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.
If you haven't completed the previous lesson(s), start this module by downloading the project package.
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 Rondonia Deforestation project.
- Add the Proposed_Road raster dataset from the Rondonia geodatabase and drag it below the Deforested Area layer.
- If necessary, turn off all layers except the Proposed_Road layer and the World Imagery basemap.
- In the Contents pane, right-click Proposed_Road and click Zoom To 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 Cities, Official Roads, and Protected Forests layers.
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.
- Turn on the Roads layer and turn off the Protected Forests layer.
- On the ribbon, on the Map tab, click Bookmarks and create a bookmark called Proposed Road.
- In the Catalog pane, right-click the Rondonia geodatabase and click New, then choose Feature Class.
The Create Feature Class menu appears. Creating a feature class allows you to create vector data. You'll base the new vector line feature on the raster image.
- In the Create Feature Class pane, for Name, type Planned_Road. For Alias, type Planned Road (without the underscore).
- For Feature Class Type, expand the menu and choose Line.
- At the bottom of the pane, click Next.
The next page allows you to add fields to the data. You want to add two fields, Name and Status, such as you found in the Roads feature class.
- For Fields, choose Click here to add a new field and rename the new field Name.
- Make sure the Data Type is Text. In the Field Properties table, for Length, type 50.
The default length for a text field is 255 characters, but you don't need that many characters for the road information you're going to input. Specifying a shorter length makes your geodatabase smaller.
- Add another text field named Status and set its length to 20.
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 use the coordinate system used by the layers already on your map, which you can quickly choose from the list.
- Click Next again. On the Spatial Reference page, under XY Coordinate Systems Available, expand Layers and choose South America Albers Equal Area Conic.
- Click Finish.
- From the Catalog pane, drag Planned_Road into the Contents pane below Roads.
Since Planned Roads is a new, empty feature class, nothing draws on the map. Next, you'll digitize data from the raster image and store it in the new feature class.
Digitize the 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. Then you'll continue digitizing the road, making one long, continuous feature.
- On the ribbon, click the Edit tab. In the Features group, click Create.
The Create Features pane appears. This pane lists templates for layers that are editable. The template displays available tools that can be used to construct or modify the feature.
- In the Create Features pane, click Planned Road.
Once selected, the options for editing a line feature expand under Planned Road. The pointer also changes to crosshairs, allowing you to draw features.
Some of the tools are also available on the Editor toolbar at the bottom of the Map window.
- On the bottom ribbon, next to the map scale, click Snapping to turn it on.
The snapping tool makes it easier to trace existing features, such as the existing unofficial roads that overlap with the proposed road. You are digitizing the proposed road feature by tracing the outline shown in the raster, so this will make part of the job easier.
- On the Editor toolbar, click Trace.
- Click the left end of the proposed road where it intersects with the existing official road.
Once you click the endpoint, the pointer immediately starts tracing any existing features you hover over. This is behavior enabled by the snapping tool.
- Trace the unofficial road that overlaps with the proposed road.
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, click the endpoint once to add a vertex.
Now that there are no more features to trace, you'll need to use a different construction method to digitize the rest of the road. You'll use a freehand method of adding individual vertices along the line to continue tracing until you reach another unofficial road.
- On the Editor toolbar, click the Line tool.
- Click along the proposed road to add vertices.
- When you reach another segment of unofficial road, click to add a vertex.
The snapping behavior will draw your pointer to the exact location of the unofficial road's endpoint. Now, you'll use the Trace tool again to finish digitizing the road.
- On the Editor toolbar, click Trace and finish tracing the road until it joins back with an official road.
- Double-click the endpoint to finish the feature.
- Click the Proposed Road bookmark to confirm that the road is fully digitized.
You should have one continuous road feature. If you're satisfied with your road, you'll save your edits to the geodatabase. Until you save your edits in the Manage Edits tab, all your edits can be undone. Simply clicking Save on the Quick Access ribbon doesn't save the new feature.
- On the ribbon, on the Edit tab, in the Manage Edits group, click Save.
- In the Save Edits window, click Yes to save all edits.
Now that you've digitized the planned road, you can remove the imagery layer from the map.
- In the Contents pane, remove the Proposed_Road layer.
- Clear the selected features.
- On the ribbon, click the Map tab. In the Navigate group, click Explore.
The pointer changes back to a hand. You can now zoom and pan the map as normal.
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 Contents pane, drag the Planned Road layer below Official Roads.
- Click the Planned Road symbol to open the Symbology pane.
- In the Symbology pane, in the Gallery tab, click Highway.
The planned road is now shown in bright orange, and stands out from the other roads on the map. Next, you'll add attribute data to the proposed road feature.
- In the Contents pane, right-click Planned Road and choose Attribute Table.
- In the Planned Road attribute table, for Name, type BR 421, and for Status, type Proposed, then press Enter.
- On the ribbon, on the Edits tab, in the Manage Edits group, click Save to save all edits.
- 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.
- In the Geoprocessing pane, on the Favorites tab, click Buffer (Analysis Tools).
- For Input Features, choose Planned Road and make sure that the Output Feature Class is Planned_Road_Buffer.
- For Distance, type 5.5 and set the Linear Unit to Kilometers.
Because there is only a single feature to buffer, this time you don't need to worry about the dissolve type.
- Click Run.
When the tool finishes, the buffer layer is added to the Contents pane and the map.
- In the Contents pane, drag Planned_Road_Buffer below Deforested Area.
- Turn Deforested Area on and compare the layers.
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.
- In the Geoprocessing pane, search for and open Erase (Analysis Tools).
- For Input Features, choose Planned_Road_Buffer, and 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.
- Name the Output Feature Class Erased_Buffer and click Run.
The tool runs and the layer is added to the map.
- In the Contents pane, click Planned_Road_Buffer twice and rename it At-Risk Area, then turn it off.
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.
- Right-click Roads_Buffer and open the Attribute Table.
- Copy the value in Percent Deforested and open the attribute table for the Erased Buffer layer.
- Click Add. In the Add Field table, for Name, type Potential_Deforestation and for Data Type, choose Double.
- For Alias, type Potential Deforestation (sq km) and click Save.
- Close the Fields tab and right-click the Potential Deforestation (sq km) field header. Choose Calculate Field.
To find the road's deforestation potential, you'll multiply the buffer area by the percentage of deforested area that you found earlier. You'll also convert the area, currently in square meters, to square kilometers, a unit more appropriate for the vast area you're describing.
- In the Calculate Field pane, for Potential_Deforestation=, create the expression Shape_Area / 1000000.
This part of the expression converts the buffered area, in square meters, to square kilometers. Next, you'll multiply by the percent of deforested area you calculated earlier.
- Add parentheses around that statement, then click multiply and paste the number copied from Roads Buffer.
- After the percentage value, divide by 100 and put parentheses around this division expression as well. The expression reads (!Shape_Area! / 1000000) * (47.995989 / 100).
- Click Run.
According to this analysis, approximately 650 square kilometers were saved by the canceling of the proposed road. Your value may be slightly different.
- Close the attribute tables, return to the Rondonia bookmark, and save the map.
In this lesson, you predicted the impact of deforestation that might stem from the proposed road. In the next lesson, you'll export the map as a layout to share.