gis 1 gis lecture 6 digitizing. gis 2 outline digitizing overview digitizing sources gis features...

GIS 1

GIS Lecture 6Digitizing

GIS 2

Outline

•Digitizing Overview•Digitizing Sources•GIS Features•Creating and Editing Shapefiles in ArcView•Spatial Adjustments•Conclusion

Copyright– Kristen S. Kurland, Carnegie Mellon University

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Digitizing Overview

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Why Digitize?

•New maps•Map features are wrong•Missing features


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Heads Down Digitizing

• Digitizing table or tablet


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Heads Up Digitizing

• Mouse on a screen• Digitizes paper maps, aerial photos, or

other images


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Digitizing Sources

•Orthophotos•GPS•Paper Maps•Scanning•Raster to Vector Conversion


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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

•Can be used to measure true distances• http://gis.esri.com/library/userconf/proc95/to150/p124.html


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DOQQs

•Digital Ortho Quarter Quadrangles•Produced by the U.S. Geological Survey (USGS)

•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.


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Image Resolutions

The amount of detail found in 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 30 meters 10 meters 10 meters 5 meters 5 meters


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Image Resolutions (Continued)

2 meters 2 meters 1 meter 1 meter


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Resources


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Resources (Continued)


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Google Maps


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Google Earth

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GPS

•Department of Defense built the global positioning system (GPS) in the 1970s

•Network of up to 24 satellites in earth orbit

•Opened it up to civilian use in the 1980s – free of charge

•GPS uses triangulation from signals from the satellites

•Base stations are fixed receivers that help GPS signals get accuracy within 1 to 5 meters

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GPS (Continued)

•Easy to input GPS data into GIS

•GPS is most valuable when there are no base maps sufficient to guide drawing new features

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National Spatial Reference System

•National network of control points (small brass disks with identifiers and marks) placed on permanent structures such as concrete posts

•National Geodetic Survey (NGS) establishes the known points

•Surveyors take measurements (angles and distances) from known locations to accurately locate points and boundaries

•By the early 1990s, GPS became the dominant method for establishing control points

•Some cities maintain their own points

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National Spatial Reference System

City of Pittsburgh control point benchmark

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Paper Maps

Historic Maps•http://www.usgsquads.com/svcs_historic_maps.htm#Historic•http://digital.library.pitt.edu/maps/-Historic maps of Pittsburgh and Allegheny County from 1872-1939


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Scanning Paper Documents

•Scanned maps and images become vectors

•Special software needed


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Raster to Vector Conversion


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Raster to Vector Conversion


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GIS Features

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Points•Nodes-Single points-Alone or at endpoints of lines

00

node

1

2 3


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Points• Vertices- Control the shape of lines

00

nodevertex

2 3

1


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Points• Tics- control points used to register the map to the

geographic space

00

nodevertextic

2 3

1


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Lines

•Connected Nodes and Vertices•Single or multipart features

00

node

1 2

line


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LinesFNODE_ TNODE_ LENGTH STREET_ STREET_ID FNAME LEFTADD1LEFTADD2RGTADD1 RGTADD2

180 129 595.106800 201 51635798 Kirkpatrick 605 699 600 698

From Node

To Node


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Line Features with VerticesFNODE_ TNODE_ LENGTH STREET_ STREET_ID FNAME LEFTADD1LEFTADD2RGTADD1 RGTADD2

180 129 595.106800 201 51635798 Kirkpatrick 605 699 600 698

Vertex


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TIGER LINE Files


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TIGER Street Centerlines


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TIGER Street Centerlines


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Polygons

•Closed lines• Single polygons or multipart features

(e.g. Hawaiian islands can be multiplepolygons as one feature)

00

node

1 2

polygon

4 3


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New Shapefiles

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Creating New Shapefiles

•Create new point, line or polygon shapefile in ArcCatalog

•Add Spatial Reference Information (optional)•Add New Shapefile in ArcMap•Edit shapefile to add features and attributes •Stop editing •Save edits as part of new shapefile


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New Shapefiles - ArcCatalog• Cannot create shapefiles in ArcMap


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Add Shapefile in ArcMap


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Heads Up Digitizing•Add base layers if necessary


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Heads Up Digitizing•Add Editor Toolbar


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Heads Up Digitizing•Start Editing


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Heads Up Digitizing•Begin Digitizing•Click Sketch Tool


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Heads Up Digitizing•Start Drawing Points

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Heads Up Digitizing•Stop Editing•Save Edits

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Heads Up Digitizing•Add attribute information

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Editing Shapefiles

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Lines and Polygons•Advanced editing tools-Editing Vertices-Snapping Functions-Multipart Features


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Editor Toolbar

Editor Toolbar Sections

- Editor drop-down

- Sketch tool

- Tasks

- Target

- Attribute Inspector

- Sketch properties


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Sketch Construction Tools

Sketch tool:• Use to create point features and to digitize the

vertices of line or polygon features. Intersection tool:

• Creates vertex at place where two segments intersect

Arc tool:• Creates a segment that is parametric (true)

curve. Only has two vertices as endpoints Midpoint tool:

• Define the location of the next vertex by on two points - the vertex is place at the midpoint of the line between these points

End Point Arc tool:• creates true curve by defining the start and end

points of the curve, then define a radius


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Sketch Construction Tools

Tangent Curve tool • Creates segments that are tangent

Distance-Distance tool• Creates a point or vertex at the intersections of two

distances from two other points

Direction-Distance tool• Like distance-distance tool except second point

uses a direction from a known point to define a bearing line

Trace tool:• Create segments that follow along existing

segments


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Tasks• Most features are

created and modified through sketches

• Tasks define what you do with a sketch

• Create features

• Modify features

• Reshape features

• Other edit tasks

• Custom tasks


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Target Layer•Target layer determines to which layer a new feature will belong

•Can edit multiple layers in an edit session

•Subtypes are also listed*


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Spatial Adjustments

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Spatial Adjustments

•Transform•Rubbersheet•Edgematch


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Transforms•Convert data from one coordinate system to another.

•Converts data from digitizer or scanner units to real world coordinates.

•Shifts data within a coordinate system, such as converting feet to meters.


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Rubbersheeting•Geometric distortions commonly occur in source maps.

•They may be introduced by imperfect registration in map compilation, lack of geodetic control in source data, or a variety of other causes.

•Rubbersheeting corrects flaws through the geometric adjustment of coordinates.


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Edgematching•The edgematching process

aligns features along the edge of one layer to features of an adjoining layer.

•The 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•Hamburg Hall Building Drawn in CAD

•Spatially Adjusted to Aerial Photo


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Summary

•Digitizing Overview•Digitizing Sources•GIS Features•Creating and Editing Shapefiles in ArcView•Spatial Adjustments•Conclusion

•More to Explore-Tutorial 6 (Introduction to Digitizing)-On-line Help (Advanced Topics)
