Now that you have explored the study area, you are ready to perform the analysis.
Using GIS, viewsheds can be calculated to show areas where observed objects, such as towers and turbines, can be seen, or, conversely, to show areas that can be seen from specified objects, such as steeples or observation decks. When performing this type of analysis for wind farms, you need to decide whether to compute the viewshed for the object from the tip of the rotor blade when it is in the most upright position, the rotor (which the blades are attached to), or some other part of the turbine. Your decision will have an impact on the viewshed analysis results.
Tall objects, such as wind turbines, solar power towers, and communication towers, will be visible for long distances in flat, open areas. In other areas, the terrain, as well as the presence of features—such as buildings, trees, and hedgerows—can make a significant difference to the actual view. The observer's visual acuity, curvature of the earth, and atmospheric refraction, which increases the visible distance to the horizon, also affect the visibility of an object. Although the earth's surface curves out of sight at a distance of about 5 kilometers (or 3 miles), the tops of tall objects may be visible above the horizon. At the same time, dust, water vapor, and pollution in the air will rarely allow you to see more than 20 kilometers (12 miles), even on a clear day. It is important to take these factors into account when assessing visual impacts through viewshed analysis.
The Create Viewshed tool takes into account atmospheric refraction and curvature of the earth, but not other factors, such as obstructing trees or buildings; therefore, viewshed analysis is often accompanied by on-the-ground investigations. For example, in the Swift Wind Farm EIA, 19 locations  were assessed to determine if their viewpoints would offer direct and uninterrupted views of the turbines. One requirement for viewshed analysis is data about the form of the landscape. In a GIS, the ground, or terrain, is commonly represented with a digital elevation model (DEM). Esri has compiled a number of elevation datasets in a format that is easy to use in your analysis and mapping. These datasets are used in the Create Viewshed tool. To learn more about the DEM data, see the Terrain layer description.
Perform the viewshed analysis
You will perform a viewshed analysis from the tip of the rotor blade, and you will compare that to a viewshed created from the base of the turbine. This will allow you to see how much the height of the object can impact the analysis.
- If necessary, open your Swift Wind Farm Viewshed Analysis map.
- Change the basemap back to Topographic.
The Wind Turbines layer is a special layer created so you can see the proposed size of the turbines when you zoom in and out. It was created as a tile layer. The Create Viewshed tool requires point features, so you'll use the Turbines layer to perform the analysis in this lesson. It represents the same turbine locations without symbols designed to represent the actual height.
- Turn off all layers at once by pressing the Ctrl key while unchecking the box of any layer that is already turned on.
- Turn on the Turbines and Swift Wind Farm layers.
- Turn on the Study Area layer and zoom to it.
This layer shows the 25-kilometer distance from the turbines that you will use in the viewshed analysis. This 25-kilometer distance was used in the EIA to encompass an area "considered appropriate to cover all potentially material landscape and visual impacts resulting from the proposed Development." 
- In the Contents pane, point to the Turbines layer and click the Perform Analysis button.
You can also access the Perform Analysis pane from the ribbon.
- In the Perform Analysis pane, click Find Locations and click Create Viewshed.
Notice that the tool displays a default height and default units.
Tip:The default units for tools may be initially set by your organization's administrator but can be edited in your profile. In addition, you may have the option to change units for specific tool settings on the user interface. Analysis results, as reflected in tables and pop-ups, will always be reported in the units set in your profile, even if you choose different units in a tool setting.
- Verify that Point features that represent observer locations is set to Turbines.
- In the Create Viewshed pane, set Height of observer locations to 126.5 meters (or 415 feet).
This is the height of the objects that will be observed, which, in this analysis, are the wind turbines.
- Set Height of other objects on the ground to 1.8 meters (or 6 feet).
This is the height of the observer, so you will assume the average height of the people who will be viewing the turbines is 1.8 meters.
- Set Maximum viewing distance to 25 kilometers (or 15 miles).
For the Create Viewshed tool, the optional Maximum viewing distance parameter is related to the resolution of the data used in the analysis and determines the extent of the viewshed. When the maximum viewing distance is less than or equal to 5 kilometers, and for areas where it is available, 10-meter resolution data will be used. If the maximum distance is greater than 5 kilometers but less than or equal to 15 kilometers, and where it is available, 30-meter resolution data will be used. Otherwise, if the distance is greater than 15 kilometers, 90-meter resolution data will be used. The maximum allowable viewing distance is 50 kilometers.
Using a distance of 25 kilometers means that the coarsest-resolution data (90 meter) will be used in your analysis.
- Change the result layer name to Viewshed and add your name or initials to make it unique in the organization.
This will prevent your analysis results from conflicting with somebody else's in your organization.
- Check the Use current map extent box.
Note:For the Create Viewshed tool, if Use current map extent is checked, only the features (turbines) that are visible in the current map extent will be analyzed. If unchecked, all features in the analysis layer will be analyzed, even if you cannot see them in the current map.
- Click Run Analysis.
When the operation finishes, the Viewshed layer is added to the map and the Contents pane.
The viewshed you created identifies the areas that can be seen from the tops of the turbines and, conversely, areas from which the tops of the turbines can be seen.
To see how the viewshed analysis varies depending on the assumptions you make, you'll compare your results with those of a second viewshed generated for the base of the turbine towers.
- In the Contents pane, point to the Viewshed yourname layer and click the Rerun Analysis button.
The Create Viewshed tool opens with the settings from the first time you ran the tool. You'll change the settings to create a second viewshed.
- Rerun the analysis with the following settings (substitute equivalent values if you are using U.S. standard units):
- Set Height of observer locations to zero.
- Set Height of other objects on the ground to 1.8 meters (5.9 feet).
- Set Maximum viewing distance to 25 kilometers (16 miles).
- Name the result layer Viewshed from Base, adding your name or initials to make it unique.
- Save the map.
In the next lesson, you'll explore the results of your analysis.