Introduction to Geographic Information Systems
Spring 2013 (INF 385T-28437)
Dr. David Arctur
Lecturer, Research Fellow
University of Texas at Austin
Lecture 6
Feb 14, 2013
Digitizing
Outline
Digitizing overview
GIS features
Digitizing features
Advanced digitizing tools
Spatial adjustments
Digitizing sources
Dataset metadata
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Lecture 6
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
digitizer
Register control points
on the map
Estimate two
conversion equations
(one for vertical and
one for horizontal
coordinates)
Digitize vectors (points,
lines, or polygons)
Tape
Control
points
Puck
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 6
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 XY event files
Points geocoded using addresses (CH 7)
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Lines
Starting and ending points with shape
vertices as needed
Vertex points
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Lines
Physical attributes of TIGER streets
Shape
From and To attributes
Node
Address
Lat/long
Length
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Polygons
Three or more lines joined to form a closed
area
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3
1
2
0
0
Single polygons or multipart features
(Hawaiian islands can be multiple polygons as
one feature)
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Polygons
Physical attributes
Shape
Area
Perimeter
Length
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Lecture 6
DIGITIZING FEATURES
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Create new feature class
Created in ArcCatalog
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Create new feature class
Add spatial reference information
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Create new feature class
Add new fields
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Create new feature class
Feature class created
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Digitize new features
Create base map
Add feature in ArcMap
Start editing
(Editor toolbar)
Digitize feature
Stop editing and save
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Create base map for digitizing
Vector features or raster images
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Add new feature class
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Add Editor toolbar
Customize, Toolbars, Editor toolbar
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Start editing
Editor, Start editing
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Begin digitizing
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Click point (tree) locations
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Stop editing
Editor, Stop editing
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Edit attribute data
Start editing, populate fields in table
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ArcGIS 10.0 10.1
Note that the “Edit Vertices” toolbar has 2 new
tools not shown in the lab book exercises,
check them out:
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Lecture 6
ADVANCED DIGITIZING
TOOLS
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Specify angle and length
Lines and polygons
Straight segment tool
Type length and
direction 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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Cartographic generalization
Bolstad, pg 138, Figure 4-9
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Generalize tool
Creates features for use at small scales with
less detail while preserving basic shapes
U.S. Census cartographic boundary files
Census TIGER water feature
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City planning rivers
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Smooth tool
Smoothes 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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Lecture 6
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, (e.g. feet to meters).
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Rubbersheeting
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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Edgematching
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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Common digitizing errors
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Undershoots, overshoots, snapping
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Spatial adjustment example
Hamburg Hall CAD Drawing
CAD drawings generally do not have
geographic coordinates
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Spatial adjustment example
Spatially match drawing to map using
displacement links
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Lecture 6
DIGITIZING SOURCES
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USGS
United States Geological Survey
Common Data Types & Standards
Digital Elevation Models (DEMs)
Digital Orthophoto Quads(DOQQs)
Digital Line Graphs(DLGs)
Digital Raster Graphics (DRGs)
Landsat Satellite Images
Land Use Land Cover (LULC)
Spatial Data Transfer Standard(SDTS)
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Orthophotography
Digital imagery in which distortion from the
camera angle and topography have been
removed, thus equalizing the distances
represented on the image
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Digital Ortho Quarter Quads—DOQQs
http://online.wr.usgs.gov/ngpo/doq/
Grayscale or color-infrared (CIR) images
1-meter ground resolution;
Cover an area measuring 3.75 minutes longitude by
3.75 minutes latitude, approximately 5 miles on each
side
Referenced to the North American Datum of 1983
(NAD83) and cast on the Universal Transverse
Mercator (UTM) projection
Interactive DOQQ ordering:
http://earthexplorer.usgs.gov/
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USGS EarthExplorer
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DOQQ image resolutions
The ground length of one pixel of the image
image with one meter resolution means that each
pixel in the image represents one square meter
on the ground.
30 meters
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10 meters
5 meters
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DOQQ image resolutions
2 meters
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1 meter
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National Elevation Data Sets
United States Geological Survey (USGS)
National Elevation Dataset (NED) Shaded
Relief Imagery Data (Free)
Maps provide highest-resolution elevation
data available across the United States, in
raster format
http://ned.usgs.gov/
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Scanning paper documents
Raster to vector conversion
Paper (historic) maps
Scanned maps and images become vectors
Special software needed
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Dataset metadata
Make your data discoverable over the web
Record what your data is about while it’s fresh!
Source: ISO 19115 – Geographic Information
Metadata Content Standard
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Editing metadata in ArcCatalog
Be a friend to other
data users besides
yourself:
Fill in metadata
for shared datasets
whenever possible!
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Summary
Digitizing overview
GIS features
Digitizing features
Advanced digitizing tools
Spatial adjustments
Digitizing sources
Dataset metadata
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