Sometimes, width of the buffer can be
determined by an attribute of the object
- E.g. buffering
residential buildings away from a street network:
- three types of
street (1, 2, 3 or major, secondary, tertiary) with the setbacks being 600 feet
from a major street, 200 feet from a secondary street, and only 100 feet from a
tertiary street
- Problems with buffer operations may occur when
buffering very convoluted lines or areas
diagram
REFERENCES
Documentation for
ARC/INFO (user manuals, Understanding GIS) provides an overview of
vector GIS functionality for a commonly available system.
Burrough, P.A.,
1986. Principles of Geographical Information Systems for Land Resources
Assessment, Clarendon, Oxford.
Chapter 5 on data analysis.
Lusardi, Frank,
1988. The Database Expert's Guide to SQL, McGraw-Hill Book Co., New York. Good introduction to Standard Query Language.
EXAM AND DISCUSSION QUESTIONS
1. Compare the
buffer function in raster and vector systems, in terms of results, options
offered by systems, and flexibility.
2. The skeleton
function moves the boundary of an area object inwards rather than outwards.
Show how this would work using a simple diagram, and what happens as the amount
of movement increases. What objects might be created by this operation, and
what uses can you devise for them?
3. What are
spurious polygons and what characteristics of data cause them?
4. Section D listed
three types of topological overlay between points, lines and areas. Are there
others? What applications might they have?
