In the previous lesson, you calculated the percentage of deforested area within 5.5 kilometers of roads. In this lesson, you'll use that percentage to estimate the area in square kilometers that would have been deforested if a proposed road were built. You'll first digitize, or draw, the proposed road as a new feature class based on an image of the road. Then, you'll buffer the road and calculate the predicted deforestation area using its attribute table.
Add an image of the proposed road to the map
The proposed road is in neither roads layer, because those layers only show existing roads. You'll add an image of the proposed road to the map and create your own feature based on the image. This image came as a raster dataset in the Rondônia geodatabase. A raster dataset is an image composed of a grid of pixels. The Imagery basemap in your map is also a raster image. Your other data is called vector data.
- If necessary, open your Amazon Deforestation map document.
- Drag the Proposed_Road layer from the Rondônia geodatabase to the Table Of Contents, placing it below the Deforested Area layer.
Instead of a symbol, the Proposed_Road layer has three color bands. These bands determine the color of each pixel in the image's raster grid.
- Turn off all layers except the Proposed_Road layer and the basemap. In the Table Of Contents, right-click the Proposed_Road layer and choose Zoom To Layer.
The map zooms to the extent of the layer.
The image shows a topographic view of the area, with features such as national boundaries, some existing roads, mountainous terrain, and rivers. The proposed road is a thick line in the middle of the image.
- Turn on the following layers: Cities, Official Roads, and Protected Forests.
The proposed road connects two official roads, with cities near both endpoints of the road. You can use the Identify tool to find out the names of each city, or you can temporarily turn on labels.
- In the Table Of Contents, right-click the Cities layer and choose Label Features.
Each city on the map is labeled with its name. The labels indicate that the cities the road will connect are Guajará-Mirim to the west and Campo Novo de Rondônia to the east. The proposed road also crosses two protected areas: the Guajará-Mirim state park and the Rio Ouro Preto Extractive Reserve. The road was prohibited because it would have gone through these areas.
- Right-click the Cities layer again and choose Label Features to turn off labels.
- Turn on the Roads layer.
Some portions of the proposed road, especially at the ends of the road, coincide with existing unofficial roads. As you draw the proposed road feature, you'll trace these existing roads where appropriate.
- On the ArcMap main menu, click Bookmarks. Create a bookmark called Proposed Road.
Create a new feature class
Using the image as a guide, you'll digitize the proposed road as a feature. You could digitize the proposed road as part of one of your roads layers, but that would alter reliable data with a feature representing a road that does not actually exist. Instead, you'll create a new feature class.
- In the Catalog, right-click the Rondônia geodatabase, point to New, and choose Feature Class.
The New Feature Class dialog box opens.
- For Name, type Planned_Road (with the underscore). For Alias, type Planned Road (with no underscore).
The name cannot be the same as any other item in the Rondônia geodatabase, so you'll go with Planned Road instead of Proposed Road.
- For Type, choose Line Features.
- At the bottom of the New Feature Class window, click Next.
Next, you'll choose the feature class's coordinate system. A coordinate system defines positions and measurement values of geographic features on a map. You'll learn more about coordinate systems later in the project. For now, you'll use the coordinate system used by the layers already on your map, which you can quickly choose from the list.
- In the list of coordinate systems, expand the Layers folder. Click South America Albers Equal Area Conic.
You can check which layers use which projection by clicking the plus sign next to the projection's name. The South America Albers Equal Area Conic projection is used by all your layers except the basemap, which uses the WGS 1984 Web Mercator (Auxiliary Sphere) projection.
- Click Next.
The next part of the New Feature Class dialog box allows you to specify XY tolerance. XY tolerance defines how close two coordinates must be to be considered the same coordinate. The default value is fine.
- Click Next.
The next parameter is for database storage configuration. You don't have to worry about this parameter.
- Click Next.
Lastly, you'll add the attribute fields. The Planned_Road feature class should have attribute fields like the existing Roads feature class. Although you can manually create fields by entering names and data types, you can also import attributes from an existing feature class.
- Click the Import button.
- In the Browse dialog box, browse to the Rondônia geodatabase. Click the Roads feature class and click Add.
The list of fields populates with the fields of the Roads feature class: Name and Status.
- Click Finish.
The Planned_Road feature class is created in the Rondônia geodatabase and added to the map as a layer. It currently has no features.
Digitize the proposed road
To add a feature of the proposed road, you'll use the image as a reference and trace the unofficial roads that coincide with the road's location. To start digitizing a feature, you'll begin an editing session.
- In the Table Of Contents, right-click the Planned Road layer, point to Edit Features, and choose Start Editing.
An editing session begins and the Editor toolbar is turned on, showing several options for editing. The Edit tool is active by default. The Edit tool selects or moves features from any editable layer. This tool is useful for modifying existing features, but doesn't create new features.
Any toolbar can be moved by dragging it around the ArcMap window. If moved to the top of the window, the toolbar will dock.
- On the Editor toolbar, click the Create Features tool.
- In the Create Features window, click the Planned Road template.
The pointer changes to crosshairs, allowing you to draw features. Before you begin, you'll explore some drawing behavior.
- Point to some of the features on the map (don't click anything yet).
The pointer is pulled toward features. This behavior is called snapping. Snapping makes it easy to connect new features to other features. However, it makes it difficult to create features that are close to other features but don't touch exactly. Because the planned road connects to the Official Roads, Roads, and Cities layers, snapping will be useful. However, you'll turn off other feature layers so you don't accidentally snap to them while digitizing.
If snapping is not enabled by default, turn on the Snapping toolbar by clicking Customize, pointing to Toolbars, and choosing Snapping. On the Snapping toolbar, click Snapping and choose Use Snapping.
- Turn off the Protected Forests layer.
The only visible layers are the basemap, the image, the Cities layer, and all three roads layers.
- On the Editor toolbar, click the Trace tool.
The Trace tool traces an existing feature, making it useful for the portions of the proposed road that overlap sections of existing roads.
- Use the scroll wheel on your mouse to zoom to the left endpoint of the proposed road.
If you use the Zoom In tool, you'll have to reselect both the Planned Road feature template and the Trace tool after you zoom in. Alternatively, use keyboard shortcuts to make certain tools active without changing the selected tool on the toolbar. Hold the C key to make the Pan tool active. While holding the C key, hold the Shift key to make the Zoom In tool active, allowing you to draw a box on the map to zoom in. When you release the C and Shift keys, the Trace tool will become active again.
- Confirm that Trace is the active tool and click the left endpoint of the proposed road, snapping to where it touches the nearby official road.
Once you click the endpoint, the pointer immediately starts tracing any existing features you hover over.
- Trace the unofficial road that coincides with the proposed road on the image.
If you reach the edge of the map extent, hold the C key to make the Pan tool active, pan to the right, and release the C key to resume tracing.
- When you reach the end of the unofficial road that overlaps with the proposed road, click the endpoint to finish tracing.
After you click, a feature with the current symbology of the Planned Road layer appears on the map. Green squares on the line indicate vertices and the red square indicates the endpoint.
The next section of the road doesn't coincide with existing features. You'll draw the next section with the Straight Segment tool.
- On the Editor toolbar, click the Straight Segment tool.
The pointer changes to crosshairs and is connected to the endpoint by a line.
- At the next bend in the proposed road, click to add a vertex.
If the cursor snaps to the nearby unofficial road when you place your vertex, temporarily disable snapping by holding the spacebar.
A new segment is added and the endpoint (the red square) moves to the location you clicked.
- Use the Straight Segment tool to draw the next section of the proposed road, placing a vertex at each bend in the road. Place the last vertex where the proposed road joins up with an unofficial road again.
For the final section of the proposed road, you'll trace another existing feature.
- Click the Trace tool. On the map, click the last vertex you added and trace until you reach the right endpoint of the proposed road, where it connects to an official road.
- Double-click the endpoint to finish the feature.
The proposed road feature is selected and the vertices disappear.
- Return to the Proposed Road bookmark and confirm the entire length of the proposed road is digitized.
If you're satisfied with your road, you'll save your edits. Until you save your edits, all of your edits can be undone by ending the editing session or exiting ArcMap. Even if you save the map document, your edits will remain unsaved.
If you don't like how your road turned out, you can either discard the edits by ending the editing session and redoing the drawing, or edit individual vertices in the road with the Edit Vertices tool on the Editor toolbar.
- On the Editor toolbar, click Editor and choose Save Edits.
Don't stop editing yet. You'll make more edits in the next section.
- Close the Create Features window. In the Table Of Contents, remove the Proposed_Road layer.
- On the Tools toolbar, click Clear Selected Features.
Symbolize and add attribute data to the proposed road
The proposed road has been digitized, but it uses the default symbology of the Planned Road layer, which is thin and difficult to see on the map. Also, while you added attribute fields when you created the feature class, those fields have no attribute data.
- In the Table Of Contents, drag the Planned Road layer below the Official Roads layer.
- Double-click the Planned Road layer to open its Layer Properties. Click the Symbology tab.
You'll give the proposed road symbology similar to that of the existing roads, but it should also be distinct and attention-grabbing. You'll import the symbology from the Official Roads layer and alter it to make it stand out.
- At the upper right of the Symbology tab, click Import.
- In the Import Symbology dialog box, confirm that Official Roads is chosen and click OK.
The orange line appears on the Symbol button. The only change you'll make to the symbol is its color.
- Click the Symbol button.
- In the Symbol Selector, for Current Symbol, change the color to Cherry Cola.
- Click OK. In the Layer Properties dialog box, click OK.
Next, you'll add attribute data to the proposed road feature.
- In the Table Of Contents, right-click the Planned Road layer and choose Open Attribute Table.
The digitized feature is the layer's only feature. Apart from the OBJECTID, SHAPE, and SHAPE_Length fields, the layer has attribute fields for Name and Status. Neither field has attribute data.
If you are in an editing session, you can directly edit attribute tables. You should be in an editing session from the previous section. If not, start an editing session using the Editor toolbar.
- In the attribute table, double-click the <Null> value in the Name field to edit it. Type BR 421 and press Enter.
BR 421 is actually the name of an existing highway that connects the major municipality of Ariquemes to Campo Novo de Rondônia. The proposed road would have extended the highway to the western part of the state.
- In the Status field, double-click the <Null> value and type Proposed. Press Enter.
- On the Editor toolbar, click Editor and choose Save Edits.
- Click Editor again and choose Stop Editing.
- Close the attribute table.
Find the potential deforestation of the road
Next, you'll estimate the deforestation the road would have caused. In the previous lesson, you determined the percentage of land that was deforested 5.5 kilometers from a selection of existing roads. To find a total area (not a percentage) of potential deforestation around the proposed road, you'll buffer the Planned Road layer to the same 5.5-kilometer distance and multiply the buffer area by the percentage of deforestation observed around existing roads. You'll also remove areas of existing deforestation so they won't be included in your total.
- On the ArcMap main menu, click Geoprocessing and choose Buffer.
- For Input Features, choose Planned Road. Confirm that the Output Feature Class will be saved to the Rondônia geodatabase with the name Planned_Road_Buffer.
- For Distance, type either 5500 (for meters) or 5.5 (for kilometers).
- Click OK.
The tool runs and the buffer is added to the map.
The Planned_Road_Buffer layer covers some areas that already have roads and deforestation, although it's difficult to tell because of the layer's order on the map.
- In the Table Of Contents, move the Planned_Road_Buffer layer below the Deforested Area layer.
- Turn on the Deforested Area layer and compare existing deforestation to the buffer.
If the Deforested Area layer is difficult to see on top of the buffer, change the buffer's symbology to a darker color.
There are some areas where deforestation has already occurred. You don't want to include already deforested areas in your analysis. You'll remove existing deforestation from the buffer with the Erase tool. The Erase tool subtracts parts of one layer that overlap another layer.
The Erase tool is not one of the tools that can be accessed from the Geoprocessing menu. Instead, you'll search for the tool.
- On the ArcMap main menu, click Geoprocessing and choose Search For Tools.
The Search window opens.
If you haven't used the Search window before, it will open on top of the Catalog. You can return to the Catalog by closing the Search window or using the tabs at the bottom of the window. Like the Catalog and Create Features windows, the Search window can be moved and docked anywhere in the ArcMap application window.
- In the search box at the top of the Search window, type Erase and press Enter. Click the result called Erase (Analysis).
The Erase dialog box opens.
- For Input Features, choose Planned_Road_Buffer. For Erase Features, choose Deforested Area.
The tool will take the Input Features and subtract from them areas that overlap with the Erase Features. In this case, it will remove areas of deforestation from the buffer.
- Confirm that the Output Feature Class will be saved to the Rondônia geodatabase. Replace the default output name of Planned_Road_Buffer_Erase with Erased_Buffer.
- Click OK.
The tool runs and the layer is added to the map.
The Erased_Buffer layer is a single multipart feature. To find out how much of this area would have been deforested, you'll multiply the area by the percentage value you obtained earlier. This value is stored in the attribute table of the Roads_Buffer layer.
- In the Table Of Contents, right-click the Roads_Buffer layer and choose Open Attribute Table.
- Right-click the value in the Percent Deforested field and choose Copy.
- Open the attribute table for the Erased Buffer layer.
You'll add a new field to estimate the deforestation of the buffer around the proposed road. You'll then multiply the total area by the percentage value you just copied. You'll also convert the area from square meters to square kilometers so the value is appropriate for what it measures.
- On the table's toolbar, click the Table Options button and choose Add Field.
- In the Add Field dialog box, for Name, type Potential_Deforestation. For Type, choose Double. For Alias, type Potential Deforestation (sq. km).
- Click OK.
The field is added to the attribute table. Next, you'll use the Field Calculator to find the estimated deforested area in square kilometers.
- Right-click the Potential Deforestation (sq. km) field name and choose Field Calculator. When the edit session warning opens, click Yes to continue.
The Field Calculator opens. The expression for this field is more complicated than the last equation because you also want to convert the square meters of the SHAPE_Area field to square kilometers. One kilometer equals 1,000 meters, which means that one square kilometer equals 1 million meters.
- Under Fields, double-click SHAPE_Area to add it to the expression box at the bottom of the Field Calculator. Click the division button and type 1000000.
- Add parentheses around the equation. After the closing parenthesis, add a multiplication symbol and paste the percentage value.
Your value is still expressed as a percentage, so you'll divide the percentage by 100.
- After the percentage value, add a division symbol and type 100. Add parentheses around this part of the expression as well.
- Click OK.
According to this analysis, approximately 623 square kilometers were saved by the cancelling of the proposed road. Your value may be slightly different.
- Close the attribute table, return to the Rondônia bookmark, and save your map.
Although your value may vary from the example, the overall story remains unchanged. If the proposed road had been constructed, several hundred square kilometers of land would have become deforested, much of that land in untouched rainforest. In addition to the deforestation that would occur directly as a result of the road, even more natural forest may be lost due to unofficial roads branching from the road or the conversion of surrounding land to private holdings.
This analysis predicted the impact of a road that was never built. The process used for the analysis could be adapted to other locations in the Amazon in order to predict the impact of future roads. Not all proposed roads end up being prohibited, so understanding the deforestation a road may cause can be a key piece of information for balancing economic and ecologic concerns.
Additional questions could guide an impact analysis. For instance, is there a difference between the deforestation caused by official and unofficial roads? Are there additional factors, such as terrain and the protection status of the land, that affect deforestation? How does changing the sample area of existing roads affect the ultimate outcome of the analysis? You can use ArcMap to explore these questions on your own.
In this lesson, you predicted the amount of deforestation caused by a proposed road. In the next lesson, you'll present your findings with a print map that can be saved and shared.