Spatial Manipulation and Analysis

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Lecture 5

• Spatial Manipulation and Analysis

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This Lecture

• Combining data.
• Spatial Analysis
• Append and Mapjoin
• Dissolve and Eliminate
• Overlay functions
• Clip, Erase, Identity, Intersect and Union
• Proximity functions
• Near, Pointdistance and Buffer
• Network Analysis.

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Combining Data

• It is likely that before analysis, youll have to
  combine several datasets.
• Weve looked at combining data that is about the
  same place. This lecture well look at combining
  geographical data that is adjacent into a single
  dataset.
• Having it in one dataset will allow us to analyse
  it.

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Append

• Makes a new Coverage by joining two or more
  Coverages usually next to each other.
• If the Feature types and Attribute Tables are the
  same, these joined too.
• Linked Polygon edges need same codes.
• First pick Coverages.

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Append
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Append and MapJoin in Arc

• Append is also an Arc command (see ArcDocs).
• Good for Arcs and Points.
• Append doesnt Clean or Build the new Coverage.
• Arc version doesnt have a clip-Coverage option.
• MapJoin is an ArcCommand that does an Append
  first, then a Clean.
• MapJoin has a clip-Coverage option.
• Best used on Polygons.

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After Appending

• Clean.
• Edge-matching Arcs that cross over adjacent
  Coverage boundaries.
• ArcEdit Matchnode command.
• MATCHNODE ltin_covergt out_cover
  match_tolerance NOEXTEND EXTEND
• See also Arc Snapcover command
• Removing sliver polygons or boundaries between
  adjacent Coverages (e.g., by Eliminate or
  Dissolve).

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Dissolve

• Merges features that share a value for an
  Attribute so there are no internal boundaries.
• E.g. dissolve on shared class in Attributes.

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Dissolve in ArcToolbox

• Data Management gt Generalization gt Dissolve
• Also an Arc command.

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Eliminate

• ArcToolbox tools (under Coverage Tools gt data
  Management) that make new Coverages from old by
  combining and deleting.
• Merges Polygons based on either the longest
  shared Arc or largest Area.
• Used to eliminate slivers e.g. merge all small
  Polygons with their neighbours with a larger
  area.
• Also merges Arcs separated by pseudo-nodes.
• Also see Arc command Eliminate in the ArcDocs.

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Eliminate

• Options to
• Ensure outer borders of Coverages kept.
• Select the Polygons to be eliminated using resel,
  nsel, asel. Or use the query builder

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Spatial Analysis

• Answer such questions as
• If smallpox has a spread rate of 10 km per day
  from a postal sorting office into the surrounding
  population, how many people will be infected in 3
  weeks?
• If we close all farms within 2km of a
  foot-and-mouth infected farm, which farms do we
  close?
• If we need to remove a 100m wide strip of trees
  to make a motorway, how much profit can we make
  by selling the wood associated with 10km of new
  road?
• All these questions can be answered by overlaying
  and cutting out bits of datasets.

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Extract Tools

• Analysis Tools gt Extract
• Clip keeps Features falling within clipping
  Coverage Polygons opposite of Erase.
• Select same kind of thing.
• Split used for splitting Features into their own
  Coverages.

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Extract Tools Clip

• Only keeps the features inside the clip feature.

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Overlay tools - Erase

• Cuts out and removes areas of Features that fall
  inside Polygons in a erase Coverage.

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ArcToolbox Erase

• Cluster Tolerance is the minimum distance between
  Points/Nodes in the output Coverages.

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Other Overlay Tools

• Combine two Coverages, deleting different
  sections.
• All overlay tools have similar dialogs to Erase.
• Attributes usually combined and repeated.

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Overlays

• Types

Input
Original
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Other Tools - Generalization

• Coverages gt Data Manegment gt Generalization gt
  Collapse Dual Lines to Centerlines
• Makes Centerlines on the basis of road outlines.
• Pick maximum and minimum distances Arc looks from
  a potential centerline point for a street edge.

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Other Tools - Generalization

• Simplify Buildings cuts down Polygons from
  architectural levels details to simpler features.
• Find Conflicts finds where simplified buildings
  fall within a certain distance of each other.
• Simplify Lines or Polygons cuts down Arcs and
  Polygons to simpler forms.

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ArcToolbox Proximity Tools

• Find or delineate Features based on distance from
  others.
• Analysis Tools gt Proximity

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Buffer

• Puts an area around a given Feature, some
  distance out from it.
• Multiple ring buffer does several bands

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Overlaid and Buffered Attributes

• All the Overlay and Buffer Tools have different
  ways of dealing with attributes.
• For example, if we split a Polygons Area, do we
  rescale the Attribute data in each or not? Do we
  even keep the data?
• Need to think carefully about whether attributes
  have been altered.
• Do you need to import the old area for a polygon
  and rescale the data associated with new Polygons
  by, e.g. assuming its evenly spread
  geographically.
• Check the ArcDocs for the ArcWorkstation commands
  to see how Attributes are dealt with.

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Create Thiessen Polygons

• Makes Thiessen Polygons, ignoring/ditching Points
  Proximal Tolerance or less apart.

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Other Tools

• Near finds the distance between a Point in one
  Coverage and the nearest Point in another, as
  long as they are below Search Radius apart.
• PointDistance finds the distance between all
  Points in one Coverage and all Points in another
  can produce very large Tables.
• PointNode assigns Attribute data associated with
  a Point in one Coverage to the closest Node
  within a search radius in another Coverage.

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Statistics

• Analysis gt Statistics
• Analyses INFO Tables and puts results in another.
• Frequency,
• Summary Statistics Means, Standard Deviation etc.

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Arc Commands

• All the above ArcToolbox tools have equivalent
  Arc commands.
• Consult the ArcDocs command list under the tool
  names.

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Combining Tools Spatial Analysis

• Using the tools in combination allows us to
  select and amalgamate data.
• This is the basis of GIS Spatial Analysis.

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Forest loss

• If we loose a 100m width of forest to build a new
  motorway, how much wood do we loose?

• We have data on the motorway route and forests.
• Buffer 50m around motorway.
• Convert buffer to Polygon Coverage.
• Clip forests from the forest Coverage using the
  new buffer Polygons.
• Aggregate the new clipped Polygons AREAs.

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Hospital markets

• Were evil NHS destroyers. We have Coverages of
  different affluence areas and a Point Coverage of
  hospitals. We want to find out the levels of
  affluence and poverty near our hospitals so we
  can convert some to private health care.

1. Make Thiessen Polygons around hospitals to see
   which one people in an area go to.
2. Use Intersect tool to combine the new Polygons
   with the affluence Polygons.
3. This makes a new dataset each Polygon having its
   affluence and a hospital reference.
4. Use Statistics tools to summarise affluence for
   each hospital.

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Summary

• Tools for appending data.
• Tools for dissolving boundaries.
• Tools for eliminating and selecting features.
• Tools for overlaying / combining.
• Tools for generalizing.
• Tools for buffering and Thiessen Polygons.
• Tools for statistics.

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Analysis in GeoDatabases

• Anything you can get to in ArcMap works for both.
• Stuff in ArcToolbox only tends to work on
  Coverages.
• Stuff in ArcWorkstation doesnt work on
  GeoDatabases at all.
• The thing GeoDatabases are good for is Network
  Analysis.

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Network Analysis

• Whats the fastest way to
• A point on an electronic circuit.
• How much will flow down
• A road or sewage pipe.
• Where is there a blockage
• in a motorway system.
• Whats downstream from
• a pollution point in a braided river system.

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Things We Can Model As Networks

• Transport Systems
• Roads, rivers, hill slopes.
• Utility Networks
• Sewage, water, electricity, gas.
• Knowledge Networks
• The spread of information, prices, policy effects.

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Networks in ArcDesktop

• Stored as Geodatabases, therefore Topology built
  during editing, and constraints added.
• We can guarantee a Network Object will only
  respond in a certain way (e.g. water pressure in
  a pipe will be under some limit).
• Stored in a special, constricted Feature Dataset
  element the Geometric Network.
• Consists of edge and junction elements which must
  be used to link each other up i.e. a edge cant
  connect directly to another edge, it must go
  through a junction.

Geometric Network
Geodatabase
Feature Class
Features
Feature Dataset
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Classes

• Junction Feature Classes inherit from
• ESRI Simple Junction Feature.
• Edge Feature Classes inherit from
• ESRI Simple Edge Feature or
• ESRI Complex Edge Feature.
• Complex Edge Features are groups of Features that
  act together.
• Network Features are like other Features, only
  they are assigned to a Geometric Network Object.

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Making Geometric Network Objects

• Right-click the Feature Dataset gt New gt Geometric
  Network wizard.

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Weights

• The Wizard allows you to say that some elements
  will have Weights attached.
• Weights control the flow rate through an edge or
  junction, e.g. stream width, or maximum traffic
  speed.
• You dont need to tell it the Feature class and
  attribute to use, just a name.
• You use the name to set an Attribute to a weight
  when you make the Feature Class.
• This way you can have the same weight name for
  different Attributes depending on the Feature,
  e.g. Maximum Flow can be used for Stream
  discharge and Traffic movement.

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Setting up Weights

• Integer is Long Integer elsewhere.

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Making Network Feature Classes

• Slight differences from other Feature Classes.
• Pick a ESRI Network Class to inherit (usually a
  Simple one)
• Pick the Network to add to (needs to be planned
  and made first).

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Sources and Sinks

• You can set a Junction to be a net Source or
  Sink, i.e. where materials enter or leave the
  system.
• This generates a flow direction for each Feature.
• This is used in combination with weights to
  determine flows.

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Setting Weights

• You get the option to set the Attributes to use
  as particular Weights.

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Table of Contents

• You should end up with something like the
  following.

Network
ArcDesktop will make default Junction Classes for
you.
But you can make your own.
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Setting Connectivity Constraints

• You can constrain which lines and junctions can
  connect in the Geometric Networks Properties.

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Adding Network Features

• You can add elements in ArcMap as usual, but you
  need to remember to add junction elements between
  edges.

• Source/Sink elements can have their AncillaryRole
  Attribute set to source/sink/neither. All
  Features also get an ENABLED Attribute which
  determines if they take part in the Network.

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Network Analysis

• Add the Utility Network Analysis toolbar. (Viewgt
  Toolbars)
• This gives you the ability to put in start and
  end flags for flow analysis.
• Also lets you block flows with flags.
• In the Analysis gt Options you can set the
  analyses to use Selected or unselected Features.
• You can also control flows by enabling and
  disabling Features in their Attributes.

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Flow and Tracing

• You can see the net result of sinks, sources and
  weights by putting on flow arrows. Flows can be
  indeterminate and uninitialized as well as
  determinate.
• However, the Trace Task list allows you to do
  more complex analysis between start and end
  flags.

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Trace Tasks

• Find Common Ancestors
• Common upstream points of flags.
• Find Connected
• Maps edges/junctions that can be reached from
  flags.
• Find Loops
• Finds Network loops.
• Find Path
• Finds the shortest path by length or the weights
  set in Options.
• Trace Downstream / Trace Upstream
• Maps areas Down/Upstream of flags.
• Can use blocking flags etc. to prevent some areas
  being accessed.

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Solve and Clear Flags

• To set an analysis running, press the Solve icon.
• To clear flags, select the menu item under
  Analysis.

Solve
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Example Find Path

• Find the quickest route from a water entering a
  glacier to an exit point on the forefield, given
  certain stream diameters etc.

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Example Trace Upstream

• Find out where pollutants could be entering a
  stream system.

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Other Examples

• Find out the maximum flow points on a Network.
• Work out where a Network is damaged.
• Calculate the effect of a Network blockage.
• Calculate the potential spread of pollutants.
• Calculate the shortest distance between two
  cities on a road network.

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Analysis in ArcWorkstation

• Geodatabases dont exist.
• Equivalent is a Network Coverage which is used in
  ArcPlot.
• The Netcover command creates a Network Coverage
  and opens the command interface for issuing other
  network commands.

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ArcWorkstation Network functionality

• Finding minimum-cost paths
• Allocating supply and demand
• Gravity modeling
• Location-allocation modeling
• Building a distance matrix
• Determining connectivity
• Assigning turn impedances
• Ordering features
• Classifying features
• Listing travel directions

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Summary

• To use Features in a Network, you have to build
  the Geometric Network.
• You can set names for the weights to be used with
  the Network.
• You can then add Feature Classes to it and assign
  the weights to Attributes.
• Features should inherit from ESRIs Simple or
  Complex Edge or Junction Classes.
• Junctions can be set as Source / Sink Objects,
  and Edges and Junctions can be given weights.

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Summary

• You add Network Features in ArcMap as usual, but
  edges should be linked by junctions.
• You can enable and disable Features and set
  Junctions as Sources / Sinks to determine the
  flows around the Network.
• You can use the flows and links to do analysis.

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Next Lecture

• Raster and 3D Data
• Practical
• Spatial Analysis task
• Assessment 1
• Detailed on the website after the practical.