NYSDOH Geomasking Tool v. 1.3
User guide
Credits
Thomas Talbot ([email protected]) and Gwen LaSelva ([email protected])
New York State Department of Health
Center for Environmental Health
Bureau of Environmental and Occupational Epidemiology
Environmental Health Surveillance Section
Introduction
The purpose of this tool is to move points randomly within a circular region around each point to preserve
privacy in when displaying point maps. Displaying residential locations on a map may lead to privacy
concerns. Health and environmental sampling data may be linked to houses using commonly available
software and publically available data. A fictional example is shown below. A map image showing point
locations was geographically registered using Google Earth. Individual house locations were obtained from
publicly available real property records.
Original Paper Map
Registered Image – Google Earth
John Doe
164 Kennewyck Circle, Slingerlands, NY 12159
Name & address from publicly available real property data
Requirements
This tool was programmed for R3.0.2 under Windows 7. R packages svDialogs, svMisc, rgdal, and tcltk are
required.
The tool requires that the points to be moved be in a shapefile with a latitude/longitude projection. In
addition, a boundary shape file in the same latitude/longitude projection is required. The movement of points
will be restricted so that all of the new points remain within the same boundaries as the old points. This can
be used to ensure that points are not moved into water bodies or that they remain within the same political
boundaries.
Using the Tool
Click on the relevant “*.bat” file to start the tool. You will see a window initially titled “RGui” open, followed
shortly by this dialog:
Select the file of points you wish to move, then click the “open” button.
This dialog will appear:
Select the boundary file you wish to use, then click the “open” button. When the points are moved, they will
be kept within the boundaries in this file. For example, we wish to show childhood blood lead levels along
with the location of a historical lead smelter. House locations are randomly moved. The new locations remain
in the same exposure areas to maintain the general point pattern. The 32 exposure areas were defined by
distance and wind direction from the smelter. (from NYSDOH. Analysis of Blood Lead Levels of Children Living
Near the Former National Lead Industries Site, Colonie, NY. 2005
After selected the boundary file, this dialog will appear:
The mimimum distance you wish to move the points can be entered into the first box, or obtained from the
data. If you wish to use a data field to specify the minimum distance, select the field from the dropdown box
after”get minimum distance from data”. If you select a field, the distance in the first box will be ignored. The
maximum distance you wish to move the points can be entered into the first box on the second line, or
obtained from the data. If you wish to use a data field to specify the maximum distance, select the field from
the dropdown box after”get maximum distance from data”. If you select a field, the distance in the first box
on the second line will be ignored. If you specify a minimum distance of zero and a maximum distance, the
following figure shows examples of how the point could be moved:
Using a minimum or maximum distance specified in the data allows you to customize the distances for each
point. For example, you could specify smaller distances in areas with higher population densities. The table
below shows example data for which the maximum distances points can be moved (Distance_m) is based on
population density.
The program may adjust minimum or maximum distances if needed to get the point to fall within the
boundaries specified or to increase the speed of the program. The actual minimums and maximums used will
be specified in the output files.
Select the distance units in the third drop-down; you can select meters, feet, kilometers, or miles. Click “next”
to continue, or “back” to select different files to work with. “Cancel” ends the program, and “Help” brings up
this file.
A confirmation dialog will appear:
Select “Yes” to continue, or “no” to select different distances or distance units to move the points.
Lastly, you will be prompted to save your file.
Enter a filename and click “Save”. In addition, a map of your points will appear. The new point data will also
be output in KML, Shapefile, and MapInfo format. The fields “min_dist” and “max_dist” will be added to the
data. They contain the actual minimum and maximum distances (in meters) used by the random number
generator.
The results can also be viewed in Google Earth or other GIS programs. For example, Google Earth is shown at
left below and ARCMap to the right.
GeoMasking Sample Data Files
We included some sample shape files for the you to try out the program. The files are in the testdata
directory.
We included an ESRI Shape file (albanypoints.shp) with 1,000 address points in Albany County.
The file has the following fields:
ID:
Address1:
City_state:
Zip5:
Popdense:
Distance_m:
ID Number from 1-1000
Street Address
The city and state
Zip Code
Population per square mile from US Census tracts.
Maximum distance in meters to move points
<1000 pop/mi2
= 250 meters
2
1000-2000 pop/mi = 500 meters
>2000 mi2
= 1000 meters
In addition we have provided boundary shape files for the county, towns and census tracts in Albany County
New York.