An overview of GIS data
Today’s Topic:
• GIS Data Models
• Data Types
• ArcGIS terminology
• Review of coordinate systems
2
2 basic types of Data Models
• Raster
• Vector
• Raster GIS
• Vector GIS
3
Raster GIS
• Raster is nothing but an array of grid cells
• Each cell is called Pixel (Picture element)
Example: IDRISI, ERDAS, GRASS
Pixel
Raster
DEM (Digital Elevation Model):
An example of Raster Data
400
450
500
600
600
300
350
450
400
450
250
200
150
150
400
0
50
100
250
450
5
Raster representation of
reservoir and highway
6
Raster GIS
• In Raster, each cell is
homogeneous
• It represents either one class or
another, nothing in between
• However, in reality, a cell is not
one thing or another.
7
Rasterizing of features
Mixel
Pixel
Raster Data
• Raster is good for
Continuous Data
• Continuous Data: When data
varies smoothly across the
area. Ex: elevation,
precipitation, etc.
• Raster model provides the
best representation for
continuously changing data
Vector Data
• Vector is good for Discrete
Data
• Discrete Data: When data
value remain same for an
area and then abruptly
changes to another value.
• Ex: landuse, soils, etc.
Vector GIS
Vector GIS is based on
POINTS
LINES &
POLYGONS
Example: ArcView, ArcGIS, Manifold, etc.
In the vector world everything is
a point, a line, or a polygon
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
Points
Lines
Polygons
Vector representation of
reservoir and highway
13
GIS Data
Most of the available GIS data have been
developed using ARC/INFO
• Arc/INFO was a flagship product from ESRI
• It was widely used all over the world
Brief history of ESRI’s GIS software evolution
Software
Data Format
Year
Arc/Info
Coverage
1980’s
ArcView
Shapefile
1990’s
ArcGIS 8 & 9
Geodatabase
2000’s
ArcGIS 10
Geodatabase
2010
Spatial Data Format
ArcGIS can work with spatial data in
multiple formats
• Shapefile
• Coverage
• Geodatabase
• Raster
• CAD
• Tables
Common data formats
Coverages
Shapefiles
Geodatabase
Shapefiles
A shapefile consists of multiple files, and
the common ones are *.dbf, *. shp, *.shx
Shapefiles in
Windows Explorer
Shapefiles in
ArcCatalog
2 types of Geodatabase
• File geodatabase (soils.gdb-newer type)
• Personal geodatabase (soils.mdb-older type)
What can be in a Geodatabase
• Feature datasets
- Collection of related feature classes
• Feature classes
• Tables
• Annotations
• Rasters
Feature dataset
Feature classes
Why Geodatabase?
It is not only a container of different types of
data but also provide enhanced functionalities
• Domains
• Rules (split and merge policies)
• Topological association
33%
66%
Other types of data
Tables
Layer file
- Does not contain data
- But stores the symbology
Layer and Layer file?
• A layer represents
geographic data
displayed in
ArcMap
• A layer references
the data on a disk
and keep it in
memory rather than
storing the data
• A layer is created when you add a dataset to ArcMap
Layer and Layer file?
• Layer is drawn with a default set
of drawing properties (symbology)
• Layer can be saved to a file as a
layer file (.lyr) to store the
symbology and shared with others
Map Documents
1. When you save a map in ArcMap, it will be
saved as a file on disk called map document
(.mxd)
2. Map documents (.mxd) do not store the data
in it except the link to the data source
3. Map documents make it easy to save, reuse,
and share your work in ArcMap
More on data
• GIS data may be projected or unprojected
• Projected or unprojected, GIS data is usually
attached to a coordinate system
• How do you determine the coordinate system
of your data?
Two types of coordinate systems
Geographic coordinates:
• Unprojected
• Map units are in decimal degrees
Projected coordinates:
• Data is projected
• Map units are in feet or meters
Projected or Unprojected ?
Projected/Unprojected?
Projected Coordinate Systems (SPC)
How do you know the UTM zone of a place?