Essentials of Geographic Information Systems Chapter 7 Geospatial Analysis I Vector Operations

Chapter Analysis I Vector Operations In Chapter Data Characteristics and Visualization , we discussed different ways to query , classify , and summarize information in attribute tables . These methods are indispensable for understanding the basic quantitative and qualitative trends of a . However , they don take particular advantage of the greatest strength of a geographic information system ( GIS ) notably the explicit spatial relationships . Spatial analysis is a fundamental component of a GIS that allows for an depth study of the topological and geometric properties of a or . In this chapter , we discuss the basic spatial analysis techniques for vector . URL books 157

Single Layer Analysis LEARNING OBJECTIVE . The objective of this section is to become familiar with concepts and terms related to the variety of single overlay analysis techniques available to analyze and manipulate the spatial attributes of a vector feature . As the name suggests , single layer analyses are those that are undertaken on an individual feature . Buffering is the process of creating an output polygon layer containing a zone ( or zones ) of a width around an input point , line , or polygon feature . Buffers are particularly suited for determining the area of around features of interest . is a suite of tools provided by many geographic information system ( GIS ) software packages that allow the user to automate many of the mundane tasks associated with manipulating GIS data . usually involves the input of one or more feature , followed by a spatially explicit analysis , and resulting in an output feature . Buffering Buffers are common vector analysis tools used to address questions of proximity in a GIS and can be used on points , lines , or ( Figure Buffers around Red Point , Line , and Polygon Features ) For instance , suppose that a natural resource manager wants to ensure that no areas are disturbed within feet of breeding habitat for the federally endangered Delhi Sands ( This species is found only in the few remaining Delhi Sands soil formations of the western United States . To accomplish this task , a protection zone ( buffer ) could be created around all the observed point locations of the species . Alternatively , the manager may decide that there is not enough location information related to this rare species and decide to protect all Delhi Sands soil formations . In this case , he or she could create a buffer around all labeled as Delhi Sands on a soil formations . In either case , the use of buffers provides a tool for determining which areas are to be maintained as preserved habitat for the endangered . URL books 168

Figure around Red Point , Line , and ( Features Lines Several buffering options are available to refine the output . For example , the buffer tool will typically buffer only selected features . If no features are selected , all features will be buffered . Two primary types of buffers are available to the GIS users constant width and variable width . Constant width users to input a value by which features are buffered ( Figure Buffers around Red Point , Line , and Polygon Features ) such as is seen in the examples in the preceding paragraph . Variable width buffers , on the other hand , call on a buffer within the attribute table to determine the buffer width for each feature in the ( Figure Additional Buffer Options around Red Features ( a ) Variable Width Buffers , Multiple Ring Buffers , Doughnut Buffer , Setback Buffer , Buffer , Dissolved Buffer ) In addition , users can choose to dissolve or not dissolve the boundaries between overlapping , coincident buffer areas . Multiple ring buffers can be made such that a series of concentric buffer zones ( much like an archery target ) are created around the originating feature at distances ( Figure Additional Buffer Options around Red Features ( a ) Variable Width Buffers , Multiple Ring Buffers , Doughnut Buffer , Setback Buffer , Buffer , Dissolved Buffer ) In the case of polygon layers , buffers can be created that include the originating polygon feature as part of the buffer or they be created as a doughnut buffer that excludes the input polygon area . Setback buffers are similar to doughnut buffers however , they only buffer the area inside of the polygon boundary . Linear features can URL books ) 169

be buffered on both sides of the line , only on the left , or only on the right . Linear features can also be buffered so that the end points of the line are rounded ( ending in a ) or ( ending in a rectangle ) Figure Additional Options around Red Features ( a ) Variable Width , Multiple Ring , Doughnut , Setback , Dissolved Buffer Operations URL books 170

is a loaded term in the field of GIS . The term can ( and should ) be widely applied to any attempt to manipulate GIS data . However , the term came into common usage due to its application to a somewhat arbitrary suite of single layer and multiple layer analytical techniques in the Wizard of software package in the . Regardless , the suite of tools available in a GIS greatly expand and simplify many of the management and manipulation processes associated with vector feature . The primary use of these tools is to automate the repetitive needs of typical spatial analyses and to assemble exact graphical representations for subsequent analysis inclusion in presentations and final mapping products . The union , intersect , symmetrical difference , and identity overlay methods discussed in Section Other Options are often used in conjunction with these tools . The following represents the most common tools . The dissolve operation combines adjacent polygon features in a single feature based on a single predetermined attribute . For example , part ( a ) Single Layer Functions shows the boundaries of seven different parcels of land , owned by four different families ( labeled through ) The dissolve tool automatically combines all adjacent features with the same attribute values . The result is an output layer with the same extent as the original but without all of the unnecessary , intervening line segments . The dissolved output layer is much easier to visually interpret when the map is classified according to the dissolved field . The append operation creates an output polygon layer by combining the spatial extent of two or more layers ( part ( of Figure Single Layer Functions ) For use with point , line , and polygon , the output layer will be the same feature type as the input layers ( which must each be the same feature type as well ) Unlike the dissolve tool , append does not remove the boundary lines between appended layers ( in the case of lines and ) Therefore , it is often useful to perform a dissolve after the use of the append tool to remove these potentially unnecessary dividing lines . Append is frequently used to mosaic data layers , such as digital US Geological Survey ( topographic maps , to create a single map for analysis display . URL books 171

The select operation creates an output layer based on a query that selects particular features from the input layer ( part ( of Figure Single Layer Functions ) The output layer contains only those features that are selected during the query . For example , a city planner may choose to perform a select on all areas that are zoned residential so he or she can quickly assess which areas in town are suitable for a proposed housing development . Finally , the merge operation combines features within a point , line , or polygon layer into a single feature with identical attribute information . Often , the original features will have different values for a given attribute . In this case , the attribute encountered is carried over into the attribute table , and the remaining attributes are lost . This operation is particularly useful when are found to be unintentionally overlapping . Merge will conveniently combine these features into a single entity . Figure ( pi ( a ) Dissolve Mil KEY TAKEAWAYS Buffers are frequently used to create zones of a specified width around points , lines , and . URL books 172

Vector buffering options include constant or variable widths , multiple rings , doughnuts , setbacks , and dissolve . Common single layer operations on vector layers include dissolve , merge , append , and select . EXERCISES . List and describe the various buffering options available in a GIS . Why might you use the various operations to answer spatial questions related to your particular field of study ?

URL books 173 Multiple Layer Analysis LEARNING OBJECTIVE . The objective of this section is to become familiar with concepts and terms related to the implementation of basic multiple layer operations and used on vector feature . Among the most powerful and commonly used tools in a geographic information system ( GIS ) is the overlay of cartographic information . In a GIS , an overlay is the process of taking two or more different thematic maps of the same area and placing them on top of one another to form a new map ( Figure A Map Overlay Combining Information from Point , Line , and Polygon Vector Layers , as Well as Raster Layers ) Inherent in this process , the overlay function combines not only the spatial features of the but also the attribute information as well . Figure Map Overlay Combining Point , Line , and Polygon Layers , as Well as Raster Layers ToMaR ( URL books ) a 174

A common example used to illustrate the overlay process is , Where is the best place to put a mall ?

Imagine you are a corporate bigwig and are tasked with determining where your company next shopping mall will be placed . How would you attack this problem ?

With a GIS at your command , answering such spatial questions begins with amassing and overlaying pertinent spatial data layers . For example , you may want to determine what areas can support the mall by accumulating information on which land parcels are for sale and which are zoned for commercial development . After collecting and overlaying the baseline information on available development zones , you can begin to determine which areas offer the most economic opportunity by collecting regional information on average household income , population density , location of proximal shopping centers , local buying habits , and more . Next , you may want to collect information on restrictions or roadblocks to development such as the cost of land , cost to develop the land , community response to development , adequacy of transportation corridors to and from the proposed mall , tax rates , and so forth . Indeed , simply collecting and overlaying spatial provides a valuable tool for visualizing and selecting the optimal site for such a business endeavor . Overlay Operations Several basic overlay processes are available in a GIS for vector , point , and . As you may be able to divine from the names , one of the overlay must always be a line or polygon layer , while the second may be point , line , or polygon . The new layer produced following the overlay operation is termed the output layer . The overlay operation requires a point input layer and a polygon overlay layer . Upon performing this operation , a new output point layer is returned that includes all the points that occur within the spatial extent of the overlay ( Figure A Map Overlay Combining Information from Point , Line , and Polygon Vector Layers , as Well as Raster Layers ) In addition , all the points in the output layer contain their original attribute information as well as the attribute information from the overlay . For example , suppose you were tasked with determining if an endangered species residing in a national park was found primarily in a particular vegetation community . The first step would be to acquire the point URL books 175

occurrence locales for the species in question , plus a polygon overlay layer showing the vegetation communities within the national park boundary . Upon performing the overlay operation , a new point is created that contains all the points that occur within the national park . The attribute table of this output point would also contain information about the vegetation communities being utilized by the species at the time of observation . A quick scan of this output layer and its attribute table would allow you to determine where the species was found in the park and to review the vegetation communities in which it occurred . This process would enable park employees to make informed management decisions regarding which onsite habitats to protect to ensure continued site utilization by the species . Figure ) I . II . no a . As its name suggests , the overlay operation is the opposite of the operation . In this case , the polygon layer is the input , while the point layer is the overlay . The polygon features that overlay these points are selected and subsequently preserved in the output layer . For example , given a point containing the locales of some type of crime and a polygon representing city blocks , a overlay operation would allow police to select the city blocks in which crimes have been known to occur and hence determine those locations where an increased police presence may be warranted . URL books 175

Figure ) A overlay operation requires line features for both the input and overlay layer . The output from this operation is a point or points located precisely at the intersection ( of the two linear ( Figure Overlay ) For example , a linear feature containing railroad tracks may be overlain on linear road network . The resulting point contains all the locales of the railroad crossings over a town road network . The attribute table for this railroad crossing point would contain information on both the railroad and the road over which it passed . Figure ( The overlay operation is similar to the overlay , with that obvious exception that a line input layer is used instead of a point input layer . In this case , each line that has any part of its extent within the overlay polygon layer will be included in the output line layer , although these lines will be truncated at the boundary of the overlay ( Figure Overlay ) For URL books 177

example , a overlay can take an input layer of interstate line segments and a polygon overlay representing city boundaries and produce a linear output layer of highway segments that fall within the city boundary . The attribute table for the output interstate line segment will contain information on the interstate name as well as the city through which they pass . Figure , The overlay operation is the opposite of the operation . In this case , the polygon layer is the input , while the line layer is the overlay . The polygon features that overlay these lines are selected and subsequently preserved in the output layer . For example , given a layer containing the path of a series of telephone and a polygon map contain city parcels , a overlay operation would allow a land assessor to select those parcels containing overhead telephone wires . Figure II ( I ( URL books 178

Finally , the overlay operation employs a polygon input and a polygon overlay . This is the most commonly used overlay operation . Using this method , the polygon input and overlay layers are combined to create an output polygon layer with the extent of the overlay . The attribute table will contain spatial data and attribute information from both the input and overlay layers ( Figure Polygon Overlay ) For example , you may choose an input polygon layer of soil types with an overlay of agricultural fields within a given county . The output polygon layer would contain information on both the location of agricultural fields and soil types throughout the county . Figure ( The overlay operations discussed previously assume that the user desires the overlain layers to be combined . This is not always the case . Overlay methods can be more complex than that and therefore employ the basic operators AND , OR , and ( see Section Measures of Central Tendency ) Depending on which operator ( are utilized , the overlay method employed will result in an intersection , union , symmetrical difference , or identity . the union overlay method employs the OR operator . A union can be used only in the case of two polygon input layers . It preserves all features , attribute information , and spatial from both input layers ( part ( a ) of Figure Vector Overlay Methods ) This overlay method is based on the operation described in Section Buffering . URL books 179

Alternatively , the intersection overlay method employs the AND operator . An intersection requires a polygon overlay , but can accept a point , line , or polygon input . The output layer covers the spatial extent of the overlay and contains features and attributes from both the input and overlay ( part ( of Figure Vector Overlay Methods ) The symmetrical difference overlay method employs the operator , which results in the opposite output as an intersection . This method requires both input layers to be . The output polygon layer produced by the symmetrical difference method represents those areas common to only one of the feature ( part ( of Figure Vector Overlay Methods ) In addition to these simple operations , the identity ( also referred to as minus ) overlay method creates an output layer with the spatial extent of the input layer ( part ( of Figure Vector Overlay Methods ) but includes attribute information from the overlay ( referred to as the identity layer , in this case ) The input layer can be points , lines , or . The identity layer must be a polygon . URL books 180

Figure Vector ( Methods ( Intersect ( Identity ( Erase Input Layer Erase Layer Input Layer Split Layer Other Options In addition to the aforementioned vector overlay methods , other common multiple layer options are available to the user . These included the clip , erase , and split tools . The clip operation is used to extract those features from an input point , line , or polygon layer that falls within the URL books 181

spatial extent of the clip layer ( part ( of Figure Vector Overlay Methods ) Following the clip , all attributes from the preserved portion of the input layer are included in the output . If any features are selected during this process , only those selected features within the clip boundary will be included in the output . For example , the clip tool could be used to clip the extent of a river by the extent of a county boundary . This would provide county managers with insight into which portions of the they are responsible to maintain . This is similar to the intersect overlay method however , the attribute information associated with the clip layer is not carried into the output layer following the overlay . The erase operation is essentially the opposite of a clip . Whereas the clip tool preserves areas within an input layer , the erase tool preserves only those areas outside the extent of the analogous erase layer ( part ( of Figure Vector Overlay Methods ) While the input layer can be a point , line , or polygon , the erase layer must be a polygon . Continuing with our clip example , county managers could then use the erase tool to erase the areas of private ownership within the county area . Officials could then focus specifically on public reaches of the for their upkeep and maintenance responsibilities . The split operation is used to divide an input layer into two or more layers based on a split layer ( part ( of Figure Vector Overlay Methods ) The split layer must be a polygon , while the input layers can be point , line , or polygon . For example , a homeowners association may choose to split up a soil series map by parcel boundaries so each homeowner has a specific soil map for their own parcel . Spatial Join A spatial join is a hybrid between an attribute operation and a vector overlay operation . Like the join attribute operation described in Section Joins and Relates , a spatial join results in the combination of two feature tables by a common attribute . Unlike the attribute operation , a spatial join determines which fields from a source layer attribute table are appended to the destination layer attribute table based on the relative locations of selected features . This relationship is explicitly based on the property of proximity or containment between the source and destination layers , rather than the URL books 182

primary or secondary keys . The proximity option is used when the source layer is a point or line feature , while the containment option is used when the source layer is a polygon feature . When employing the proximity ( or nearest ) option , a record for each feature in the source layer attribute table is appended to the closest given feature in the destination layer attribute table . The proximity option will typically add a numerical to the destination layer attribute table , called Distance , within which the measured distance between the source and destination feature is placed . For example , suppose a city agency had a point showing all known polluters in town and a line of all the river segments within the municipal boundary . This agency could then perform a based spatial join to determine the nearest river segment that would most likely be affected by each polluter . When using the containment ( or inside ) option , a record for each feature in the polygon source layer attribute table is appended to the record in the destination layer attribute table that it contains . If a destination layer feature ( point , line , or polygon ) is not completely contained within a source polygon , no value will be appended . For example , suppose a pool cleaning business wanted to hone its marketing services by providing only to homes that owned a pool . They could obtain a point containing the location of every pool in the county and a polygon parcel map for that same area . That business could then conduct a spatial join to append the parcel information to the pool locales . This would provide them with information on the each land parcel that contained a pool and they could subsequently send their mailers only to those homes . Overlay Errors Although overlays are one of the most important tools in a GIS analyst toolbox , there are some problems that can arise when using this methodology . In particular , slivers are a common error produced when two slightly vector layers are overlain ( Figure Slivers ) This misalignment can come from several sources including digitization errors , interpretation errors , or source map errors ( Chang 2008 ) For example , most vegetation and soil maps are created from field survey data , satellite images , and aerial photography . While you can imagine that the boundaries of soils and vegetation frequently coincide , the fact that they were most likely created by different researchers at different times suggests URL books 183

that their boundaries will not perfectly overlap . To ameliorate this problem , GIS software incorporates tolerance option that forces nearby lines to be snapped together if they fall within a distance . Care must be taken when assigning cluster tolerance . Too strict a setting will not snap shared boundaries , while too lenient a setting will snap unintended , neighboring boundaries together ( Wang and 1995 ) Figure Polygon at time Polygon at time Polygon composite Sliver from overlay operation A second potential source of error associated with the overlay process is error propagation . Error propagation arises when inaccuracies are present in the original input and overlay layers and are propagated through to the output layer ( 1975 ) These errors can be related to positional inaccuracies of the points , lines , or . Alternatively , they can arise from attribute errors in the original data table ( Regardless of the source , error propagation represents a common problem in overlay analysis , the impact of which depends largely on the accuracy and precision requirements of the project at hand . KEY TAKEAWAYS Overlay processes place two or more thematic maps on top of one another to form a new map . Overlay operations available for use with vector data include the , and models . URL books ( 184

Union , intersection , symmetrical difference , and identity are common operations used to combine information from various overlain . EXERCISES . From your own field of study , describe three theoretical data layers that could be overlain to create a new , output map that answers a complex spatial question such as , Where is the best place to put a mail ?

Go online and find the vector related to the question you just proposed . Chang , 2008 . Introduction to Geographic Information Systems . New York . Wang , and . 1995 . A Study of the Impact of Automated Editing on Polygon Overlay Analysis Computers and 21 . 1975 . The Accuracy of Map Landscape Planning . URL books 185