Create Graduated Symbol and Density Maps from Geocoded data in ArcGIS Desktop

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Introduction

This document aims to show how to carry out simple GIS Anlaysis using ArcGIS Desktop.

Aim

The overall aim of the processes is to create a Graduated Symbol map and a Density map. This process will demonstrate Geocoding postcodes and then performing analysis using spatial joins and creating map outputs.

Required Datasets

• • A shapefile of boundaries of Wards downloaded from Digimap Ordnance Survey Data Download.

  • Code-Point Open data for Edinburgh downloaded from Digimap Ordnance Survey Data Download to give a list of all postcode units with centroids in Edinburgh.

  • Census population information for each postcode unit. We will use Geoconvert (http://geoconvert.mimas.ac.uk/index.html) to get Census population data for postcodes using the code-point postcodes as input.

  • A background OS map.

Summary of Processes

1) Create an Address Locator in ArcGIS Desktop

This will be used to geocode the postcodes, enabling them to be mapped.

• Download postcode data for Edinburgh from Digimap:

• • Use the Code-Point Open data for Edinburgh
  • The data is supplied in CSV format in a zip archive
  • Unzip the data from the archive

• Add the postcode data to ArcGIS using the Add Data... function

• Right click on the layer in the Table of Contents --> Display XY Data…

• Choose FIELD3 and FIELD4 for the coordinate fields

• Set the coordinate system to British National Grid

• Click OK (Ignore warning about no Object-ID field)

• The data will be add to ArcMap as an Events layer

• Right click on the events layer then Data --> Export Data… to export the data as a new Shapefile. (e.g. code-point.shp)

• Add the new shapefile to ArcGIS

• Remove the events and csv data table as they’re no longer required.

Next, create an Address Locator in ArcGIS using the downloaded Shapefile of postcodes for Edinburgh.

• In ArcToolbox navigate to Geocoding Tools --> Create Address Locator,
• Use the ‘General - Single Field’ style as the Address Locator Style
• Select the shapefile created above as your Reference Data,
• Set the ‘Field1’ field from the postcode data as the Key Field.
• Specify a suitable name location (i.e. a new folder) for the ‘Output Address Locator’ e.g EH_Address_locator.
• Click OK

2) Get Population by Postcode Unit

• Go to: http://geoconvert.mimas.ac.uk/index.html

• Start Geoconvert

• Select “Information about Postcodes” and select Next

• Select 2011 UK Census postcode lookup and select Next

• Select “Postcode populations” --> “Usually resident persons” and select Next

  • NOTE : You can also get the location of postcodes directly from Geoconvert but we will not do that as we want to show the generic geocoding functionality in ArcGIS.

• Supply a File of Postcodes, Comma separated with no header and select the location of the Code-Point Open csv dataset (eh.csv). Select Next

• Note some postcodes won’t be matched because they are not residential addresses and so have no census data.

• Select the Postcode Metadata and Save the file e.g. EH_matched.csv

3) Geocode Spreadsheet of Population by Postcode

Geocode the Population potcodes data

• Open the Geocode Addresses tool: ArcToolBox --> Geocoding Tools --> Geocode Addresses.

• In the Input Table field browse to the .csv spreadsheet of population points that have been downloaded from Geoconvert.

• Select the Address Locator created in step 3 above in the Input Address Locator field.

• In the Input Address Fields table select “Single Field” and ensure that the Alias Name field name is set to the column that contains the postcodes in the spreadsheet (standard_postcode).

• Specify a suitable output file location and name (e.g. Geocoded_pop).

The geocoded Population dataset will be added to ArcMap once the process has completed.

• Open the attribute table of the geocoded population data and check the points have all the necessary attributes (including one called TOT_PERS11 which is the total number of persons at that postcode)

4) Clip the data to an existing Spatial Extent

We only want a map showing the population within the boundary of the City of Edinburgh. The EH postcode extends outside of the City of Edinburgh.

• First create a layer of just the City of Edinburgh wards

• Load the Ward boundary dataset (district_borough_unitary_ward.shp)

• Use Selection --> Select by Attribute and Select * FROM district_borough_unitary_ward WHERE “FILE_Name” = “CITY_OF_EDINBURGH”

• Create a layer from selected features by right clicking on the Ward layer then Selection --> Create layer from Selected Features

• Second, start editing the Geocoded Population dataset

• Select all Geocoded population points inside the Ward boundaries using Select by Location

  • Target layer: Geocoded_pop

  • Source layer: district_borough_unitary_ward selection

  • Spatial selection method: are within the source layer feature

• Invert/Switch selection (this will change the selection to all those features outside the boundaries). Right click on Geocoded_pop layer--> Selection --> Switch Selection

• Delete the selected postcode points outside of Edinburgh by using Edit --> Cut

• Stop Editing on the Geocoded_pop layer and Save edits

5) Create Map of Population per Ward

Do a Spatial Join to calculate the population within each Ward area from all the Postcodes in that Ward. Add ‘counts’ to all the Ward polygons of points that match the criteria.

• Right click on the Wards (polygon) dataset and select Joins and Relates --> Join…

• Select ‘Join data from another layer based on spatial location’ in the top drop down menu.

• In Step 1 select the geocoded population dataset

• In Step 2 select the first toggle button (Each polygon will be given a summary…)

• Select Sum from the list of how you want the attributes to be summarized

• In Step 3 specify an appropriate name for the output Shapefile (e.g. wards_pop_counts)

• Click OK and a new Shapefile will be created that has the counts of geocoded points matching the query criteria on each polygon (the value will be in the attribute “Sum_TOT_PE”.

6) Symbolize Ward Population Data

Set up symbology (e.g. gradient, points symbols etc.) on the newly created dataset using the different attributes for the different maps:

• Bring up the Properties dialog for the layer by double clicking on it or right click --> Properties

• Symbology tab --> Quantities --> Graduated Symbols and select Sum_TOT_PE for the Value

• Change Background colour to be Hollow

• Add Labels to map based on Ward names

• Right click on the Wards layer and select Symbology then toggle the “Label features in this layer” On

• Set up the properties of the labels e.g the Label Field should be Name, Text size 8pt and Halo should be 1px

• Use Maplex for better positioning

7) Population Density Map

Create a population density map from the geocoded population points.

• Use ArcToolbox --> Spatial Analyst Tools --> Density --> Point Density

• Input point features: Geocoded_pop

• Population field: TOT_PERS11

• Set Output Cell size to be 50m

• Set Neighbourhood Radius to be 1000m

• Click OK

• The population density map will be loaded into ArcMap

• Set up Symbology for the generated Density map by selecting the Layer Properties --> Symbology tab

• Use a Classified symbology by classifying the result into 9 classes using Natural Breaks

• Choose a suitable colour ramp (e.g. blue)

• Change the first value to be “No Color”

• Click OK

• Set Transparency of Layer to be 50%: right click on layer and select Properties… --> Display tab and change Transparency to 50%

8) Add a Basemap

• Add Basemaps – OS Open Background from the Add Basemap menu
  

9) Final Map

The final map looks similar to that below showing both the Graduated Symbol symbols and the density map.