### Geographic coordinate systems

```Geographic Information Systems for Resource
Management
FW 5550
Lecture 2
Arc-Node Topological Model
Most widely used vector data model.
Two units of construction
Arc is defined as a straight line vectors that start and
end at a node.
A node is an intersection point where two or more
arcs meet. Can also occur at the end of a “dangling”
arc (i.e. an arc that is not connected to another arc).
Topology- explicit representation of spatial relationships in
the vector data model. What about raster data model?
Georeferencing
Topology allows geocoding and address matching.
Explicit vs. Implicit Representation
Explicit representation- vector data. Location is
described by X, Y coordinates. Precise location.
Implicit representationraster data. Location is
described by pixel
coordinates.
Location of coordinatecenter of pixel. “Wiggle
Room”
From: MSU Extension
How well boundaries
are represented in
raster is function of
spatial resolution.
A triangulated irregular network (TIN) is a vector
format data structure used for the representation of a
continuous surface.
Composed of irregularly distributed nodes and arcs with
3-D coordinates (x,y, and z) that are arranged in a
network of non-overlapping triangles.
TINs are often derived from a digital elevation model
(DEM).
BTW- what’s wrong with this color scheme?
Using a TIN to model
elevation
Each triangle within the
TIN assumes a constant
value (elevation).
Areas of little change
(flat) will have fewer and
larger triangles.
Areas of rapid change
(steep slopes) will have
smaller, more frequent
triangles.
Continuous Data vs Discrete Data
Continuous data- thematic information that varies
without discrete steps
Discrete data- thematic information that represents
phenomena with distinct boundaries.
http://support.esri.com/en/knowledgebase/GISDictionary
/search
Spatial or Geographic Reference Systems Provide
The capability to situate measurements on a geometric
body, such as the earth; establish a point of origin and
orientation of reference axes; determine geometric
meaning of the measurements and units of the
measurements.
Bottom line- need X,Y coordinates to define location
The geographic coordinate system provides a
measure of absolute location as opposed to relative
Once information is linked to a geographic coordinate
system, it is referred to as spatial information.
Coordinate systems fall into two categories- geographic
and projected. It can be can be a simple grid or a
complex mathematical representation.
Geographic coordinate systems cover the earth’s
entire surface and are a “curved system”.
Projected coordinate system takes features found on
earth’s curved surface and projects them to a flat
surface and yet maintains topology. Also referred to as
“planar”.
Key Concepts- Week 1
1. What is a geographic information system?
2. How can earth surface features be represented in a
GIS?
3. What is an arc? What is a node?
4. What is the difference be explicit and implicit
representation?
5. Do 911 emergency systems use absolute or relative
address locations in rural areas? Why?
6. What 3 geometric shapes are used in map
projections?
7. Are topographic contour lines discrete or continuous
data?
8. Why does a TIN portray areas of rapidly changing
elevation more accurately than a DEM?
```