Basic Concepts of GIS
January 29, 2013
What is GIS?
“A powerful set of tools for
collecting, storing, retrieving,
transforming and displaying
spatial data”
A Geographic Information System requires:
(1) Data containing locational information
(2) An ability to store or readily compute the spatial
interrelations that describe how the data are connected (also
known as topology)
Having information about how the data was collected, its
precision and accuracy (metadata) is preferable, but not
essential.
Topology
Computational geometry – branch of mathematics that
deals with the spatial relationships between and among
objects
All features/objects are one of the following types:
0 dimensions - point (node)
1 dimension - line (arc, link)
2 dimensions - area (polygon)
3 dimensions - volume
Spatial relations example
Examples of Spatial/Topological Relations:
Belongs (New Castle, Kent and Sussex belong to Delaware)
Comprises (Delaware comprises New Castle, Kent and Sussex)
Located in (The city of Wilmington is located in New Castle)
Borders on (New Castle borders on Kent but not Sussex)
Is a certain distance from (Wilmington is 47 miles from Dover)
Is within a certain distance of (Dover is within 60 miles of every
point in the state of Delaware)
Until about 1995, a GIS stored topological information
explicitly:
Node topology - a listing of nodes and the links that meet
at each node
Link topology - a listing of links and the nodes that bound
them, as well as the polygons to the left and right of each
link
Polygon topology - a listing of polygons and the links that
define them
Since that time, topological information is not stored, but
calculated on the fly using various algorithms (more on
this later!)
The shape file has long been the standard file type for GIS
information
Consists of a minimum of three parts:
- .shp – the feature geometry itself
- .shx – shape index format; a positional index to
allow seeking forwards and backwards quickly
- .dbf – the data attributes
A shape file contains a collection of points, lines, or
polygons – never a mixture!
Find the distance from Albany to Buffalo:
GIS vs. non-GIS
GIS program, or Google Earth: Simple calculation
executable through a drop-down menu
Non-GIS: Obtain the coordinates for Albany and Buffalo.
Type in the Pythagorean distance formula. If your
coordinates are in latitude and longitude, add cosine
terms to account for the curvature of the earth. Add
additional trigonometric terms to account for the nonspherical nature of the earth.
Find the counties that border Albany County:
GIS program: Simple calculation executable through a dropdown menu
Regular database (brute force approach): Create a table of all
coordinate points defining the borders of every county in New
York. Match Albany county coordinates against all county
coordinates. (An exact match is not good enough if coverage
is not clean – or if the Hudson River is stored as a separate
object – you would then need to write code to find points that
are within some small distance of one another.)
Google Earth: No obvious solution
2 Broad Models of Reality:
1. Space is occupied by entities: real-world phenomena that
are not divisible into phenomena of the same kind.
Examples:
County
Building
Disease case
2. Space is occupied by continuously varying attributes.
Examples:
Air pollution
Temperature
Elevation
Vector vs.
raster
2 Broad Models of Reality:
1. Space is occupied by entities: real-world phenomena that
are not divisible into phenomena of the same kind.
Examples:
County
Building
Disease case
Vector model
2. Space is occupied by continuously varying attributes.
Examples:
Air pollution
Temperature
Elevation
Raster model
Vector and raster representations of a river:
Some basic GIS functions:
Distance
Perimeter
Area
Centroid
Centroid
Distanc
e
Area
and
Perim
eter
Area and Perimeter
Centroid
Some more basic GIS functions:
Intersect
Buffer
Polygon Overlay
Combine/Split Objects
Intersect
Select all from SchoolDist, ZIP where
SchoolDist.obj intersects ZIP.obj
Buffer
Buffer
+Intersect
Polygon
Overlay
Combine/split objects
Advanced Spatial Analysis Functions in GIS:
Shortest Path
Routing
Location-Allocation
Nearest Neighbor
Spatial autocorrelation
Spatial interpolation
Advanced
functions
Shortest path
Routing
Location-Allocation
Nearest neighbor
Spatial autocorrelation
High
None
Spatial interpolation
+
+
+
+
+
+
+
+ +
+
+
++
+
+
+
Now to look at some algorithms…
Point in polygon: Ray-tracing and the even-odd rule
Many points in many polygons: The grid-based method
Polygon intersection: The bounding-box rectangle method
Shortest path: Dykstra’s algorithm