GETTING STARTED WITH MAPPING AND STATISTICS
NEIGHBORHOOD HOUSING GROUP EXAMPLE
A local housing non-profit wants to make the case that it is possible to develop new affordable housing while maintaining historic buildings within their neighborhood. They know from their own experience that goals for additional housing can be met without demolishing historic buildings but do not have visuals or statistics to support this claim.
This scenario provides an example of how GIS and spatial statistics tools can help community groups, and is also a real world example of the work performed by the consultants affiliated with SpatialJustice.org
Steps for creating maps and statistics of neighborhood properties
- Compile relevant spatial data
- Compose map overlaying relevant data
- Produce statistics based on spatial relationships
- Produce map theme visualizing group’s message
ACQUIRING SPATIAL DATA
The group has determined that their goal is to map and analyze how much space is available for development on lots not currently occupied by a historic building. This requires the following data:
- Property data that includes lot locations, areas, and the construction date of any buildings
- Zoning data on allowable building heights and density
- Neighborhood boundaries to establish the study area
- Street names and locations for reference
Fortunately, all of the data can be obtained with permission from city and county agencies in this case. The use of public data can frequently save time and money but its available varies greatly from place to place. Check with US Census, and city and county GIS or tax assessment departments for data in your area.
We download our data from city and county sources in Shapefile and Database formats. The image at right depicts a sampling of our data sources.
- Shapefile is the most common spatial data format and can be used by many GIS software packages. A single data source is actually made up of several files with different extensions (see “parcels” at right).
- Database (.dbf) files can be used to add additional information to shapefiles in many GIS applications. Our detailed property data was originally downloaded in a comma-delimited format and converted to a Database file using the SAS statistical package.
COMPOSING A MAP
GIS processing steps
- “Add Vector Layer” (Shapefiles) for land parcels, neighborhood boundaries, and zoning.
- “Join” Database table of detailed property information to the land parcels layer using a common field
- “Spatial Join” to add the zoning information to the land parcels.
- “Select by Attributes” to pick out the neighborhood of focus from the neighborhoods layer.
- “Save Selection as Shapefile” to create a layer of only one neighborhood’s boundary.
- “Select by Location” to identify all land parcels within the neighborhood boundary.
- “Save Selection as Shapefile” to create a layer of only the parcels falling within the neighborhood boundary.
We now have map containing all necessary data that is focused solely on our neighborhood of focus. The GIS processing steps combine data sources based on spatial position and create information that we can visualize on a map or export back to a table or database.
Create Map Themes for visual impact
The data contained in a GIS project can be visualized in many ways. We can control the way the map looks by adjusting the map’s “symbology.”
- First, we add appropriate titles to each layer
- The neighborhood boundary layer is displayed as a hollow symbol with a simple outline.
- For the land parcels, we classify the continuous values of the construction year into three categories. We select our choice of color and title for each category
We now have a good “draft” map of our neighborhood that can be used to visualize the locations of historic properties. For a more professional looking map, the ArcGIS software package offers many additional ways to fine tune the map’s appearance.
Calculating statistics for a report
In the previous steps we have added new information to our data set of land parcels using the “Join” and “Spatial Join” operations. We have also narrowed down our data set to include only properties within our neighborhood of interest. This modified data set was saved as a shapefile using the “Save Selection as Shapefile” command. One part of this shapefile is a database table (.dbf) that can be opened in a database or statistical program. In this instance we opened the table in Excel to perform some additional calculations on the data and prepare tables for our final report. (Caution: Create a copy of the database file before modifying it to avoid corrupting the shapefile.)
CREATING A MORE PROFESSIONAL LOOKING MAP
The ArcView GIS package allows for some additional modifications to our map that will give it more of a professional appearance. ArcView can also be used to perform all of the GIS procedures above and much more. See the CSISS ArcGIS Cookbook for more examples.
Composing a map for printing
Using ArcView’s Layout View we can add and modify map elements like titles, legend, and comments. The additional symbology elements available in ArcView allow for more highly developed themes. In this case we are displaying a cross-hatch for historic buildings overlaid on graduated color scheme representing density (floor area ratio). This map is used to visualize the distribution of land that could be developed at higher densities as well as the land with historic buildings on site. Our community groups is using this map to target properties for preservation while directing the development of more affordable housing to alternative locations.
Posting a map for a web mash-up
Using ArcView, we can export one more map layers from the shapefile format to the KML format used in web-based mapping applications like Google Maps. We create an interactive Google Map of our neighborhood properties as follows:
- In the “Layer Properties”, open the “Display” tab and set to 50% transparency
- Open the “Fields” tab and check boxes for only the fields we want to display online
- Open the “HTML Pop-up” tab and turn on the HTML pop-up table
- Using the Toolbox “Conversions Tools,” “Export Layer to KML”
- Upload the KMZ (compressed version of KML) file to any web server
- Enter the URL of the KMZ file in the search box in Google Maps to display the map in a web browser!