Introduction to Geographic Information Systems
Fall 2013 (INF 385T-28620)
Dr. David Arctur
Research Fellow, Adjunct Faculty
University of Texas at Austin
Lecture 7
October 10, 2013
Digitizing
Outline

Digitizing overview

GIS features

Digitizing features in ArcMap

Advanced digitizing tools

Transforms and spatial adjustments
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Lecture 7
DIGITIZING OVERVIEW
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Why digitize?

New maps

Map features are wrong

Missing features

Other?
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Heads-down digitizing

Digitizing tablets
 Used to digitize hard copy maps into GIS
 Transform wire intersections into
coordinates of the tablet’s coordinate
system
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Steps for heads-down digitizing




Tape map to the
digitizing tablet
Register control
points on the map
Estimate two
conversion equations
(one for vertical and
one for horizontal
coordinates)
Digitize vectors
(points, lines, or
polygons)
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Control
points
Puck
Tape
Map
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Heads-up digitizing


Mouse on a screen
Digitizes paper maps, aerial photos, or other images
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Lecture 7
GIS FEATURES
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Points

Single features

Nodes
 Line endpoints

Vertex points
 Controls shapes of lines
and polygons
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Creating points

Heads-up digitizing
 Single point features
 Sketch tool and snapping features


Points added as x,y event files (chapter 5)
Points added using geocoding functions
(chapter 8)
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Lines

Starting and ending points with shape vertex
points as needed
Vertex points
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Line attributes


Shape
From and To attributes
 Node
 Address (street centerlines)
 Lat/long

Length
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Polygons


Three or more lines joined to form a closed
area
Single polygons or multipart features
 Hawaiian Islands can be multiple polygons as
one feature
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Polygon attributes




Shape
Area
Perimeter
Length
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Constructions tools
Points
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Lines
Polygons
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Lecture 7
DIGITIZING FEATURES IN
ARCMAP
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Create new feature class

In ArcCatalog
 Navigate to desired location
 Create new feature class or shapefile
 Add spatial reference information
 Add new fields (optional)
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Digitize new features

In ArcMap





Create base map
Add feature class
Start editing
Digitize feature
Stop editing and save
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Create basemap


Vector features or raster images
Add features
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Start editing

Start editing
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Begin digitizing
pick points
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Stop editing


Editor toolbar, Stop Editing
Save edits
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Edit attribute data

Start editing, populate fields in table (create
fields if necessary)
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Lecture 7
ADVANCED DIGITIZING
TOOLS
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Specify angle and length

Lines and polygons
 Straight segment tool

Type length and angle
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Snapping tools

Snapping toolbar
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Trace tool


Good for complicated shapes
Editor toolbar, Trace tool
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Generalize tool


Creates features for use at small scales with
less detail while preserving basic shapes
US Census cartographic boundary files
Census TIGER water features
Generalized/small scale
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Modified rivers
Smoothed/large scale
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Smooth tool

Smooths sharp angles in polygon outlines to
improve aesthetic or cartographic quality
Smooth before
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Smooth after
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Cut polygons


Creates two polygons from one original
Cut polygon tool
Polygon before
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Polygon after
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Move and rotate

Adjusts features
Polygon before
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Polygon after
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Lecture 7
TRANSFORMS AND SPATIAL
ADJUSTMENTS
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Transforms

Converts data
 From one coordinate system to another
 From digitizer or scanner units to real world coordinates

Shifts data
 Within a coordinate system (for example, feet to meters)
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Rubber sheeting

Geometric distortions commonly occur in
source maps
 May be introduced by imperfect registration in map
compilation
 Lack of geodetic control in source data
 Variety of other causes

Corrects flaws through the geometric
adjustment of coordinates
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Edge matching

Aligns features along the edge of one layer
to features of an adjoining layer
 Layer with the less accurate features is adjusted,
while the adjoining layer is used as the control.
 Attribute transfer is typically used to copy
attributes from a less accurate layer to a more
accurate one.
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Spatial adjustment example

CAD drawings (buildings)
 Generally do not have geographic coordinates
Hamburg Hall
CAD drawing
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Hamburg Hall
Ortho photo
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Spatial adjustment example

Spatially match drawing to map using
displacement links
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Summary

Digitizing overview

GIS features

Digitizing features in ArcMap

Advanced digitizing tools

Transforms and spatial adjustments
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