Transcript Steps in Building Data

Digitizing
• There are three primary methods for
digitizing spatial information:
• Manual Methods include:
• Tablet Digitizing
• Heads-up Digitizing
• An Automated Method includes:
• Scanning and Vectorization
Manual Digitizing
Ref: Getting Started with GIS, by K. Clarke
How is it done?
• Although methods vary, the typical steps for
digitizing a paper map require the following:
– Physical Preparation
• Digitizer Set up
• Map preparation
– Digital Preparation
• Map registration
– Digital Collection
• Feature collection
• Feature correction
• Save, save, save
Creating New Data (Cont’d)
• Manual Digitizing:
– Digitizing tables used (tedious process)
– Electronic mesh senses cursor position
– Paper maps (e.g. Topo Maps) are sources of most
GIS themes that we use
– Point or stream mode digitizing done
– Now-a-days mainly done in 3rd world
– Need to record ref. coor. converted to real-world
coor. (GIS coverage)
– Digitizing polygons need labels (ID)
– Careful planning needed before you digitize
Heads up Digitizing
• Digitizing and scanning
Scanning
Ref: Getting Started with GIS, by K. Clarke
Creating New Data (Cont’d)
• Scanning:
– Scan a map into binary grid file
– Can be converted back to vector thru tracing (i.e.
line thinning extracting topo-reconstruction or
smoothing)
– Software available to clean scanned images
– Resolution is an issue (e.g. a line = 5-6 pixels)
– Digital maps are as good as the source map
Creating New Data (Cont’d)
• On-Screen (head-up) Digitizing:
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Relatively easy to do
Good for updating/editing changes
You can add points, lines, polygons
You don’t have to register points as in manual
digitizing
• Geometric Transformation:
– Digitized maps have same coor. As source map
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You may have to convert to real-world coor. &
projection using control points.
Raster to Vector Conversion
•Able Software R2V
•AutoDesk Raster
Design
•ESRI ArcScan
Filters
Rubber Sheeting: Spatial database editing software that
attempts to correct errors by stretching a map to fit known control
points or monuments.
Why Topology Matters
• Topology allows automated error
detection and elimination
• Need to build/clean topology for
digitized/scanned maps
• Unconnected nodes are snapped
• Slivers from digitizing are eliminated
• Topology makes map overlay feasible
• GIS operations are done w/o accessing
the point files
Why do I want Topology?
Gaps and slivers
Overshoots and undershoots
Non-Topology
Ref: Introduction to GIS by K. Chang
Creating New Data
• Remotely sensed
data
• Georef; 1m x 1m
resolution; good as a
background map for
other themes
Ref: Getting Started with GIS, by K. Clarke
Creating New Data (Cont’d)
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GPS: differential GPS
have cm-scale resolution
– Measures ranges
(distances of receivers
by 24 NAVSTAR
satellites using radio
signals & atomic
clocks)
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Ref. or base stations can
reduce noise
Diff. GPS comes w/
software to download data
Hand-held GPS data can be
downloaded too
Ref: Getting Started with GIS, by K. Clarke
Creating New Data (Cont’d)
• Field Survey Data:
– defines parcels by
azimuth & distances
– Uses Total Station,
Stadia, Laser Ranging,
and COGO software
Ref: Getting Started with GIS, by K. Clarke
Methods of Collecting Data
• Existing data, downloads
from field
• Keyboard
• COGO
Creating New Data (Cont’d)
• Street Addresses:
– Point features, street
segment length,
even/odd house
numbers with side
descriptions (e.g. TIGER
files) can be geocoded
• TEXT Files: lat/long in
dec. degrees cane
converted to an event
theme
Ref: Getting Started with GIS, by K. Clarke
Geocoding
What is Conflation?
• the process of
transferring attributes
from a source line
layer based on one
level of accuracy and
precision to a target
line layer of a different
precision and
accuracy.
Format & Exchanging of GIS Data
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Can import different data formats or use utility programs
(e.g Import 71) to convert data
GIS accept TIF, GIF, JPEG, AutoCAD data formats
(needs right extensions) which are not georeferenced
Vector to Raster conversion is easy, opposite is hard
Data exchange by import/export involves errors in
attributes & geometry
ESRI New Product called Data Interoperability
Efficient data transfer is important for the future of GIS
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OpenGIS® Specifications on
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GML Geographic Markup Language
WMS Web Map Service
WFS Web Feature Service