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Lecture 5. Spatial Manipulation and Analysis. This Lecture. Spatial manipulation Combining data. Append Dissolve and Eliminate Spatial Analysis Overlay functions Clip, Erase, Identity, Intersect and Union Proximity functions Near, Pointdistance and Buffer Network Analysis.

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Lecture 5

Spatial Manipulation and Analysis


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This Lecture

  • Spatial manipulation

    • Combining data.

    • Append

    • Dissolve and Eliminate

  • Spatial Analysis

  • Overlay functions

    • Clip, Erase, Identity, Intersect and Union

  • Proximity functions

    • Near, Pointdistance and Buffer

  • Network Analysis.


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Selecting the right toolCoverages vs Feature Classes

  • ArcToolbox has Tools for Coverages and Tools for Geodatabases/Feature Classes

  • Need to choose correct Tool

  • Get to know your way around ArcToolbox

  • Analysis Tools will work for both models in general

  • However, a Tool is not always available in one of the data models


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Combining Data

  • It is likely that before analysis, you’ll have to combine several datasets.

  • We’ve looked at combining data that is about the same place. You also may need to combine geographical data that are adjacent into a single dataset.

  • Having it in one dataset will allow us to analyse it.


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Append - Coverages

  • 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

Choose append Method

Decide how tics and coverage features will be numbered


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Append – Geodatabases


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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.

A

A

C

B

C

A

B

B

C


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Dissolve in ArcToolbox

  • Data Management > Generalization > Dissolve.


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Eliminate

  • ArcToolbox tools (under Data Management > Generalization) that make new Feature Classes 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.


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Eliminate

Options to…

  • Ensure outer borders of Coverages/Feature classes kept.


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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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Analysis Tools in ArcToolbox


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Extract Tools

  • Analysis Tools > Extract

  • Clip: keeps Features falling within clipping Coverage Polygons – opposite of Erase (an Overlay tool).

  • Select: same kind of thing.

  • Split: used for splitting Features into their own Coverages/Feature classes


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Extract Tools: Clip

  • Only keeps the features inside the clip feature.


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Extract Tools: Split

  • Clips the input Features/coverages and stores them in multiple output feature classes/output coverages


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Overlay

  • Combination of attributes

    • e.g. a property developer may be interested in land NOT in flood zone but for sale

  • Overlay = Take two layers and create a third which meet certain criteria

  • One layer must be a polygon feature class/coverage

  • Other layer can be a polygon, line or point feature class

  • Pre-GIS it would have been done using transparent maps and a light table


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Overlay tools - Erase

Original Coverage

Erase Coverage

Final Coverage

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


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ArcToolbox Erase


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Other Overlay Tools

  • Combine two Coverages/feature classes, deleting different sections.

  • All overlay tools have similar dialogs to Erase.

  • Attributes usually combined and repeated.

A

A

A

1

2

A1

A2

B1

B2

B

B3

B4

3

4

A3

A4


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Insersect

(AND)

Union

(OR)

Overlays

Output

Original

Input

  • Types

Identity

(Input

determines

output area)


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Other Tools - Generalization(Coverage Tools only)

  • Coverages > Data Management > Generalization > 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

  • Smooth a line: smoothes a line to improve its aesthetic or cartographic quality – uses smoothing algorithm

  • Simplify Buildings: cuts down Polygons from architectural levels details to simpler features (coverages only).

  • Find Conflicts: finds where simplified buildings fall within a certain distance of each other (coverages only).

  • Simplify Lines or Polygons: cuts down Arcs and Polygons to simpler forms.


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Proximity

  • Nearness in terms of physical distance

    • E.g. House buying – close to nice things such as friends, shops, countryside but long way from unpleasant things such as a smelly factory

    • E.g. Power Station may need to be close to infrastructure (roads etc) and water but far away from protected natural beauty spots


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Proximity in GIS

  • GIS implements proximity using buffers

    • A set distance around a point (circle)

    • A set distance on one side or both sides of a line (polygon)

  • ArcGIS creates a buffer and then allocates a field within the attribute which defines its status (inside or outside the buffer)


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ArcToolbox Proximity Tools

  • Find or delineate Features based on distance from others.

  • Analysis Tools > 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 Polygon’s 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 it’s evenly spread geographically.


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Create Thiessen Polygons

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


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Other Proximity Tools

  • Near: finds the distance between a Point in one Feature Class and the nearest Point in another, as long as they are below Search Radius apart.

  • PointDistance: finds the distance between all Points in one Feature Class and all Points in another – can produce very large Tables.


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Near


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Point Distance


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Statistics

  • In ArcToolbox: Analysis > Statistics

  • Analyses Attribute 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

  • We’re evil NHS destroyers. We have layers of different affluence areas and a Point layer 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.

  • Make Thiessen Polygons around hospitals to see which one people in an area go to.

  • Use Intersect tool to combine the new Polygons with the affluence Polygons.

  • This makes a new dataset each Polygon having its affluence and a hospital reference.

  • 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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Spatial Operations

  • You have been set the task of identifying a suitable site to move the University of Leeds to a new, non-residential location so that there is more space to expand and accommodate all the students on campus

  • You have access to land use data, river data, road data and site status

  • You know that the site must be non-residential, not susceptible to flooding so at least 0.5km from a river, within 1km of a motorway or major road to allow easy access and the site must be for sale

  • What spatial operations are required to site the University?

  • Try and complete the boxes with general spatial operations which might define the scheme required to site the University


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Site status maps

Land use maps

Identify and collect data

Transport maps

River maps

Convert data to digital format

Land use

Site status

River Map

Road map

Non-residential areas

Sites for Sale

River buffer

Major roads

No Flood

Road buffer

New roads

Overlay

Answer map

Calculate areas

Final map of suitable sites


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Analysis in GeoDatabases

  • Many, but not all, analysis tools in ArcToolbox work for both Coverages and Feature Classes

  • Stuff in ArcWorkstation doesn’t work on GeoDatabases at all.

  • The thing GeoDatabases are good for is Network Analysis.


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Network Analysis

  • What’s 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.

  • What’s 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 can’t connect directly to another edge, it must go through a junction.

Geometric Network

Geodatabase

Feature Dataset

Feature Class

Features


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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 > New > 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 don’t 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 Network’s 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. (View> 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 > 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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Network Analysis in ArcWorkstation

  • Geodatabases don’t 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.

  • This is not covered in this module


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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 ESRI’s 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


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