A model is a

1
What is a model?
A model is a synthesis of data information
about how the data interact.
Geographical data are locations and attributes at
locations ...
2
What results from the locational referencing of
geographic attribute data? From the infinite
numbers of objects, their characteristics and
relationships in the world, a limited set is
selected. The result is a conceptual model of
reality.
locational data (x,y) attribute data (z) a
spatial data model
3
The conceptual model of reality becomes, in a
GIS, a logical data model encoded in digital
form. This allows us to visualize it and work
with it using computer technology.
4
Locational Systems
Methods of describing locations on the earths
surface or on maps of the earths surface
1. Geographical co-ordinates (f, l)
2. UTM co-ordinates (E, N)
5
Latitude, f, of a point the angular distance
north or south of the equator Longitude, l, of a
point the angular distance east or west of a
designated meridian
Meridian a line of constant longitude Parallel
a line of constant latitude
6
l, f
Co-ordinates of point P
Equator
f
0º
Prime Meridian
l
0º
7
Ellipsoid a description of the shape of the
earth
Peter H. Dana, The Geographer's Craft Project,
Department of Geography, The University of
Colorado at Boulder. See http//www.colorado.edu/
geography/gcraft/notes/coordsys/gif/refellip.gif
8
Datum a reference system for surveying and
mapping
vertical datum a level surface to which
elevations are referenced horizontal datum a
network of control points based on a description
of the earths shape, used as a reference for
position
9
North American horizontal datums 1. NAD 27 -
based on the Clarke 1866 ellipsoid 2. NAD 83 -
based on the GRS80 (WGS84) ellipsoid
10
The Universal Transverse Mercator co-ordinate
system
The UTM is both a map projection and a
rectangular grid system for spatial referencing
11
a UTM zone is 6º wide
How many zones are there in total?
12
There are 60 zones, numbered 1-60 from 180º West
to East
Ottawa is near the central meridian of zone 18.
13
Superimposed on the zone is a rectangular grid.
The basic grid lines are 100,000 metres (100 km)
apart. The grid lines coincide with the equator
and the central meridian of the zone.
14
The False Origin for the northern hemisphere for
UTM co-ordinates (eastings and northings) is
500,000 metres west of the intersection of the
equator and the central meridian of the zone.
False origin(0,0)
500,000 m
15
MTM (Modified Transverse Mercator)
co-ordinates Instead of 60 x 6º zones, the MTM
system has 120 x 3º zones. The False Origin is
304,800 metres from the central meridian of the
zone. MTM co-ordinates are used in Canada in
Quebec and Nova Scotia.
16
Spatial data structures
All GISs store maps and map data as numbers.
There are two main models for structuring these
data

• raster
• vector

17
Raster data use a grid or cell structure to hold
the data. Vector data composed of points
represented by exact co-ordinates which define
points, lines, polygons.
18
point - pair of co-ordinates
end node
line - series of points
polyline - multiple arcs
start node
vertex
end node
start node
19
polygon - series of lines enclosing an area
vertex
start/end node
vertex
20
Kenneth E. Foote and Donald J. Huebner, The
Geographer's Craft Project, Department of
Geography, The University of Colorado
21
Kenneth E. Foote and Donald J. Huebner, The
Geographer's Craft Project, Department of
Geography, The University of Colorado
22
Raster structure

• Advantages
• Simple data structure
• Compatible with remotely sensed or scanned data
  
• Simple spatial analysis procedures
• 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

Kenneth E. Foote and Donald J. Huebner, The
Geographer's Craft Project, Department of
Geography, The University of Colorado
23
Vector structure

• Advantages
• Requires less disk storage space
• Topological relationships are readily maintained
  
• Graphical output more closely resembles
  hand-drawn maps
• 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

Kenneth E. Foote and Donald J. Huebner, The
Geographer's Craft Project, Department of
Geography, The University of Colorado