Different kinds of data

Example of Elevation

TopographicDEM (Digital Elevation Model)

How can elevation be represented in a GIS?

GIS Data Specifics

Getting into the guts of GIS data structure …

Basic structure of Vector and Raster:

PointsLinesPolygons

Attributes

AttributesGrid cells

Remember the idea of a Data Model:

Keep the concept ofdata compression andmachine code in mind…

The Roles of Raster v. Vector Data

CHARACTERISTIC RASTER VECTOR Data structure

Usually simple Usually complex

Storage requirements Large for most data sets without compression

Small for most data sets

Coordinate conversion May be slow due to data volumes, and may require resampling

Simple

Analysis Easy for continuous data, simple for many layer combinations

Preferred for network analyses, many other spatial operations more complex

Positional precision Floor set by cell size Limited only by quality of positional measurements

Accessibility Easy to modify or program, due to simple data structure

Often complex

Display and output Good for images, but discrete features may show “stairstep” edges

Map-like, with continuous curves, poor for images

Going Back to Elevation…

Cartography

Precision (?)of isolines

Calculations

Triangular IrregularNetwork

Data compression

Converting from vector to raster

Converting from raster to vector

Editing!

Yes, you can change data, but do it very VERY carefully!

• Vector data editing• Geoprocessing• Hand digitizing• Attribute table

• Raster adjustments• Calculations• Classification• Deletion of classes• Resampling

Always work in a disposable data copy and save Save SAVE!

Hand digitizing

For example:Creating vector feature classes

based off aerial imagery

Snapping

What this looks like in ArcMap:

Warning:Opening an editingsession in ArcMapchanges how your.mxd functions!

Feature being editedappears with greenbox vertices:

Topology

Topology: The study of geometric properties that do not change as the forms are bent, stretched or undergo similar transformations

Issues with editing – topology rule fixes

Slivers!

Vector feature topology and tables

These tables allowfor fast indexingand searching!

Network example for vector data:NHD (National Hydrography Dataset)

Nodes

Stream Order

Modeling Flow

Direct editing of raster data is limited…

The “Delete Selected”button is what youcan use to edit in araster table

… instead, change rasters with processing!

More Raster Processing

Resampling

Classification

Raster Compression Examples

Run-length coding

Quad-tree compression

Working with vector data attribute tables

Edit attributes directlyHand editing in vector data attribute tables is possible

Joins and Relates – a powerful technique! (Excel is useful here)

Join Types:

One to One

One to Many

Managing Data

DBMS:Database Management SystemControl points for editing

Potential conflicts

Our shapefilesNot centralized

Shapefiles <> File Geodatabase

spatial index format (.sbx)attribute format (.dbf)projection format (.prj)spatial index format (.sbn)shape format (.shp)shape index format (.shx)geospatial metadata (.xml)

For Further Reference!

Author:Paul Bolstad