SPATIAL DATA EDITING SUG613/GLS613
CONTENTS
Introduction To Topology Topological Data Structure Topological Relationships Between Spatial Objects Spatial Data Editing Location Errors Topological Errors Topological Editing Polygon Rules, Line Rules & Point Rules Topology In ArcGIS Non-topological Editing Other Editing Operations
TOPOLOGY
• The topologic model is often confusing to initial users of GIS.
“Topology is a mathematical approach that allows us to structure data based on the principles of feature adjacency and feature connectivity. It is in fact the mathematical method used to define spatial relationships”
• Without a topologic data structure in a vector
based GIS most data manipulation and analysis functions would not be practical or feasible.
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Common Topological Data Structure
• This model contains two basic entities, the arc
and the node. • The arc is a series of points, joined by straight line segments, that start and end at a node. • The node is an intersection point where two or more arcs meet. Nodes also occur at the end of a dangling arc, e.g. an arc that does not connect to another arc such as a dead end street. • Isolated nodes, not connected to arcs represent point features. A polygon feature is comprised of a closed chain of arcs. Prepared by: Sr Dr Siti Aekbal Salleh
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Simply define
• Node : an intersect of more than two lines or strings, or start and end point of string with node number
• Chain : a line or a string with chain number, start and end node number, left and right neighbored polygons
• Polygon : an area with polygon number, series of chains that form the area in clockwise order (minus sign is assigned in case of anti-clockwise order). Prepared by: Sr Dr Siti Aekbal Salleh
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Geometry and Topology
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Topological Relationships between Spatial Objects
• In practical applications of GIS, all possible relationships in spatial data should be used logically with more complicated data structures. The following topology relationships are commonly defined.
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Point-Point Relationship "is within" : within a certain distance "is nearest to" : nearest to a certain point Point-Line Relationships "on line" : a point on a line "is nearest to" : a point nearest to a line
Point-area Relationships "is contained in’’ : a point in an area "on border of area" : a point on border of an area Prepared by: Sr Dr Siti Aekbal Salleh
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Cont… • Line-Line Relationships "intersects" : two lines intersect "crosses" : two lines cross without an intersect "flow into" : a stream flows into the river
• Line-Area Relationship "intersects" : a line intersects an area "borders" : a line is a part of border of an area
• Area-Area Relationships "overlaps" : two areas overlap "is within" : an island within an area "is adjacent to" : two area share a common boundary Prepared by: Sr Dr Siti Aekbal Salleh
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Example of Topological Relationship
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Topological Rules
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SPATIAL DATA EDITING
• Spatial data editing – process of adding, deleting, and modifying features or simply, to remove digitizing errors
•
Two type of digitizing errors:
• •
Location errors – relate to geometric inaccuracies of spatial features e.g. missing polygons or distorted line Topological errors – relate to logical inconsistencies between spatial features e.g. dangling lines and unclosed polygons
• Main challenge in spatial data editing: determining which method to use for a given data set
Advantages vs. Disadvantages • The advantages of the topological data model are to avoid duplication in digitizing common boundaries of two polygons and to solve problems when the two versions of the common boundary do not coincide.
• The disadvantages are to have to build very correct topological data sets without any single error and to be unable to represent islands in a polygon.
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Location errors • Location errors using secondary data sources (e.g. paper map)
• •
Identify by comparing digitized map with source map Scale error
• •
e.g. 1:50000 less reliable than 1:5000 e.g. smaller map scale, map details decreases and degree of line generation increases
Location errors • Cause for digitizing errors • • •
Human errors in manual digitizing Errors in scanning and tracing Errors in converting digitized map into real-world coordinates
Figure 8.1 Common types of digitizing errors from tracing. The thin lines are lines on the source map, and the thick lines are lines from tracing.
Location errors • Location errors using primary data sources (e.g. GPS & Remote Sensing)
• • •
Resolution of the measuring instrument determines the accuracy of spatial data Range from >1 m to 1 km for satellite images
Range from several mm to > 20 meters
Spatial data accuracy standards • Spatial data accuracy – how close the recorded location of a spatial feature is to its ground location
• Spatial data precision – how exactly the location is recorded
Topological errors • Violate the topological relationships • Method to perform topological errors: • •
E.g. ESRI Coverage – connectivity, area definition & contiguity
E.g. ESRI Geodatabase – 25 topology rules (refer to Table 4.1)
• If digitized features did not follow these relationships, they would have topological errors
Topological errors • Topological errors with geometric features • •
Polygon features
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Line features
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e.g. unclosed polygons, gaps between polygons & overlapping polygons
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do not meet perfectly at a point (node) – e.g. undershoot & overshoot a.k.a dangling node Pseudo node appears along a continuous line & divides separate lines Direction of a line – e.g. hydrologic analysis & traffic flow
Point features
•
Polygon should have one label point
Figure 8.2 (a) An unclosed polygon, (b) a gap between two polygons, and (c) overlapped polygons.
Figure 8.3 An overshoot (left) and an undershoot (right). Both types of errors result in dangling nodes.
Figure 8.4 Pseudo nodes, shown by the diamond symbol, are nodes that are not located at line intersections.
Figure 8.5 The from-node and to-node of an arc determine the arc’s direction.
Figure 8.6 Multiple labels can be caused by unclosed polygons.
Topological errors •
Topological errors between layers
• • •
Coverage model – only work with single coverage Geodatabase model – work for two or more layers
Common error between two polygon layers:
• Boundary are not coincident
•
Common error between two line layers:
• Layer do not connect with another layer at the end points • Overlapping line features • Line features not covered by another set of line features
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Common error between points layers:
• Not fall along line features in another layer
Figure 8.7 The outline boundaries of two layers, one shown in a thicker line and the other a thinner line, are not coincident at the top.
Figure 8.8 Line features from one layer do not connect perfectly with those from another layer at end points. (b) is an enlargement of the top error in (a).
Topological editing • • •
Ensures digitized spatial features follow topological relationships
Use topological-based GIS package software – detect, display & remove
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E.g. ArcGIS, AutoCAD Map & MGE
Topological editing in ArcGIS
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Topological editing on coverages
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Arcedit in ArcInfo Workstation
Topological editing using a map topology
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Map topology & topology tools in the topology toolbar & Topology tasks in editor toolbar
Topological editing using topology rules
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Defined through the properties dialog of a feature dataset in a geodatabase
Ten Polygon Rules
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Thirteen Lines Rules
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Four Point Rules
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Geodatabase topology rules and topology error fixes ArcGIS Geodatabase Topology Rules
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Topology in ArcGIS
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Example of Designing Geodatabase Topology
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The topology process (continue)
The topology process
Managing errors •
Options • Ignore the error • Mark as an exception • Fix the error
• Errors cannot be deleted •
Only fixed or marked as exceptions
Error inspector
• Has standard fixes for some rule violations • Choices depend on the error
Topological editing on coverages • Starts by constructing topology (for entire features) • E.g. Clean command in ArcGIS • •
• •
Builds topology, applied dangle length and fuzzy tolerence Dangle length – specifies minimum length for dangling arcs Fuzzy tolerance – specifies minimum distance between points (vertices) Clean command must used cautiously
Topological editing on coverages • Snap point (nodesnap), snap lines (arcsnap) & extend for individual features
• Can combine the above snapping command with basic editing tools
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E.g. delete, move, add, split, unsplit & flip
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Topological editing on coverages E.g.
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Dangles - remove undershoot by extending the dangling arc to meet with target arc & remove overshoot by deleting the extension Duplicate arcs – select extra arcs and delete or delete all and redigitize Wrong arc directions – Alter the direction by flipping the arc
Pseudo node – setting two arcs on each side to have same ID value and unsplitting them Label errors - adding new label points with proper IDs or deleting extra label points Reshaping Arcs – move, add or delete points (vertices) that make up the arc
Figure 8.13 The allowable extend distance can remove the dangle by extending it to the line on the right.
Figure 8.14 To remove the overshoot a, first select it and then delete it.
Figure 8.15 To remove the pseudo node, select a and b, assign the same ID value to both, and un-split them.
Non-topological editing • • •
Variety of basic editing operation that can modify simple features and can create new features from existing features Major difference with topological editing - not involved topology Edit existing features
Extend/trim lines Delete/move features Reshaping features Split lines and polygons
•
Create features from existing features
Merge features Buffer features Union features Intersect features
Figure 8.16 After a polygon of a shapefile is moved, a void area appears in its location.
Figure 8.17 Reshape a line by moving a vertex (a), deleting a vertex (b), and adding a vertex (c).
Figure 8.18 Sketch a line across the polygon boundary to split the polygon into two.
Figure 8.19 Merge four selected polygons into one.
Other editing operations
• Edgematching • Line simplification and smoothing
Figure 8.20 Edgematching matches the lines of two adjacent layers (top) so that the lines are continuous across the border (bottom).
Figure 8.21 Mismatches of lines from two adjacent layers are only visible after zooming in.
Figure 8.24 Line smoothing smoothes a line by generating new vertices mathematically and add them to the line.
Land Management Information Center at Minnesota Planning: NSSDA statistic http://www.lmic.state.mn.us/ Laser-Scan: Radius Topology http://www.laser-scan.com/technologies/radius/radius_topology/ Ordnance Survey: Master Map http://www.ordnancesurvey.co.uk/oswebsite/