Raster/Vector Analysis for Land Use Studies
by Bill Lloyd and Frank Gossette
In this assignment you will learn how Urban and
Regional
Planners can work with potential land developers to identify suitable
sites
for new construction projects. Government planners can impose
environmental
restrictions on new buildings and use GIS to help determine which sites
are appropriate for development and which are not. CLICK
HERE to
download the data sets necessary for the exercise.
In this exercise you will learn how to do a simple
site suitability study. You want to identify areas of single
family residential
land use located on steep slopes. To accomplish this goal, you will
employ
the following concepts and procedures:
-
Converting a U.S.G.S. digital elevation model into a grid data
set
-
Modifying grid legends
-
Setting grid analysis properties
-
Converting a polygon theme to a grid theme
-
Querying a grid
-
Converting an elevation grid to a slope grid
-
Using the map calculator to multiply two grids
You'll be working in an area covered by the San Juan Capistrano 7.5
minute
quadrangle. Steep slopes are more than an idle curiosity in this part
of
California: last week several large homes were destroyed as they slid
down
the El Nino soaked hillsides of this area.
SETUP
UnZIP the following files into your working directory:
-
landuse2.shp
-
landuse2.shx
-
landuse2.dbf
-
landuse.avl
-
lugrid.avl
-
sjc.dem
- Some of these files are "legacy"
files from ArcView 3.x -- you may wish to use that program to get the
details of the legends, etc. Otherwise, work in ArcGIS!
Defining the Analysis Strategy
Now that you have the two datasets in your view, you are ready to
begin
the analysis. Here are the steps needed to identify areas of single
family
residential land use on steep slopes:
- Convert the land use polygon data to land use grid data
- Subset the land use grid to a single family residential grid
-
Convert the elevation grid to a slope grid
-
Convert the slope grid to a steep-slope grid
-
Overlay the steep-slope grid and the single family residential grid to
identify the locations of single family residential land use on steep
slopes
- Take a moment to explore the Lugrid theme. If you click with the
Identify
(i) tool you'll see that theValue field contains an integer,
while a S_value field contains the land use description. Grid values
must
be numeric, but you told Arcview to use a descriptive character field
as
the value field. Arcview deals with this by assigning integer values
for
each unique description and appending the character description as an
S_value
field in the new grid's value attribute table. The actual data
represent
the land use value found at the center of each cell in the analysis
grid.
Make the new lugrid theme look
like the old landuse2.shp
theme. The major difference is that the color coding is applied to
30-meter
land use grid cells instead of land use polygons.
Create a Single Family Residential Land Use Grid
Now you want to isolate land use cells that are classified as single
family residential. A grid that isolates one value out of a range of
values
is sometimes called a binary grid. Each cell that contains the
target
value is assigned a new value of one (true); each cell that does not
contain
the target is assigned a new value of zero (false). You use the Map
Query function to find all cells that satisfy a given condition:
Create a Slope Grid
Since your goal is to relate single family residential land use to
steep
slopes, not to elevation, you must convert the elevation grid to a
slope
grid. ARC provides a method for this:
Remember:
this operation only works
if the horizontal units of the DEM are the same as the vertical units.
Creating a Steep Slopes Grid
For the purposes of this exercise, we'll define any slope over 20%
as
a steep slope. You need to create a binary grid
with values of one representing cells with slopes greater than 20%:
Create a Grid of SFR on Steep Slopes
Calculating Areas
- With grid data, you only need to know the number of cells and the
area
of each cell to calculate areas.
April 2005