EXERCISE 4 – Keeping Your Feet Dry
Environmental Resources 372:362
Intermediate Environmental Geomatics
Due Monday, February 23rd
Prologue: Data
Scale
As an example of how the
scale at which data is produced affects its accuracy and utility, let's take a
quick look at the
Now load \\ad-rsc\data\databank\njdep2\admin\state
into the data frame. Next load …intgeo\avdata\usa\states and restrict this
data layer just to display
Think for a second about the coast of
Water, Water . . .
One of the most frequent uses of GIS is to find areas on the earth’s surface that meet certain conditions. For example, you want to buy a house – where would be the best place to look? Well, you might start with a few simple criteria to start narrowing down the possibilities. Such things as distance to work, location of good schools, and avoiding congested roads seem a reasonable place to begin. You can make the treasure hunt for the ‘right house’ as complex as you want as long as you have access to the data. (Remember, acquisition of data can be the most time-consuming part of any GIS endeavor.)
Today you are going to do a similar hunt – right in your back yard. We are going to work with the Lawrence Brook watershed to see what types of land use tend to be found in flood prone areas. We might be interested in this type of question for a number of reasons – you want a house with a dry basement (good luck), you want to reduce your flood insurance payments, or you are a city manager who is interested in long term management of flood prone areas to minimize flood damage downstream.Oh, Where to Begin…..
I. Saving the Selected
First, we want to create a separate layer of water and floodprone areas in the Lawrence Brook watershed. Add landuse and flood from \\ad-rsc\data\teach\intgeo\Classwork\avdata\lbwpuse and become familiar with
these layers and their attributes. Select those polygons that are water or flood prone.
Once you have created your
selection,export this as a new layer to your class folder (right-click on layer name, Data | Export Data) - it will contain just the portion of the flood layer
that meets the conditions you specified. To keep things straight, rename the
new layer to something you will remember.
II. Buffer for Safety
Generally, one would be interested in completely avoiding flood prone areas, which can be accomplished by having a buffer around these areas (let’s say 100 feet). To buffer the coverage we've just created, we'll venture into ArcToolbox anduse the Buffer Tool (under the Analysis Tools | Proximity). In the Buffer tool, select the appropriate input features, save the output to your own directory, fill in the appropriate buffer distance, and set the dissolve type to "all." Keep the rest of the defaults.
From the original flood layer, we now have not only flood prone areas but also buffers around them. Move the buffer layer under the flood prone layer in the TOC (Table of Contents) so you can clearly see the buffer you just created.
It might be interesting to know the current land use of the areas that are inside the buffer. To find this out, we can ‘clip’ the land use layer to the areas overlapping the buffer. Clipping extracts features from one layer using the features from another layer as a cookie cutter. Again, we’ll find the tool we need in ArcToolbox. Go to Analysis Tools | Extract and select Clip. Use this tool to clip the land use layer by the buffered flood prone areas you created – careful, be sure you have the correct input and clip features in the tool. To keep things tidy and so as not to get confused, be sure to give your new layer a logical name. Run the tool and add the new layer to your map.
After you clip, open the output table of the layer you just created. Sort by area and then select the polygon with the largest area. Now examine the selected feature closely in the data display window. Notice something odd? There's more than one polygon associated with that feature. How can this be? Somewhat confusingly, a single feature (represented by a single record in the attribute table) can be made up more than one polygon. The clip tool creates one feature in the output layer for each clipped feature in the original layer. Depending on how the layer to be clipped overlaps with the cookie cutter layer, there might one or there might be many polygons for each feature in the output. This isn't a problem for us, but it is something we should be aware of.
What is a problem for us is that the numbers in the ‘AREA’ field are from the polygons in the original land use coverage and don’t reflect the actual area of our clipped land use polygons. You can verify this by checking the area of the corresponding polygon in the original landuse layer.
What to do? You’re lucky we’re using ArcGIS 9.3 because this used to be much more annoying to fix. ArcGIS now comes with a handy tool to calculate the geometry (area, perimeter, etc) of polygons. (1) Open the attribute table and clear any selected features in the clipped land use layer. Add a field called Acres. Make it type ‘Double’ with a precision and scale of 0. (2) Right-click on the field name and select Calculate Geometry. Set the property you wish to calculate to Area. Use the coordinate system of the data source. Set your units to acres. Hit Ok. Now you’ve got the actual area of your polygons measured in acres.
If you don’t believe me, repeat the process, but calculate the units as square feet. By comparing the original area field in your clipped output to the new square footage field, you can then see which polygons in the clipped output differ in area from the original land use layer. If we had used the original area field, ArcGIS would have grossly overestimated the area.
III. Creating a Tally
There is a lot of information in this new layer and, while you can get a general sense of what the predominant land use is, let’s be a little more specific. We want to find out how much acreage there is for each land use type in flood prone areas. Fortunately, ArcGIS includes a tool that allows us to summarize the data in (almost) any field of an attribute table.
In the attribute table of land use in flood prone areas (your clipped land use layer), right-click on the field LUCODE and select Summarize. Fill out the form so that the different land use types will be summarized by the sum of the areas. Be sure you use the correct area field! Click OK and add the output to your map. This will create a table that shows the cumulative acreage for each unique value in LUCODE.
Assignment 4
a. Create a map of land use in the buffered flood prone areas following the rules of map-making. Use the LUCODE for your map, but replace the numbers in the legend with appropriate verbal descriptions of the land use class. Use the following descriptions (it's up to you to match them with the appropriate LUCODE values): Barren, Water, Forest/Shrub, Wetland, Developed, Agriculture. Be sure to use these descriptions instead of the numerical code on the maps and in the questions that follow. l.
b. Which land use (using LUCODE) has the most features in flood prone areas? Which land use (again using LUCODE) has the largest area inside the buffer? Answer in acres, please.
c. Create a separate map using the same information, but this time let’s assume we really don’t care about buffers and we are only interested in the land use that’s in the original, unbuffered flood prone areas. With this new condition, which land use type has the most features in flood prone areas? Which land use type has the largest area inside the buffer? In addition, which land use type changed the most when we did not include a buffer? Remember to convert your areas to acres.
d. Create another map, but this time let's be really sensitive to flood prone areas and create a buffer three times the size of the original buffer. Which land use types are most affected compared to when you used no buffer? Use the questions in parts (b) and (c) to answer this question.
Your work is due before start of class on Monday, February 23rd. ***Don’t forget to answer the questions that go along with your maps!!!***