An overview of GIS data

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Today’s Topic:

• Data Types

• ArcGIS terminology

• Review of coordinate systems

• GIS Data Models

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2 basic types of Data Models• Raster• Vector

• Raster GIS• Vector GIS

Raster GIS• Raster is nothing but an array of grid cells• Each cell is called Pixel (Picture element)Example: IDRISI, ERDAS, GRASS

Pixel

Raster

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DEM (Digital Elevation Model):An example of Raster Data

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Raster representation of reservoir and highway

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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.

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 GISVector GIS is based on

POINTSLINES &POLYGONS

Example: ArcView, ArcGIS, Manifold, etc.

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Points Lines

Polygons

In the vector world everything is a point, a line, or a polygon

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Vector representation of reservoir and highway

GIS DataMost 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 FormatArcGIS can work with spatial data in multiple formats• Shapefile• Coverage• Geodatabase

• Raster• CAD• Tables

Common data formatsShapefilesCoverages Geodatabase

Shapefiles

Shapefiles in Windows Explorer

Shapefiles in ArcCatalog

A shapefile consists of multiple files, and the common ones are *.dbf, *. shp, *.shx

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 classesFeature dataset

Feature classes

• Feature classes• Tables• Annotations• Rasters

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

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Other types of dataTables

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)

3. Map documents make it easy to save, reuse, and share your work in ArcMap

2. Map documents (.mxd) do not store the data in it except the link to the data source

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 systemsGeographic 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?