Chapter 9. Attribute joins

Transcription

1 Chapter 9 Spatial Joins 9-1 Copyright McGraw-Hill Education. Permission required for reproduction or display. Attribute joins Recall that Attribute joins: involve combining two attribute tables together to create one table Join two tables one-to-one by common a field that exists in both tables The field names can be different, but The field values must match 2 GIS TUTORIAL 1 - Basic Workbook 1

2 Destination table Attribute joins Common field Source table Joined table 4-3 Attribute joins When joining tables, the default option is to keep all records in the join table 9-4 2

3 Attribute joins With, the keep all records option If a record in the target table doesn't have a match in the join table, that record is given null values for all the fields being appended into the target table from the join table 9-5 Attribute joins The other option is to keep only matching records in the join table 9-6 3

4 Attribute joins with the keep only matching records option If a record in the target table doesn't have a match in the join table, that record is removed from the resultant target table 9-7 Attribute joins GIS TUTORIAL 1 - Basic Workbook 8 4

5 Making Permanent joins Attribute joins are temporary and can be removed Export data (or Table) to make joins permanent 9GIS TUTORIAL 1 - Basic Workbook SPATIAL JOINS 10 GIS TUTORIAL 1 - Basic Workbook 5

6 Spatial Joins Spatial Join: Joining two feature classes based on a common spatial relationship One feature inside another One feature closest to another Output is a new feature class Output is permanent (unlike attribute table joins) 9-11 Spatial Joins Lets you append additional data from the join layer features to the source layer features, so you can: Symbolize the source layer features with joined layer feature data Generate counts and summaries of the number of joined layer features spatially related to the source feature layer

7 Spatial join example You have census block group centroids (points) with housing fuel data You want to know housing fuel data by city neighborhood (polygon) The two layers have no attribute fields in common So, a spatial join is needed 13 GIS TUTORIAL 1 - Basic Workbook Spatial join: example Spatially join points to polygons Spatially joins point features (block centroids) within polygons (neighborhoods) Joins using the shape field (not a common attribute field) GIS 14TUTORIAL 1 - Basic Workbook 7

8 Spatial join: example Right click polygon layer (neighborhoods) GIS 15TUTORIAL 1 - Basic Workbook Spatial joins: example Result: New feature class (in this case polygons) Summary data from the joined layer GIS 16TUTORIAL 1 - Basic Workbook 8

9 Spatial join inputs and outputs Input: Destination feature class Input: Source feature class Neighborhoods Census Block Centroids Output: New joined feature class Neighborhoods Types of spatial joins Every join involves two geometry types Points to points Polygons to lines Lines to points, etc. Each combination offers two possible join types a simple join, or a summarized join

10 Simple Summarized Inside Schools Counties Which county is each school in? Counties Schools How many schools in each of the counties? Distance Hotels Attractions Which attraction is closest to each hotel? How far is it? Hotels Attractions How many attractions are closest to each hotel? 9-19 Several possible combinations Geometry Type Join Type Example Points to Points Lines to Points Polygons to Points Simple distance Summarized distance Simple distance Summarized inside Simple inside Simple distance Find the hospital closest to each town. Find all the towns closer to one hospital than to any other hospital. Find the water main closest to the proposed building site. Find the total voltage of all electric lines meeting at a substation. Find the soil type that underlies each gas station. Find the lake that is closest to each campground

11 Simple distance Ex: Points to Points Find the source feature that is the closest to the destination feature. Find the hospital closest to each town. Destination: hospitals Summarized distance Summarize the attributes of all the source features that are closer to the destination feature than to any other. Find all the towns closer to one hospital than to any other hospital Simple distance Ex: Polygons to Points Find the county that is closest to each hospital and give the hospital the county attributes. Destination: hospitals Simple inside Find the county that each hospital is inside and give the hospital that county s attributes

12 Ex: Polygons to Lines Simple distance Find the park that is closest to each road and give the road the park attributes. Summarized inside Give the interstate the total population of all the counties that it crosses Ex: Polygons to Polygons Simple inside Give each urban area the attributes of the county that it falls inside. Summarized inside Give each county the summarized attributes of the urban areas that fall inside it. Notice that in this case we need to switch the destination layer for the join to make sense

13 Ex: Polygons to Polygons Simple inside Find the park that each lake is inside and give the lake the attributes of the park. Summarized inside Give the park the total area of all the lakes that fall inside it. Notice that in this case we need to switch the destination layer for the join to make sense. Also notice that some lakes do not fall cleanly inside one park or another. Not all joins are capable of giving valid results. Think! 9-25 A note on polygon distances In measuring distances for polygons, the centroid (geometric center) of the polygon is used as a point to measure distance with Ex: For each of the counties, the spatial join would measure the distance from the hospital (point) to the centroid (a point) of the county (polygon) If the county contains a hospital, the distance is zero Pennington County has three hospitals all with a distance of zero, so one is randomly chosen to match

14 Other options These examples are only a few of the possible combinations the rest are shown in your text. If this seems complicated, don t worry. ArcGIS figures out the two possibilities and presents you with a choice one usually makes sense after a little thought Polygons to points Example: ATM robberies (points) need neighborhood name 28 GIS TUTORIAL 1 - Basic Workbook 14

15 Polygon to points join result Neighborhood name appended to each point feature 29 GIS TUTORIAL 1 - Basic Workbook Points to Points Example: What is the distance of a burglary to the nearest commercial property? 30 GIS TUTORIAL 1 - Basic Workbook 15

16 Points to points join result Distance to nearest commercial property shows on each burglary point 31 GIS TUTORIAL 1 - Basic Workbook Distance joins and coordinate systems

17 Distance joins and coordinate systems When using distance joins always make sure your source layers are in a planar coordinate system that conserves (does not distort) distance, i.e.: UTM State Plane Feet NOT WGS 84 or Web Mercator Both input layers should be in the same coordinate system 9-33 Source coordinate system Look again at the join of hospitals to counties. What happens if the source layers are in a GCS?

18 Distance join units The source data were in a GCS with units of decimal degrees. Distances are recorded in stored units Decimal degrees cannot be easily converted to miles or km because the conversion factor varies with latitude Better to use a projected coordinate system 9-35 Beware GCS SD State Plane Projected on the fly The data frame coordinate system may be different from the source data coordinate system. Setting the data frame coordinate system is not enough to fix the problem. You must project the source data using the Project tool before performing a distance join

19 ArcGIS Demo In-Class Demo (The following slides are for reference if you missed the demo) 9-37 How to join Right-click destination table Set Join type to spatial Choose source table Choose join type Specify output file

20 Two choices Based on the two geometries and the destination, ArcMap picks the possible two join types. You just need to pick the right one. Usually one is simple and one is summarized Choosing summary statistics Unlike the Summarize command that pairs stats with specific fields, in joins you only pick the type(s) of statistics. A new stats field will be generated for every numeric field in the table. Choose only what you need or your output may have too many fields!

21 Setting up a join Think about the layers involved How do I want the output layer/table to look? Which is the destination layer? Is this a distance join or an inside join? Which is the input layer What is the cardinality? Do I need a simple join or a summarized join? 9-41 Example #1 Find all congressional districts that have had more than 10 earthquake deaths

22 How do I want the output to look? Which is the destination layer? Is this a distance join or an inside join? What is the cardinality? Do I need a simple join or a summarized join? 9-43 Example #2 Develop a pollution risk index for rivers based on the total number of people in the adjacent counties

23 How do I want the output to look? Which is the destination layer? Is this a distance join or an inside join? What is the cardinality? Do I need a simple join or a summarized join? 9-45 Example #3 Create a table showing the volcano closest to each city in the US

24 How do I want the output to look? Which is the destination layer? Is this a distance join or an inside join? What is the cardinality? Do I need a simple join or a summarized join?