Demo: Using DIS Mapping and Analysis Program (DMAP) To Compute Disease Rates With Variable Spatial Filters and Test For Statistical Significance

Use a GIS to Produce Disease Rate Maps with Variable Spatial Filters

Two basic steps:

A. Use buffer analysis capabilities of a GIS to compute health events and disease rates within a specified filter size.

B. Use digital elevation modeling capabilities of a GIS to produce contour maps of observed disease rates.

Step A:

Use Buffer analysis capabilities of GIS to compute health events and disease rates within a specified filter.

  1. Create a spatial grid for the study area.
  2. Geocode grid intersection locations.
  3. Geocode locations of numerator and denominator health events.
  4. Compute the number of health events within a specified radius of each grid location.
  5. Compute disease rate at each grid intersection location.
1. Create a spatial grid

The program GRID.EXE is used for generating a spatial grid on the study area. This program requires as input the latitude and longitude coordinates of the northwest and southeast corners of study area. The user is required to choose the size of the grid in miles (e.g. 0.4 mile). The output from the program is a text file containing the ID, latitude and longitude coordinates of grid intersections.

2. Geocode grid intersection locations

The geocoding capabilities of GIS software can be used to convert the output text file to a geographic file based on coordinate information. The following steps are necessary for geocoding a file containing coordinate data in TransCAD:

  1. Open the output file from the previous step in Microsoft Excel and save it as a comma-delimited (*.CSV) file, before opening it in TransCAD.
  2. In TransCAD, go to File-Open and open the comma-delimited file containing the coordinates of the grid points.
  3. Choose Tools-Locate by Coordinate to display the Locate by Coordinate dialog box. From the options available, choose 'FIELD_1' as the ID Field, 'FIELD_2' as the X Field, and 'FIELD_3' as the Y Field. Enter an appropriate name for the map layer (e.g. Grid points) and click OK when you are done.
  4. In the Save As dialog box, enter a name for the geographic file to store the grid information, and click OK.

A new geographic file containing a grid location for each record in the data file is created and displayed as a new map layer.

3. Geocode locations of numerator and denominator health events

The locations of health events can be geocoded in a GIS on the basis of street address information or coordinate information. If coordinates of these events are available, follow instructions in step 2. If street address information is available:

*For more information on Address Matching.

4. Compute the number of numerator and denominator events within a specified radius (filter size) of each grid location.

The GIS database at this stage should contain the following map layers, from the geographic files created in the previous steps:

  • geocoded grid point locations;

  • geocoded numerator health events; and

  • geocoded denominator health events.

    The buffer analysis capabilities of GIS are used for computing the health events located within a specified radius of each grid intersection. The following steps are necessary in TransCAD:

    1. Go to the dataview window of the map layer containing grid point locations ('Grid points'. Select Modify-Table and create three blank columns (fields) for storing the numerator events, denominator events, and disease rates associated with each record, with appropriate column headings (e.g. numerator, denominator, disease rate).
    2. Select the 'numerator' column by clicking in the name of the field.
    3. Choose Edit-Fill to display the Fill dialog box, and choose Aggregate from the Fill Method list to display the Aggregate Data dialog box.
    4. In the Aggregate Data dialog box, choose the layer containing numerator events from the From Layer drop-down list and 'All Features' from the Include list.
    5. Enter the size of the spatial filter or circular buffer in the Band Size box, and choose the units (e.g. miles) from the drop-down list.
    6. In the Aggregate Settings box, choose 'Count' from the Fill With drop-down list.
    7. Click OK to return to the Fill dialog box, and OK again. TransCAD fills the column with the number of events within the specified filter or band size (for each record).
    8. Repeat steps 2 to 7 to compute the denominator events in the 'denominator' column.

    5. Compute disease rate at each grid point.

    Computation of the disease rate essentially consists of using a formula that involves dividing numerator column by the denominator column, and storing the results of the formula in the Ôdisease rateÕ column created in the previous step. The following steps are necessary:

    1. Select the 'disease rate' column by the clicking on the name to select the entire field.
    2. Choose Edit-Fill to display the Fill dialog box, and choose Formula from the Fill Method list to display the Formula dialog box.
    3. Enter a formula, or use the Formula Builder to select the fields and operators necessary to create the formula (e.g. numerator*1000/denominator).
    4. Click OK to return to the Fill dialog box, and OK again. TransCAD evaluates the formula and fills the highlighted column with the results.

    Step B:

    Use digital elevation modeling (DEM) capabilities of a GIS to produce contour maps of disease rates.

    1. Create a TIN digital elevation model

    2. Use DEM analysis to generate contours

    1. Create a TIN Digital Elevation Model

    Some GIS software applications enable the user to create and maintain three dimensional datasets. These data sets comprise an array of points that are defined by their geographic location (longitude and latitude) and a z value. The z value may define the elevation, or the 'height' of another data value (e.g. disease rate) over a geographic region.

    A Digital Elevation Model (DEM) is a specialized database that represents the relief of a surface between points of known elevation. A DEM is created by capturing an array of elevations that are used as input to an interpolation routine which will approximate the surface between sample points. The process of interpolation typically results in the creation of a commonly used DEM structure known as a triangulated irregular network (TIN). TIN structures are based on triangular elements with vertices at sample elevations, and are derived through a form of interpolation that assumes that the surface between any three points is a plane.

    The GIS software uses triangulation to interpolate surfaces between the grid intersections containing disease rate information, and generate a TIN data structure. The TIN is then used as an input to other surface visualization and analysis functions, such as contour generation. The following steps are necessary in TransCAD to create a TIN digital elevation model based on grid intersections containing disease rate information:

    1. Make sure that the map layer containing the grid point locations is the working layer. The disease rate information (computed in step A) should be stored in the one of the columns in the dataview window associated with the map layer.

    2. Choose Tools-TINS to display the Create TIN dialog box.

    3. In the Create TIN dialog box, choose 'All entities' from the Selection list, the name of the column (heading) containing the disease rate information from the Field list, and '1' as the Level of interpolation.

    4. Click OK when you are done.

    TransCAD creates a triangulated irregular network DEM and displays the TIN toolbox.

    2. Use DEM analysis to generate contours

    The DEM analysis capabilities of a GIS can be used to generate lines that represent the elevation of a data value. These lines, referred to as contour lines, are imaginary lines that pass through points of equal elevation (disease rate). TransCAD allows the user to choose the elevation difference or contour level that separates contour lines. It also supports the creation of contours as both line and area features. In other words, disease rates can be visually displayed in the form of:

  • contour lines, labeled with corresponding values;
    see figure 1

  • contour areas, shaded with darker colors representing higher rates.
    see figure 2

    The following steps are necessary to generate contours in TransCAD:

    1. Click the contour icon (4th from left) in the TIN toolbox to display the Contour Generation dialog box.

    2. In the Contour Generation dialog box, choose whether to create a Line or Area contour layer.

    3. Choose Automatic or Manual definition of the contour interval.

    4. Enter the number of contours or contour level data as follows:

    Database Type Contour Interval Enter:
    Area Automatic Number of contours
    Area Manual Fixed contour interval
    Line Automatic Number of contours
    Line Manual Fixed contour interval

    5. Click OK when you are done. The Save As dialog box is displayed.

    6. Enter a name for the geographic file you are creating to represent the contour layer, and click OK.

    A contour layer is created and the map is redrawn to display the content of the layer on the screen. The dataview associated with this map layer contains information for each line or area created. The contents of this table can be used to label contour features, shade contour areas, and create maps showing contour intervals. The contour layer can also be overlayed on another map layer (e.g. the street network of the study area, or grid intersection locations) to identify specific locations associated with disease rates.

    Go back to previous section of Demo.

    Go back to main Demo page.