Coordinate Systems
• Global Coordinate System – Latitude, Longitude
and elevation
• UTM – eastings and northings, reference points
are the equator and the central meridians (60 of
them for 60 zones) Extends from 84° N to 80 °S
• State Plane – State specific, northings and eastings
NAD 27 is based on feet, NAD 83 is based on
meters.
• There are many many more.
Reference Ellipsoids and Datums
• Clark 1866 spheroid – NAD 27
(Sometimes Clark 1880?)
•
•
•
•
GRS 80 spheroid – NAD 83
WGS 84 spheriod – NAD 83
Sphere – for world projections
Many many more.
Types of Projections
with examples
• Planar (Azimuthal) - North Polar Stereographic
• Conic – Lambert Conformal Conic, Albers
Equal Area, Polyconic
• Cylindrical – Mercator and Transverse Mercator
GIS is composed of layers
• Layers
–
–
–
–
land/water
roads
urban areas
pollution levels
• Data can be
represented by
VECTORS, or
• Data can be
represented by
RASTERS
Vector GIS
• Every object is given a
location
• Database consists of
points, lines and
polygons
Raster GIS
• Every location
given an object
• Grid
– Cells or pixels
Creating A Raster
• Overlay grid on real world
• give each cell a value
Cell Values
• Assume only one value per cell in any given
layer
• How do you decide what value to give the
cell?
– Value of greatest proportion?
– Value of the most important feature?
– Some systems allow for percent composition,
edge effect
Cell Values
•
•
•
•
Integer
Real Numbers
Alphabetic
Interpret using legend
Characteristics of a Raster
• Resolution
– smallest unit discernible
– often grid size, but not always
• Orientation
– angle that grid makes with true north
• Value
– information stored in cell
Characteristics of a Raster
• Zones
– areas of continuous values
• e.g. county, land parcels, etc.
• Class - term used to describe all the zones
of same value in a layer
A
B
Zones
A
A
A Class
Characteristics of a Raster
• Location
–each cell has a unique location
• often defined by bottom left corner
• X, Y coordinate
Storing the Information
• Full length encoding
– store every single cell’s value individually
1
1
1
1
1
1
1
2
encoded as
1
1
2
2
1111111211221112
1
1
1
2
Storing the Information
• Run Length encoding
– compress data
1
1
1
1
1
1
1
2
encoded as
1
1
2
2
711221223112
1
1
1
2
Storing the Information
• Quadtree Encoding – compress data
Break area into quads:
if HOMOGENEOUS, encode
if non-homogeneous, break down further
Map
A A A B
2
A A A B
A A B B
A A A B
0
3
1
e.g. 0 and 3 is homogenous
1 and 2 are not: these are broken down further
0
A
1
2
A
3
0 1 2 3 0 1 2 3
A B B B A B A B
Sources of Raster Data
• Scanned Images
– Aerial photographs
– maps
• Satellite images
• Many packages work on RECTIFYING these
images
– photograph’s scale is not constant across image
Vector GIS
• Every object is given a
location
• Database consists of
points, lines and
polygons
Creating a Vector GIS
• Define Points
–
–
–
–
fundamental property of a vector GIS
no dimensions, but they have a location
can have attributes associated with it
used for utility poles, sampling points, wells
etc.
Point_id
1
Well_id
0234-1
Depth
100
Creating a Vector GIS
• Define Arcs (lines)
–
–
–
–
line that joins points
also called chains and edges
has length and direction
attributes can include what is on right and left
side
– used to represent road, utility lines, rivers, etc.
Creating a Vector GIS
• Two Methods of Defining Polygons
– Polygon Storage
– Arc Storage
– Used to represent lakes, landuse categories,
forest stands, etc.
Polygon Storage
• Store Polygon as series of points, starting
and ending at same point
0,1
1,1
(1,0) (1,1) (0,1) (1,0)
1,0
• Each line encoded twice
• Difficult to dissolve boundary
Polygon Storage - Use Arcs
(more common)
•
•
•
•
0,2
d
0,0
Every arc stored as a sequence of points
Every polygon stored as series of arcs
Boundaries only stored once
Nodes are points where arcs meet or end
d
A
4,2
c B
a
4,0
6,2
g
e
f
6,0
Arc ID
a
b
c
d
e
f
g
Start
(0,0)
(4,0)
(4,2)
(0,2)
(4,0)
(6,0)
(6,2)
End
(4,0)
(4,2)
(0,2)
(0,0)
(6,0)
(6,2)
(4,2)
Polygon ID
A
B
Number of
Arcs
4
4
Arc ID's
a, b, c, d
c, e, f, g
Topology
• The geometric relationship between objects
located in space.
– Adjacency
– Containment
– Connectivity
Raster and Vector Summary
• Vectors have advantage of accuracy but not
good with continuous fields
• Vectors were used first - digitizing
• Earliest include ASCII (x,y coordinates but
got too large) then binary took over.
• Raster not good with lines or points but
good with continuous coverage areas.
• Raster has the mixed pixel problem.
Raster Advantages:
• Simple data structure
• Compatible with remotely sensed or scanned data
• Simple spatial analysis procedures
Raster Disadvantages:
• Requires greater storage space on computer
• Depending on pixel size, graphical output may be less
pleasing
• Projection transformations are more difficult
• More difficult to represent topological relationships
Vector Advantages
• Requires less disk storage space
• Topological relationships are readily maintained
• Graphical output more closely resembles handdrawn maps
Vector Disadvantages
• More complex data structure
• Not as compatible with remotely sensed data
• Software and hardware are often more expensive
• Some spatial analysis procedures may be more difficult
• Overlaying multiple vector maps is often time consuming
Maps as numbers
• Binary 0000 1111
– Eight bits in a row are termed a byte
– 256 conbinations or 0 to 255
• ASCII text- American Standard Code for
Information Interchange - 256 standard
meanings for the values that fall into one
byte. (letters, numbers, special characters)
Vector Data Formats
• DXF Digital Exchange Format (Autocad)
ASCIIw/binary code mantains layers
• ArcView Shapefiles 9 (.shp)
• HPGL Hewlett-Packard Graphics Language
– A device-specific but industry standard language for
defining vector graphics in page coordinates (ASCII) no topology
• Adobe PostScript ASCII
Vector Data Formats
• DLG Digital Line Graph - USGS
(1:100,000 & 1:24,000) ASCII UTM
• TIGER U.S. Census Bureau
(Topologically Integrated Geographic Encoding
and Referencing)
– Geocoded block address-matching
– Topology correct, but accuracy problems
Raster Data Formats
•
•
•
•
•
DRG Digital Raster Graph
TIF Tagged Interchange Format
GIF Graphics Interchange Format
JPEG Joint Photographic Experts Group
GeoTiff – Geographical referenced TIF
Files