In the previous lesson, you familiarized yourself with the study area and visually analyzed the deforestation you could see from the Imagery basemap. In this lesson, you'll take your analysis a step farther. First, you'll add a layer showing the extent of deforestation. Then, you'll quantify the percentage of land that is deforested within a certain distance from roads. By finding the relationship between existing roads and deforestation, you'll later be able to estimate the deforestation prevented by the proposed road's prohibition.
Although you looked at deforestation in the last lesson, now you'll begin a more thorough exploration in order to understand existing deforestation patterns in Rondônia. You'll start by adding a layer that shows deforestation in the state.
- If necessary, open your Amazon Deforestation map document in ArcMap.
- In the Catalog window, drag the Deforested_Area layer to the Table of Contents, below the Roads layer.
- In the Table Of Contents, click the symbol for the Deforested Area layer. In the Symbol Selector, change the symbol's Fill Color to Lime Dust.
- Change the Outline Color to No Color.
- Click OK.
The layer has only one feature. This is a multipart feature: a feature composed of many noncontiguous elements. Deforestation typically occurs in small parcels, not large unbroken swaths. At this map scale, it's difficult to see deforestation in much detail. You'll zoom in to look closer, but first you'll bookmark the current map extent. With a bookmark, you can quickly navigate to specific map extents.
- On the ArcMap main menu, click Bookmarks and choose Create Bookmark.
The Create Bookmark dialog box opens.
- Change the bookmark name to Rondonia and click OK.
- On the Tools toolbar, click the Zoom In tool.
When you move the pointer over the map, it becomes a magnifying glass.
- Draw a rectangle around the area of deforestation in northwestern Rondônia.
The map zooms to the extent of the rectangle. Once zoomed in, you can see the deforestation in more detail. Deforestation is often patchy, like in the following image:
Typically, small strips of forest are deforested through a process called slash-and-burn agriculture. In slash-and-burn-agriculture, small farmers cut and burn plots of forest to create fields. The burned biomass serves as fertilizer for agriculture on the cleared land. This agricultural technique has been practiced for centuries throughout the world, including the Amazon. In small amount, slash-and-burn agriculture can be sustainable. But when widespread, massive areas are cleared in a small amount of time, it can drastically affect an ecosystem.
Next, you'll compare the Deforested Area layer to the visible deforestation on the Imagery basemap. To see both the layer and the basemap at the same time, you'll use transparency.
- In the Table Of Contents, double-click the Deforested Area layer.
- In the Layer Properties dialog box, on the Display tab, change the Transparent setting to 50 percent.
The higher the percentage, the more transparent the layer will be.
- Click OK.
If you have difficulty seeing the basemap through the transparent layer, you can also turn the Deforested Area layer on and off to compare.
You can see where visible deforestation overlaps with areas in the Deforested Area layer. However, some parts of the layer do not seem to coincide with deforestation visible on the basemap, especially areas close to untouched rainforest. Deforestation is a constantly changing process and the basemap may not have been updated to show its most recent occurrences.
If you want to know the last time the basemap or the Deforested Area layer was updated, you can view the layer's metadata. Metadata explains when data was collected and by whom it was collected, among other things. To see the metadata of a part of the basemap, use the Identify tool to click the map (make sure you are identifying from the World_Imagery layer by choosing it at the top of the Identify dialog box). The SRC_DATE2 field gives the date of the imagery for that portion of the basemap. To see the metadata for the Deforested Area layer or any other feature layer on your map, right-click the layer in the Table of Contents and choose Data > View Item Description.
Another noticeable pattern is that deforestation sometimes ends abruptly with fairly sharp boundaries, as in the following image:
In the above example, the boundaries are defined by protected areas, which either prohibit or greatly restrict deforestation. Protected areas come in two types: protected forests and indigenous territory. Feature classes for both categories are in the Project Data geodatabase. You are welcome to add them both to explore, but for this project you only need the data on protected forests.
- On the ArcMap main menu, click Bookmarks and click Rondonia.
- In the Catalog window, add the Protected_Forests layer to the Table Of Contents, below the Roads layer.
The Protected Forests layer covers the layers under it. You'll change the symbology and make it transparent.
- In the Table Of Contents, click the symbol for the Protected Forests layer. In the Symbol Selector, scroll through the list of symbols and click the Pink symbol.
- Click OK. Open the Layer Properties dialog box of the Protected Forests layer.
- Change the transparency to 60 percent and click OK.
The protected areas seem to be an effective deterrent of deforestation. Now that you can see one of the preventative measures against deforestation, you'll investigate its causes. In the previous lesson, you turned off the Roads layer because the dense road network obscured other layers on your map. Now, you'll take a closer look at those roads to see how they relate to deforestation.
- In the Table Of Contents, check the Roads layer to turn it on.
- Zoom back to the area in the northwest part of the state.
If you have not navigated anywhere else or closed ArcMap since the last time you zoomed in, you can use the Go Back To Previous Extent button on the Tools toolbar.
- Pan and zoom around the map, paying attention to the distribution of roads and deforestation.
There is a strong relationship between roads and deforestation. In fact, 95 percent of deforestation in the Amazon rainforest occurs within 5.5 kilometers of a road. Roads allow access to the otherwise impenetrable rainforest and facilitate the transportation of lumber. Unlike official roads, which connect cities, unofficial roads access deeper areas of the rainforest and connect rural properties.
Your goal is to estimate how much deforestation a proposed road would cause if its construction were allowed. To make this estimate, you'll first find how much deforestation is associated with existing roads.
- Return to the Rondonia bookmark.
Select a sample area to analyze
Before you begin your analysis, you'll select a sample area of the existing road network. The road network is massive, with over 20,000 features. Performing analysis on the entire thing would take a lot of time. Selecting a sample area may affect your results slightly, but not much.
- In the Table Of Contents, turn off all layers except the Roads layer and the Imagery basemap.
To quickly turn off all layers, right-click Layers at the top of the Table Of Contents and choose Turn All Layers Off. Then, turn on the Roads layer and the basemap.
To select a sample of the Roads layer, you'll use the Select tool. The Select tool selects features from all selectable layers. Turning off the other layers makes them unable to be selected, ensuring that only features in the Roads layer will be selected. The basemap has no features, so it has nothing to select.
- On the Tools toolbar, click the Select Features tool.
If you click the arrow next to the Select Features tool, you can choose from several selection methods. Clicking the Select Features tool will use the Select by Rectangle method by default.
- On the map, draw a box around the northwest portion of the state.
Your selection does not have to match the above image exactly.
The features within the box are selected in the map.
Some roads that extend outside the selection area are selected. If part of a feature is in the selection area, the entire feature is selected.
- On the ArcMap main menu, click Selection and choose Zoom To Selected Features.
The map zooms to the extent of your selection. Some of the selected roads, particularly on the northern end of your selection, are not near visible deforestation (you can turn on the Deforested Area layer to confirm). Other roads are inside very dense deforestation. Overall, your selection gives a general representation of the roads in this area.
Find deforestation near existing roads
Next, you'll begin your analysis. To estimate the impact of the proposed road, you'll first determine the impact of existing roads. You'll measure this impact by the percentage of area around roads that is deforested. To find this percentage, you'll create a buffer around your roads selection and compare the buffer's area to the area of deforestation inside the buffer.
- On the ArcMap main menu, click Geoprocessing.
The Geoprocessing menu expands. This menu contains six of the most commonly used geoprocessing tools, as well as options to find and use tools. Geoprocessing tools are operations that manipulate geographic data. Typically, they take an existing dataset as their input and run a process to transform it into a new output.
- In the Geoprocessing menu, click Buffer.
The Buffer dialog box opens. In this dialog box, you'll set your input dataset and a few parameters needed to run the tool. One of those parameters sets the distance of your buffer, or how far away from the input features the buffer area will extend. You already know that 95 percent of deforestation in the Amazon occurs within 5.5 kilometers of roads. This is a good distance for your buffer, as relatively little deforestation occurs beyond this distance.
For an explanation of either the tool or a parameter in the tool, click the Show Help button in the lower right of the tool window.
- In the Buffer dialog box, for Input Features, choose Roads.
When you choose a layer as your Input Features, only selected features are used as the input. If no features are selected, all features are used.
If you want to use a layer that is not in your Table Of Contents as your input, use the Browse button and browse to the location of the data.
- For Output Feature Class, confirm that the output location is the Rondônia geodatabase.
The name of the output feature class is at the end of the text string. In this case, it is Roads_Buffer. You can change the default name by replacing it in the text box, but this name is fine.
- For Distance, in the Linear unit text box, type 5500.
The default measurement unit is meters; 5,500 meters equals 5.5 kilometers. Alternatively, change the measurement unit to kilometers and type 5.5 in the box.
The only other parameter you need to change is the Dissolve Type. By default, the Buffer tool creates a buffer for each feature in the input layer. Because your Roads layer selection has many features and those features are very close together, the Buffer tool would create a large number of overlapping buffer features. By changing the Dissolve Type parameter, the Buffer tool will create a single feature as its output.
- For Dissolve Type, choose All.
- At the bottom of the Buffer dialog box, click OK.
The Buffer tool runs. When the process finishes, the result layer is added to the map.
When the output of a geoprocessing tool is added to the map, it has random default symbology. Your symbology may be different than that in the example images.
Next, you'll find how much deforestation falls within this buffer zone.
- In the Table Of Contents, drag the Roads_Buffer layer below the Deforested Area layer. Turn on the Deforested Area layer.
- On the Tools toolbar, click the Clear Selected Features button to deselect all features.
- Compare the Deforested Area and Roads_Buffer layers.
If the default symbology of the Roads_Buffer layer makes the Deforested Area layer difficult to see, change the symbology of the buffer layer.
A significant portion of the buffer overlaps with the Deforested Area layer, although not uniformly. The northwestern part of the buffer has many areas that are near roads but have relatively little deforestation.
To calculate the percentage of the buffer that is deforested, you'll need a layer of deforestation within the buffer. You can create this layer using a geoprocessing tool called Clip. The Clip tool clips the extent of one layer to the extent of another.
- On the ArcMap main menu, click Geoprocessing and choose Clip.
The Clip dialog box opens.
- For Input Features, choose Deforested Area.
- For Clip Features, choose Roads_Buffer.
- For Output Feature Class, confirm that the output will be saved to the Rondônia geodatabase with the name Deforested_Area_Clip.
- Click OK.
After the tool finishes, the layer is added to the map.
Calculate the percentage of deforested area near roads
You've created two layers. One shows the area within 5.5 kilometers of the roads in your sample area. The other shows deforestation within that buffer. Your next goal is to calculate the percentage of the buffer that is deforested. To find the percentage, you'll calculate a new attribute field.
- In the Table Of Contents, right-click the Deforested_Area_Clip layer and choose Open Attribute Table.
The layer has two fields of geometry that are automatically created for all polygon feature classes: Shape_Length (perimeter) and Shape_Area.
- Examine the Shape_Area field.
Because the area is based on the sample roads you selected, your area values (and all subsequent values you calculate from them) will probably differ from the example images.
The unit of measurement is not given in the table, but specified in the layer's coordinate system information.
- Leave the attribute table open and open the Layer Properties dialog box of the Deforested_Area_Clip layer.
- On the Source tab, under Data Source, scroll down to the projection information.
The projection of the data is South America Albers Equal Area. Output layers made from geoprocessing tools use the same projection as their input. Below the projection name is technical information about the projection, including its linear unit (the unit of measurement used by the projection). For the South America Albers projection, the linear unit is meter. Thus, the Shape_Area field in the attribute table is in square meters.
- Close the Layer Properties dialog box and return to the attribute table.
- Right-click the Shape_Area value and choose Copy.
Copying the area will allow you to use it while you look at the Roads_Buffer table.
- In the Table of Contents, right-click the Roads_Buffer layer and choose Open Attribute Table.
The table opens in a new tab on the table that is already open. Like the Deforested Area Clip layer, the Roads Buffer layer has a Shape_Area attribute. You'll add a new field and use both area values to find the percentage.
- On the table's toolbar, click the Table Options button and choose Add Field.
- In the Add Field dialog box, for Name, type Percent_Deforested. For Type, choose Double.
The field type determines what kind of values the field can have. The Double type allows numerals with decimals.
Field names can only have text, numerals, and underscores. You can create a field alias that contains other characters. The field alias is displayed in the table, while the actual name is stored in the data.
- Under Field Properties, click the box next to Alias and type Percent Deforested, with no underscore.
- Click OK.
The Percent Deforested field is added to the end of the attribute table. It has <Null> as its value. <Null> is the default value for new fields and is equivalent to having no value. To add a value, you can edit a table cell directly or create an equation to calculate values.
- Right-click the Percent Deforested field and choose Field Calculator.
A warning explains that you are about to calculate outside of an edit session.
- Click Yes to ignore the warning.
The Field Calculator opens. The Field Calculator is similar to the Select By Attributes dialog box in that it prompts you to build an expression. It gives you standard mathematical operators and functions as well as existing fields in the attribute table to help build your expression. To find the percentage, you'll divide the area of the Deforested_Area_Clip layer by the area of the Roads_Buffer layer and multiply the result by 100.
- In the Field Calculator, in the text box at the bottom of the window, paste the area value that you copied from the Deforested_Area_Clip attribute table.
- In the list of mathematical operations, click the division button.
- Under Fields, double-click Shape_Area.
The Shape_Area field is added to the equation after the division symbol. To express the value as a percentage, you'll multiply it by 100.
- In the list of operations, click the multiplication button to add it to the equation.
- In the equation, after the multiplication symbol, type 100.
The initial area value in your expression may be different than the example.
- At the bottom of the Field Calculator, click OK.
The Percent Deforested field is calculated. Based on the example area values, the value returned was about 46 percent. Your value may differ, but probably not by more than a couple of percentage points.
You now know the percentage of land within 5.5 kilometers of roads that is deforested. If a new road were built in this sample area, you could predict that a similar percentage of land within 5.5 kilometers of that road would become deforested.
Now that you have this value, the Deforested_Area_Clip layer is no longer necessary. You'll remove it from your map. You'll keep the Roads_Buffer layer because it has the percentage value in its attributes, but you'll turn off the layer.
- Close the attribute table. In the Table Of Contents, turn off the Roads_Buffer layer. Right-click the Deforested_Area_Clip layer and choose Remove.
The layer is removed from the map. The feature class still exists in the Rondônia geodatabase.
- Return to the Rondonia bookmark and save the map.
In this lesson, you looked at deforestation in Rondônia and determined its relationship to roads. Ultimately, you determined the percentage of land around roads that is deforested. In the next lesson, you'll apply that percentage to the proposed road to estimate how much deforestation was prevented by the road's prohibition.