Transcript Introduction to GIS

DIGITIZING AND
SCANNING
OUTLINE:
 geocoding
 digitizing terms and methods
 scanning methods
 adding attributes
DIGITIZING AND SCANNING

we can buy data, steal it, download it over the
internet or collect it ourselves
GEOCODING DATA
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conversion of spatial information into digital
form
involves capturing the map, and sometimes also
capturing the attributes
often involves address matching
method can influence the structure and error
associated with the spatial information which
results
GEOCODING DATA

digitizing

scanning
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field data collection (remote sensing, geographic
databases, field work)
GEOMATIC DATA
Two aspects of geodata:

Spatial: points, lines, and polygons that are
referenced to a position on the earth

Attribute: characteristics of the spatial data
DIGITIZING TERMS
Terms
Arc
Example
Description
Line feature: a node at each end;
vertices at each change of direction.
Node
Endpoint of an arc (also found at
intersections between lines).
Vertex
A point on an arc that signals a
change of direction.
Pseudo Node
On an (island) arc that connects to
itself.
Dangling Node
Arc endpoint that is not connected.
Label point
Identifies a point feature or polygon.
Tic
Geographic control point; coverage
features can be registered to the
same coordinate system.
DIGITIZING
AND SCANNING
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convert map (analog) data to digital data.
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collecting the geometry, attributes, topology.
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establishing a geographic frame of reference.
VECTOR/MANUAL DIGITIZING
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captures map data by tracing lines from a map by
hand
components:
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electronically-sensitive tablet or digitizing surface
(sizes 12" x 18" -> 36" x 48").
curser with crosshairs
and keypad
computer interface
VECTOR/MANUAL DIGITIZING
VECTOR/MANUAL DIGITIZING
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trace paper map line by line (vertices and nodes)
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one layer at a time
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if you have a map with roads, parks, building
– do all roads first
control or reference points (3-6) – help line up
work from day to day, layer to layer
VECTOR/MANUAL DIGITIZING
Vector digitizing steps:
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fix to tablet
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digitize control
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determine coordinate transformation
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trace features
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proof plot
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edit
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clean and build (code and add attributes)
VECTOR/MANUAL DIGITIZING
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smoothness of lines depends on number of vertices
created
DIGITIZING TABLE

no point in having a table that is much more
precise than an operator can point to
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digitizing tables need to be calibrated regularly
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if map is larger than table, will have problems
trying to take the digitized pieces and knit them
together
VECTOR DIGITIZING
Point mode: mouse used to record points, points are
digitized by placing the cross hairs over the point one
wishes to digitize and pressing the button.
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record point features
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minimizing the size of the data file
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accuracy mostly controlled by skill of operators
and line weights
VECTOR DIGITIZING
Line mode: (automated mode) – points are recorded at
a fixed rate
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straight lines usually have few points recorded.
Complex areas more points
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more operator time, data storage capability and
computer processing time
PROBLEMS
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labour intensive
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operator fatigue; requirements for accuracy and
errors.
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topology requirements force extra specifications
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sheet matching and registration.
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specialized equipment
DIGITIZING ERRORS
1.
Missing line
DIGITIZING ERRORS
2.
Undershoot
DIGITIZING ERRORS
3.
Overshoot
DIGITIZING ERRORS
4.
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Multiple Identifies (attributes)
associated with undershoots
or missing lines
DIGITIZING ERRORS
5.
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No Identifiers
from simple omission
DIGITIZING ERRORS
6.
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Extraneous Linework
creates unlabelled polygons
DIGITIZING CONTROLS
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a lot of visual inspection
use software to verify relationships expected to
occur
attributes verified by other relationships or brute
force inspection.
DIGITIZING CONTROLS
Fuzzy tolerance - minimum distance between all
(nodes and vertices) arc coordinates.
 typical minimum distance between coordinates
.002 inches.
 handles small overshoots or undershoots,
automatic sliver removal, and coordinate thinning
of arcs.
 exercise caution when using the fuzzy tolerance
DIGITIZING CONTROLS
Dangle Length - minimum distance between arc
coordinates.
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a dangling arc has the same polygon on its left
and right side.
Node match tolerance (snap tolerance) - minimum
distance between node features.
Weed tolerance - distance between coordinates
(vertices) within each arc
HEADS-UP DIGITIZING
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looking at monitor
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creating new maps by editing existing layers
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create zip codes from blocks, create bus line
from streets
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caution – new maps only as accurate as original
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precision dependent on
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resolution of the screen (at normal zoom)
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resolution of the data on the screen
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how easily the operator can point to things.
SCANNING
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device that converts
maps into digital form
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originally single sensor
on a drum
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uses CCD arrays to
record information in
binary form (a semiconductor that translates
photons into counts of
electrons)
SCANNING
SCANNING
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places a map on a glass plate, and passes a light
beam over it
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measures the reflected light intensity
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result is a grid of pixels
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image size and resolution are important
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use software to clean up image and transform it
from image to data set
SCANNING EXAMPLE
SCANNING
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caution – scanner will include the whole map, not
individual layers (also include coffee stains, map
folds, pencil marks…)
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individual layers must be re-traced in computer or
extra layers must be digitally erased (cleaned)
ADDING ATTRIBUTES
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simple attributes – can be coded at time of digitizing
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cursor buttons or menu
extensive attributes – held in a relational database
management system (RDMS)
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attach feature codes to graphic entities.
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attach unique identifiers then enter no-spatial data
afterwards.
AVOIDING DIGITIZING/SCANNING

you can avoid digitizing by finding a source for the
digital database "somewhere" out there.
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remote sensing or geographical databases