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IT 3205 - GEOGRAPHIC INFORMATION SYSTEMS

LAB 10 – PROXIMITY TOOLS

The purpose of this laboratory is to introduce vertical (overlay) and horizontal (buffering) spatial

operations or techniques. The INTERSECT (or UNION) command brings the attributes from the

overlaying shapefile and combines the attribute tables into the intersected (or unioned)

shapefile as well as intersecting the shapefile graphically.

BUFFERING

Buffering and clipping are two of the most common operations in cartographic modeling. A

buffer zone is an area that is within a given distance from a map feature. Points, lines, or

polygons can be buffered. Buffers are used to identify areas surrounding geographic features.

For example, you may wish to keep septic systems over 100 meters away from streams, locate

housing within a quarter mile of existing roads, keep hiking trails away from seasonally flooded

rivers, or make sure most of your city is within some maximum distance from a fire station or

school.

When you buffer on a set of features, the output is a set of polygons. (Buffering points or lines

creates a new coverage that is a polygon coverage). These polygons define an inside region, an

area less than the specified buffer distance from the features of interest (e.g., less than 300

meters from a stream), and an outside region, an area more than the specified buffer distance

from the features of interest.

These inside and outside regions are typically distinguished by different codes in an attribute

table. You should know the specific codes assigned for the software system you use.

Fixed Distance Buffers

A fixed distance buffer identifies the inside and outside region for a fixed distance away

from a set of features. Fixed distance buffering may be applied to points, line, or

polygon input, and creates polygon output.

The figure on the below shows the buffer area (in blue) for a road network (thicker black

lines). It separates the inside area (blue) from the outside area (white). There will be a

corresponding table with some coding to indicate the in/out status of each polygon.



Note there is a third status possible for each polygon. We may note those that are out,

but “surrounded” in that they are contained within the bounds of an “in” polygon.

Variable Buffers

Another variation on buffering will change the buffer distance depending on feature

attributes (see the figure, below).

A GIS project may require buffering those lakes to map a minimum distance from shore

for installing septic systems. However, the acceptable distance for septic systems may

depend on lake size. A large lake could have a system within 100 meters of lakeshore,

but a small lake needs a setback of 25 meters.

A variable distance buffer could buffer the lakes coverage by using an attribute that

specifies the size class. Different sized buffers would be applied to each lake depending

on the size class attribute.

You will create a variable distance buffer in this lab, with the buffer distance defined by

stream order.

Variable distance buffering requires some way of specifying the distance. This is most

often done with an attribute in a table. The buffering operation uses the table entry to

determine buffer distance around a feature. A numeric data item must be created to

specify the buffer distance, as shown in the column named DIST in the table below. Each

lake with an attribute for surface of 1 will be assigned a buffer with distance 100, a

surface value of 2 a buffer distance of 50, and a surface value of 3, a buffer distance of

25. Examples of lakes with the three different surface values and resulting buffers are

shown in the figure above.



Setup

We will be working with the swamp creek water sheds in Berks County, Pennsylvania.

Download and extract lab_10 data to your flash drive.

Adding data

1. Open ArcCatalog and connect to your Lab_10 folder.

2. Create a new personal geodatabase called Lab_10 under the data folder and import the

following shape files:

streams

wtrshed

Select both the streams and wtrshed shapefiles



Click ok and they should now be added to the Lab_10 data base

Close ArcCatalog

3. Open a new ArcMap session and begin the session with a new empty map document.

Set you default geodatabase to the Lab_10.mdb created above. Let’s save our map

document immediately as “lab10.mxd” in the Lab_10 folder.

4. Rename the data frame view to “All Watersheds” by right clicking and accessing the

Properties option selection underneath the View menu and typing “All Watersheds” in

the Name box.

5. Set the coordinate system to NAD 1983 StatePlane Pennsylvania South FIPS 3702.

6. Add the wtrshed and streams shape files from the Lab_10 geodatabase.



7. Rename the layer wtrshed polygon to “Watersheds” by right clicking on each layer,

accessing the Properties option on the popup menu, making the General tab active and

typing into the name box. You may also want to change the appearance of these layers

by double clicking on their symbols in the table of contents and changing the symbology.

Your map should look similar to the graphic below.

Selecting Features

Select by Attributes

1. Let's create a new shapefile of only those streams that are located in the Swamp Creek

Watershed through an attribute selection process. First, let’s access the Select By

Attributes option under the Selection menu.



2. Set the initial layer to Watersheds and working with the Fields and Unique Values boxes,

build a query that reads "NAME" = 'Swamp Creek'. Remember that you’ll have to choose

the “Get unique values” button to see a list of attribute values for the NAME field.

Choose Apply.

You can close the Select By Attributes box after choosing Apply. This process should

select the Swamp Creek Watershed boundary meaning that the entire watershed area is

selected.



Selection by Location

1. Selection by location is another way of using a shape to select features within another

shape. This menu choice is available under the Selection menu‐ go ahead and activate

this option. Leave the first default as is but make sure Streams is the layer of interest in

the second option box. Make sure that intersect is in the third option box and that

Watersheds is the layer indicated in the fourth. Choose Apply.

You can close the dialog box after you select Apply. This operation should select all

streams in the Swamp Creek Watershed.



2. Let’s convert these selected set into a shapefile that we can work on. Right click on the

streams layer, choose Data from the popup menu, and then select Export Data.

A dialog box should open. We don’t need to change any of the defaults but let’s make

sure that we’re saving to your Lab_10.mdb location and that we give it a name that

makes sense: swckstrms.shp. Choose OK.



Answer yes when prompted to add the map layer.

This will add the new streams layer to your data frame. You may not be able to see the

new streams due to the selection color taking precedence over other display colors. You

can fix this by choosing Clear Selected Features from the Selection menu.



3. Turn off the Stream Layer. You will notice that several streams extend into adjoining

watersheds. This is due to the way we selected the streams. By selecting by intersection

by source layer feature the segments of the streams that intersected the swamp creek

watershed came along with the data. So let’s cleanup the swckstrms feature class. This

can be done by either editing the feature class or clipping the feature class. So let’s clip

the swckstrms feature with the watershed feature class.

4. Select by Attributes the Swamp Creek watershed as we did before.

5. Open ArcToolbox chose the clip tool

6. Select the input features from swckstrms and have the clip features Watersheds. The

output feature class should be placed in you Lab_10.mdb and use the default name

swckstrms_clip



Answer yes when prompted to add the map layer. Remove the layer swckstrms and

clear selected features. You will notice now the stream sections in the other watersheds

on the swckstrms_clip have been removed.

7. Create a new data frame for the new layer by choosing Data Frame from the insert

menu. Rename the new data frame to Swamp Creek by right clicking on it in the table of

contents, choosing Properties, typing into the Name box, and choosing OK.

Next copy the new layer (swckstrms_clip) from the All Watersheds data frame by right

clicking on it and choosing copy, and then right clicking on the Swamp Creek data frame

and choosing Paste Layer (or by simply clicking and dragging the layer with your mouse).

You can then remove the layer from the All Watersheds data frame by right clicking on

the layer and choosing Remove. Rename the swckstrms layer to Swamp Creek Streams

by right clicking on it, accessing Properties, and typing into the name box.

You may want to change the display of the Swamp Creek Streams layer by double

clicking on its symbol in the table of contents and selecting from among the size and

color choices.



Save your map document!

Buffer

Buffering is a very important proximity tool for many GIS packages, particular in Forestry as we

often establish zones or management areas by measuring a set distance away from a stream or

land cover type. To demonstrate the use of buffering tools within ArcGIS, we’re going to buffer

streams based on stream order information. We will create a buffer around the Swamp Creek

Streams shapefile you just created. We’ll buffer first order streams by 100 feet and second and

third streams by 200 feet and fourth order streams by 300 feet. You will need to create a buffer

field then populate the buffer field by using the field calculator. The field calculator is a very

versatile tool that allows you to perform mathematical calculations.

Strahler number

In mathematics, the Strahler number or Horton–Strahler number of a mathematical tree

is a numerical measure of its branching complexity.

These numbers were first developed in hydrology by Robert E. Horton (1945) and Arthur

Newell Strahler (1952, 1957); in this application, they are referred to as the Strahler

stream order and are used to define stream size based on a hierarchy of tributaries.

Back to buffering

1. Begin by opening the attribute table for the Swamp Creek Streams: right click on

the layer and choose Open Attribute Table. Create a new field by clicking on the

Options button and selecting Add Field. An input box should appear. In the

name field, enter buffer. In the type field, accept the default of Short Integer.



2. Choose OK.

3. Select a subset of the records in this database that include all stream segments

that are classified as a first order stream by:

a. Clicking on the Options button.

b. Choosing Select by Attributes…



A dialog box should open. There is a field in this table named STRAHLER in your

table. Double click on this field in the Fields box, single click on the = operator,

click on the “Get unique values” button, and double click on 1 in the Unique

values box. Your input should match the graphic below‐ when it does, choose

Apply.

This operation should result in all the first order streams being selected. Now,

any calculations that we do on other fields should only occur in the selected

streams. Add a buffer value for the first order streams by right clicking on the

top of the buffer variable we just created and choosing Field Calculator from the

options. You’ll be warned about your editing process by a dialog box‐ choose

Yes.



c. The Field Calculator should open. Type 100 into the input box and

choose OK.

Take a look at your table to see if anything’s changed‐ be careful not to click on

any of the table cells!

Add buffer values for the other streams by finding your Select by Attributes

input box and choosing the Clear button. Next, build a query by selecting Fields,

operators, and values that reads "STRAHLER" = 2 OR "STRAHLER" = 3. You

should be able to do this without any typing from the keyboard. Choose Apply.



Add a buffer value for the second and third order streams by right clicking on

the top of the buffer field and choosing Calculate Values from the options. You’ll

be warned about your editing process by a dialog box‐ choose Yes.

The Field Calculator should open. Type 200 into the input box and choose OK.

Take a look at your table to see if anything’s changed‐ again, be careful not to

click on any of the table cells!

Now let’s repeat for the fourth order streams

This process should’ve created a buffer field and values for us to run our

buffering operation. Let’s look briefly at the results.

Let’s create some buffers.

Our first task will be to make sure that the coordinate units are set to feet.

Access the properties of the Swamp Creek data frame and set both the map and

display units to feet.



Open the ArcToolbox and choose:

1. Analysis Tools

2. Proximity, and

3. Double click on the buffer option.

4. This should open the Buffer dialog. Fill out the options using the graphic

below as a guide. Make sure that the Dissolve Type is set to all.



A new layer will be add to the map called swckstrms_Clip_Buffer and should

look like this:

Save your work

Once you’ve saved your map document, rename the new layer to Swamp Creek Buffer,

and examine the buffer results to see if they match our expectations.

Let’s also add field to calculate acreage:

1. Choose Add Field from the Options button

2. Creating a new filed named acres, of a float type

3. Run a calculation on the new acres field to create an acreage amount (divide

area by 43,560).

Your buffer table should look similar to the graphic below.



Dissolving a Layer

We also have a shapefile that contains surficial geology for our watershed areas. We’re going to

open this file and clip it to the extent of the Swamp Creek Watershed so that we can do some

processing.

Return to your All Watersheds data frame by right clicking on it and selecting Activate from the

popup menu. Clear any selections you might have by using the Clear Selected Features option

under the Selection Menu. Make the Swamp Creek Watershed the active polygon by using the

Select by Attributes option under the Selection menu or using the Select Features button and

clicking on the Swamp Creek Watershed to select it.

Create a new feature of this watershed area by right clicking on the Watersheds layer, choosing

Data, then Export Data. Write the output to your workspace folder and name it swchshed.

Now, open the geology shapefile into the All Watersheds data frame by using the Add Data

button.

Let’s trim the geology file by accessing the Clip option in the ArcToolbox

1. Click on Analysis Tools

2. Open the Extract group

3. Double click on Clip

4. In the next dialog box, select geology as the input layer to clip, Watersheds as

the polygon clip layer, and direct the output to your Lab10 personal

geodatabase with the name swckgeology.



Choose the OK button.

The new layer should appear in your All Watersheds layer. If it looks OK, click on it and

drag it to the Swamp Creek data frame. Rename the new layer as Swamp Creek Geology

and remove its double from the All watersheds data frame. Activate the Swamp Creek

data frame and choose the Full Extent button.

You can examine the values of a field in a layer by changing its symbology. Practice this

by right clicking on the Swamp Creek Geology, choosing Properties, and then the

Symbology tab.

1. In the upper left of the properties input box, click on Categories and select the

Unique Values option.

2. Change the Value Field to AGE.

3. Choose the Add All Values button.

4. Uncheck the box next to the first symbol in the SYMBOL box.

5. Choose Apply to see the unique values in AGE.



You can individually change the colors for each unique value by clicking on the symbols in this

dialog box and selecting an alternative. Let’s close the Layer Properties dialog.

Dissolve

Notice that we have four Age classes but that some redundancy exists in the number of

polygons‐ some of the geology classes contain multiple polygon boundaries that border each

other. Using the new shapefile, we are going to perform a dissolve operation to reduce data

redundancy.

In the ArcToolbox, go to:

1. Data Management Tools

2. Generalization

3. Dissolve (double click)



In the next dialog box, make Swamp Creek Geology the input layer to dissolve,

AGE the attribute to dissolve on, and send the output to your Lab10 folder with

the name swckagegeo. Use the graphic below as a guide and choose OK when

ready.



Rename the new layer Swamp Creek Dissolved Geology, and change its

symbology so that the unique values of AGE are displayed, just like we did with

the previous geology file. Take a look at this new layer. You might toggle its

visibility on and off to more closely examine the changes. You should notice that

there are fewer polygons.

MAPS TO TURN IN:

Swamp Creek Watershed, with streams and buffers

Swamp Creek

buffering - gis.weavsplace.comgis.weavsplace.com/labs/lab 10.pdf · lab 10 – proximity tools...

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