EXERCISE 6 – BASIC ANALYSIS:
MAP ALGEBRA AND OVERLAYS
Environmental Resources 372:362
Intermediate Environmental Geomatics
The Lawrence Brook
Watershed turns out to be a relatively small watershed, about 46 square miles.
Its boundary (give or take) is in the coverage lbw (//ad-rsc/data/teach/intgeo/ClassWork/avdata/lbwpuse/). Let's imagine that the watershed
group there wants to build a new environmental center somewhere within that
boundary. They figure it should be accessible, so let's keep it within 200' of
a road.
Use ArcToolbox/Analysis
Tools/Proximity to Buffer the roads (//ad-rsc/data/teach/intgeo/ClassWork/avdata/lbwpuse/lbwstreets)
of the watershed by 200 feet. (Call it rdbuf200, and set the Dissolve
Type to All).
But as an environmental
center, it shouldn't be too close to a road. Let's keep it at least 50 feet
away from a road.
Buffer the roads of the
watershed by 50 feet. (Call it rdbuf50 (and set the Dissolve Type to All
again))
Take a peek at the tables
for each buffer layer. You'll
notice that each has field called
Add
the Editor toolbar to ArcMap.
In the toolbar’s Editor menu, select Start Editing to start an Edit session (you can’t
edit individual cells in the table without doing this step). Be sure to select the folder that allows
you edit layers of the type Shapefile (that’s
the data layer type of the buffer output).
Now open the attribute table for rdbuff50. Type 50 in the ID
field of the first record. Now
open the attribute table for rdbuff200 and type 200 in the Id field of the
first record. Close the attribute
tables, save your edits and end the edit session.
Now, let's combine these
two different layers to make a single layer.
In ArcToolbox/Analysis
Tools/Overlay, Union the two buffer layers. (into rdcomb)
Look at rdcomb’s attribute table. You’ll see an Id and an Id_1
field. One will have a value of 200
hundred in both records, one a value of 50 in one record. Areas within the 50 foot buffer of the
roads have the value of 50 in their field.
Areas within the 200 foot buffer but not within the 50 foot buffer have
a value of 0 in this field.
The "chosen site"
needs to be on land that is not already developed but can developed. Sounds like we need land use information, so let's combine the landuse and the road buffers.
In ArcToolbox, Union landuse
and rdcomb. (into lbwmess)
That still doesn't seem so
specific, so let's say it should be within 500' of a stream. After all, it IS a
watershed group.
Use ArcToolox
to Buffer the streams layer of the watershed by 500 feet. (Call it streambuf500, and yes, Dissolve Type should be All)
Change the value in the streambuff500 Id field to 500. Don’t forget to start a new edit
session.
In ArcToolbox, Union lbwmess
and streambuf500 layers. (into lbwtotal)
Look at the attribute table
for lbwtotal. Note that there are several fields with
names that begin with Id, each one containing a different buffer distance. Which field represents which buffer
depends on the order of your unions and the order in which you added the layers
when you performed each union. Confused? You should be. That’s why we needed to ensure
that each Id field had a unique value in it before unioning. Obviously, if we had buffered both the
streams and the roads by 200 feet, we would have had to have used a different
value for the streambuff Id field. This is one reason why it’s
important to take notes on your work, especially when you’re working on
big projects and are creating lots of layers (hint, hint).
The lbwtotal layer is a bit frightful. Let's
try restricting the polygons to just those that meet our criteria. We can do
this using the Select tool in ArcToolbox/Analysis
Tools/Extract. Unlike in ArcMap, when you use the
Select command in ArcToolbox you create a new layer
with based on your query.
In
ArcToolbox, Select the polygons in lbwtotal that are inside the stream and 200' road
buffers (have a 500 and 200 in their respective Id fields), but outside the 50'
road buffer (have a 0 in their 50’ buffer Id field), and have a LUCUDE of
4 for forest. (call it envcenters). Click on the “SQL” button in
the Select tool to get to the dialogue box where you can add your select
criteria.
Do you still have some
options? How would you pick? What if we only looked at the largest sites?
Weight and Rate
What if the tiny slivers of
sites our analysis left us with were deemed unsuitable? Could we create a
larger scoring system that describes the entire watershed?
Add the following fields
(as integers) to lbwtotal:
Landscore
Streamscore
Rd50score
Rd200score
Totalscore
Open the attribute table
and Select (using the Select by Attributes under the Table Options) all of the
polygons inside the 500 foot stream buffer. Then Calculate the streamscore = 10 for all of those. The calculation will be
performed only on the selected records.
Repeat this for each of the
roads buffers. Make to sure select those polygons outside the 50 foot buffer
and those that are inside the 200 foot buffer.
For the landuse,
make the forests (LUCODE=4) a 10, the farms (LUCODE=2) a 7 and the rest a 2.
Clear the selection, then
calculate an additive total (i.e., landscore+streamscore+rd50score+rd200score)
in the totalscore field. Use the totalscore
field to view the coverage with a graduated color scheme. (It'll look better if
you eliminate the outlines around the polygons.)
For a different view, we can differentially weight the criteria, allowing us to stress the more important ones and downplay the less important ones. Calculate a weighted total (i.e., 5* landscore+7*streamscore+2*rd50score+3*rd200score) in the totalscore field. Take a look at the results in ArcMap using a graduated color scheme.
Assignment 7a
Lawrence Brook
What would happen if you
used a different set of buffers? What if you "scored" areas as more
suitable or less suitable, instead of discarding all areas that are unsuitable?
What if you introduced some address-matched data, like nearby schools? What if
you tried to locate the center near wetlands or lakes? The criteria you could
use for choosing sites are limited only by the available data.
Develop a site-selection system for the environmental center. Hand in a map showing your selected site. You must use different buffers than we used AND you must use at least a little address matched data. Use the street.mxc address locator in Y:/intgeo/rowan/avtrn/middlese/. You must also use a weight and rate system. Your map MUST be grayscale (design it in black/white/grays AND print on the black and white printer (ljup)) and should clearly explain your rationale and scoring on your map.
Assignment 7b
Kenny's Little Kernels
Namtrac Food Products, Inc. has developed a
frozen bag of sugar-coated veggies that kids love called Kenny's Little
Kernels. However, to make it a profitable venture, they figure they need to
produce and sell their product entirely within a single country. They've asked
you to help pick that country. But since they would also consider expanding to
many other countries (without shipping product across borders) you should
provide a scoring system that ranks all countries in the order that they might
proceed.
"Well," you thought, "What would improve their chances of
success?" A business book you found suggested:
Copy the demog.dbf file from //ad-rsc/data/teach/intgeo/ClassWork/avdat/world/tables into your directory so that you can modify it. If you so choose, you can copy the whole world directory. Be sure to the use the cntry04 data layer for your geographic data.
ONE solution would work
like this…
Add some fields to the
table and save it under a different name.
Calculate a
"score" from 1 to 3 in each of the new fields to reflect the degree
to which the record satisfies that condition (criterion). Some demographic data
which might be useful include: Long life expectation (b - good production
capability), High population (a - strong customer base), Stable Growth Rate
(a), Lots of kids (a), and High convergence of ag. and urban land (a &
b).
Add a field for the total
formula score and calculate the formula score with weights included. While you
are at it, add a few different fields, and try a few different formulas.
Save the edited table.
Join/relate it to the world
attribute table and map it.
Come up with your own
global solution. You don't need to limit yourself to factors (a) and (b) above,
but at least some of your scoring criteria should reflect the importance of these
factors. Maps will be graded in part based on their readability and
composition. PRINT ONE BLACK AND WHITE MAP. It should be highly
communicative. What were your criteria? What is your final decision?
Assignment due Monday, March 24th. And remember, maps should be black and
white.