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.
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
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.
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.
Append Choose append Method Decide how tics and coverage features will be numbered
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
Dissolve in ArcToolbox • Data Management > Generalization > Dissolve.
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.
Eliminate Options to… • Ensure outer borders of Coverages/Feature classes kept.
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.
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
Extract Tools: Clip • Only keeps the features inside the clip feature.
Extract Tools: Split • Clips the input Features/coverages and stores them in multiple output feature classes/output coverages
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
Overlay tools - Erase Original Coverage Erase Coverage Final Coverage • Cuts out and removes areas of Features that fall inside Polygons in a erase Coverage.
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
Insersect (AND) Union (OR) Overlays Output Original Input • Types Identity (Input determines output area)
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.
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.
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
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)
ArcToolbox Proximity Tools • Find or delineate Features based on distance from others. • Analysis Tools > Proximity
Buffer • Puts an area around a given Feature, some distance out from it. • Multiple ring buffer – does several bands
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.
Create Thiessen Polygons • Makes Thiessen Polygons, ignoring/ditching Points Proximal Tolerance or less apart.
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.
Statistics • In ArcToolbox: Analysis > Statistics • Analyses Attribute Tables and puts results in another. • Frequency, • Summary Statistics: Means, Standard Deviation etc.
Arc Commands • All the above ArcToolbox tools have equivalent Arc commands. • Consult the ArcDocs command list under the tool names.
Combining Tools: Spatial Analysis • Using the tools in combination allows us to select and amalgamate data. • This is the basis of GIS Spatial Analysis.
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.
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.
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.
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
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
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.
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.
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.
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
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.
Making Geometric Network Objects • Right-click the Feature Dataset > New > Geometric Network wizard.
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”.
Setting up Weights • Integer is Long Integer elsewhere.
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).
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.