Transformations - Spatial Mathematics: Theory and Practice through Mapping - page 80

spatial analysis, you need to define the data's coordinate system according
to what was indicated on the TIGER download site. Otherwise, a buffer on a
point in the northern part of your data set would not have the same dimen-
sions as a buffer in the southern part of your data set, due to the curvature
of the Earth.
Launch the ArcMap application from ArcCatalog to launch ArcMap with a blank
map document. Access ArcToolbox → Data Management Tools → Projections
and Transformations → Define Projection, (or just use “search” to search and
find the Define Projection tool) and define the projection on your tgr08031lpt.
shp shapefile. This is your landmark points data set. Define the map projec-
tion as Geographic Coordinate System → World → WGS 84 (World Geodetic
System is what WGS stands for). Do the same for the other layers. For the
other layers, you can save time by using the Import function when you are
defining the projection, to import the defined coordinates for the tgr08031lpt
file and applying this projection to the other layers. With the map projected
using WGS 84, buffers you calculate will be geodesic buffers. Buffers around
points will be ellipses.
Add all of the layers (shape files) that you downloaded earlier to your data
frame: Roads, block group boundaries, landmark points, and landmark poly-
gons. Rename the appropriate layers to “roads,” “block group boundaries,”
“landmark points,” and “landmark polygons” as appropriate, so that the file
names are more intuitive. Name your data frame “Denver County.” Save your
map document with a logical name, such as “Internetcafesites,” and place
it into the appropriate folder. Your map document is stored as an “mxd”
file. What are your map units for your data? Your map units are in decimal
degrees. What is the coordinate system of your spatial data? The coordinate
system is geographic, latitude-longitude. Set your display units to kilometers.
The display units are what will be displayed in your scale and when you do
any sort of measurement on your map. Your data sets are now ready for use in
analysis. Figure 3.10 shows these data sets mapped in WGS 84 with display
units set to kilometers.
3.5.5 Analyzing your data: Buffers
In this chapter, we focus on the part of this activity that involves building
buffers to determine proximity to schools. The full activity, linked below, will
guide you through other aspects of problem solution such as finding regions
with dense groupings of potential users and nearness to busy roads. We will
visit these again in this topic in later chapters.
To find educational proximity zones, first find out where the high schools are
located. Examine the table for your point landmark features. Census Feature
Class Code (CFCC) of D43 indicates if a point landmark feature is a school.