Online WhatIf? Data Preparation Guide

Online WhatIf: Input data preparation

Online WhatIf (OWI) can be configured to work for any study area for which the required data is available. Land use categories, suitability factors, population and employment data, policy options, and all other information considered in the model needs to be combined into a single shapefile that will be uploaded into OWI.

The fundamental spatial unit of analysis in OWI is polygons. Each polygon must be uniform or homogeneous with respect to all attributes. For instance, all points within a given polygon must have the same land use class, be located in the same municipality, and be within the same buffer distance of an existing or proposed bus stop, and so on. We will refer to this spatial unit of analysis (the polygons making up the study area) as Uniform Analysis Zones (UAZs).

UAZs are created by combining all the relevant datasets using appropriate GIS overlay functions and techniques.

It is absolutely essential to have a layer for the study area’s existing land use. A variety of additional layers can be added, depending on the data availability, the analysis and policy needs, the level of detail desired, and the kind of secondary applications you will be doing (e.g. transportation planning).

At a minimum, you will want at least three land feature layers for the Suitability component of the analysis and two or three additional infrastructure or land use control layers for the Allocation component. You will also need a sub-area boundary layer that can be used to extract and project population and employment data.

An actual OWI application may incorporate a dozen GIS layers. The following is a simplified example which contains only three GIS layers: Grassland Quality, Land Use and Planning Scheme (zoning). However, the procedure for preparing the UAZ file is similar regardless the number of layers considered. The following steps are required to convert GIS layers into a WhatIf UAZ file:

  • Combining GIS layers;
  • Removing multi-part Features;
  • Removing slivers;
  • Creating/correcting topology;
  • Deleting unnecessary data fields;
  • Creating the UAZ shapefile.

The process of creating a UAZ file will be described using ArcGIS 10.2. Please note that it is possible to follow a similar process in most of the GIS tools available, however we will concentrate on ArcGIS since it seems to be the tool of preference in most Planning Institutions around Australia. Nevertheless, an experienced GIS Officer/Analyst should be able to derive a similar process using their GIS of choice by following the method described in this document.

Sample GIS Layers

The process of creating a UAZ file begins by using the Union (Analysis) in ArcGIS. This overlay technique calculates the geometric union of any number of polygon layers (vector format). Please note that if any of the layers to be combined is not in the polygon feature data format then the necessary geoprocessing technique should be employed to convert it.

In this example, Grassland Quality is a raster dataset. Thus, before doing a Union (Analysis) the Grassland raster dataset was converted to a polygon feature data using the RasterToPolygon functionality in ArcGIS (ArcToolbox → Conversion Tools → From raster → Raster to Polygon). Each group of contiguous cells of same value in the raster dataset becomes polygons in the output. Figure 1 shows the raster image (left) and what it looks like once converted to polygons (right).

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Grassland Quality

The Grassland layer contains the native vegetation of the study area, namely grassland. The study area has been assessed and classified in 3 categories: High Quality, Medium Quality and Low Quality, as shown in Figures 2 and 3. Figure 2 shows the current state of the grassland on a map of the study area; Figure 3 shows the table of attributes associated with the map.

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Land Use Layer

The Land Use Layer includes ten land use classes: Agricultural, Commercial, Educational, Hospital/Medical, Industrial, Parkland, Residential – High Density, Residential – Medium Density, Residential – Low Density, and Transport. The polygons representing each of these land use classes are illustrated on the map in figure 4. The land use classification is stored in a field called “LU” and their respective size (area in hectares) is stored in a field called Area_Ha, as shown in Figure 5.

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The Planning Scheme Layer

The planning scheme layer, or the zoning layer, is the representation of statutory objectives, policies and provisions for the use, development and protection of the land. This layer is an important instrument in land use planning and will determine future urban developments. The polygons representing land zoning are shown in Figure 6. The zoning classification is stored in a field called “ZONE_CODE” and “DESC” as shown in Figure 7.

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[Click to Enlarge]

Combining GIS Layers

To combine all the GIS layers into one single shapefile:

  • Open ArcCatalog;
  • Click on the ArcToolbox → Analysis Tool→ Overlay→ Union;
  • Use the Input Features selection tool to select the Grassland Quality, Land Use and Planning Scheme polygon-based layers;
  • Use the Output Feature Class selection tool to specify the name for the Union file
  • Select ALL from the JoinAttributes option list;
  • Click on the Environments button and expand the Output Coordinates Option to specify the Coordinate System of the output file;
  • Select “As Specified Below” from the Output Coordinate System option list and search for GCS_WGS_1984 (EPSG4326) Geographic Coordinate System by browsing the Spatial Reference Properties dialog box: Geographic Coordinate SystemWorldWGS 1984;
  • Click on the OK button to accept environmental settings; and
  • Click on the OK button to run the Union (Analysis) tool.

 

Removing Multi-part Features

The Union (Analysis) will likely create multi-part features that should be removed. To do so, you should:

  • Select the ArcToolbox→ Data Management Tools Features→ Multipart to Singlepart option;
  • Select the UNION.shp file from the Input Features selection tool;
  • Specify the name for the new shapefile, e.g., UNION_singlepart.shp, in the Output Feature Class text box; and
  • Click on OK.

NB: If the newly created shapefile has more records than the original file, then the original file contained multi-part features.

Removing Slivers

The process of combining GIS layers into a single layer and then eliminating multi-part polygons may create polygons that are extremely small in size (slivers). It is generally desirable to eliminate some of the smallest polygons. This will remove UAZs that are too small to be developable, might avoid the unnecessary fragmentation of land and will speed up the analysis process.

 

Selecting Polygons to Eliminate

Before eliminating the polygons that are below a particular size threshold you must use the following procedure to select the polygons you wish to remove:

  • Open ArcMap and add the shapefile you want to eliminate the small polygons (slivers), i.e. UNION_singlepart.shp;
  • In the main Menu Bar go to Selection Select by Attributes option;
  • Select your UNION file, e.g., UNION_singlepart.shp for the Layer option and Create a new selection for the Method option;
  • Select the field that contains the UAZ areas, e.g., Area_Ha, from the Fields list;
  • Specify the criterion for selecting the polygons to be eliminated, e.g., Area_Ha < 0.3 in the SELECT * FROM text box; and
  • Click on the OK and Close buttons to select all of the polygons that are smaller than the specified size limit, e.g., smaller than 0.3 hectares in this example.

 

Deleting Small Polygons

To eliminate all the polygons selected in the step above, you can use the following procedure in ArcMap:

  • Open ArcCatalog;
  • Select the ArcToolbox→ Data Management  Generalization Eliminate option;
  • Use the Input layer selection tool to select the UNION_singlepart.shp layer;
  • Use the Output Feature Class selection tool to specify the name of the output file, e.g., UNION_elim;
  • You can select the Eliminating polygon by border option to specify that the deleted polygons will be merged with neighbouring polygons with the largest shared border (if you do not select this option, the deleted polygons will be merged with the largest neighbouring polygon); and
  • Click on the OK button to create a new UNION_elim coverage.

This procedure eliminates many—but not all—of the polygons that are less than 0.3 hectares in size.

Creating and Validating the topology

Topology is the arrangement that defines how point, line, and polygon features share coincident geometry. For example, adjacent UAZ polygons share common boundaries. To guarantee the topological integrity of the UAZ file we strongly recommend the creation and validation of a topological data model. At a minimum, the “must not have gaps” and “must not overlap” topology rules should be checked and validated. The procedure to create and validate topology will not be described here. Rather, we encourage this procedure to be undertaken by a skilled GIS Analyst or we encourage the user to follow the ArcGIS Help documentation on the topic.

Deleting unnecessary data fields

The process of combining the various GIS data layers will create a large number of data fields. Of these, only the subset of data fields will be required by OWI. The remaining fields should be removed:

  • Select the ArcToolbox  Data Management Tools  Field  Delete Field option;
  • Select your UNION_elim file with the Input Table selection tool;
  • Select the fields you wish to delete from your UNION_elim file from the list displayed in the Drop Field text box; and
  • Click on OK.

 

Creating the UAZ shapefile

The OWI Setup module requires a compressed (zip) shapefile as data input format. As a result, if you have created your UNION_elim file as a coverage or as a geodatabase file you must convert it to a shapefile before compressing it into a Zip file. To do so, you must:

  • Open ArcCatalog and locate the UNION_elim coverage or geodatabase file;
  • Right click on the UNION_elim file and Select the Export → To Shapefile (single) option
  • Use the Output Location to specify the folder in which the shapefile will be created;
  • Use the Output Feature Class to specify the name of the shapefile, e.g. UAZ_Union.shp;
  • Click on the Environments button and expand the Output Coordinates Option to specify the Coordinate System of the shapefile;
  • Select “As Specified Below” from the Output Coordinate System option list and search for GCS_WGS_1984 (EPSG4326) Geographic Coordinate System by browsing the Spatial Reference Properties dialog box: Geographic Coordinate System World WGS 1984;
  • Click on the OK button to accept environmental settings; and
  • Click on the OK button to create the shapefile;
  • Open Windows Explore and locate the shapefile created;
  • Select the files:
  •  .shp
  •  .dbf
  •  .prj
  •  .shx
  • Compress the files listed above into a ZIP file.