Cross-Agency Comparison

As mentioned in the “Background” section, the BLM’s approach for delineating IAUs for NSHTs begins with a viewshed analysis from the trail segment and then captures its outer perimeter to create a polygon. To learn more about best practices for developing viewsheds, we turned to Federal scenery management programs that often use viewsheds during inventory. Aside from the BLM’s guidance for inventory of NSHTs, the FS and NPS each possess their own national visual resource management programs with agency-specific manuals or documentation for conducting inventories (refer to table 1).

As of 2024, the BLM is the only agency that has written operational guidance for inventorying NSHTs. Volumes 1 and 2 of BLM Technical Reference 6280-1 (BLM, 2020a, b) aim to develop a consistent and repeatable method for conducting an inventory of NSHTs—they create a standardized process for all national trails and can be used as a model for any agency attempting inventory efforts of NSHTs. At the time of this report (2024), only the BLM and FS have created reports regarding the inventory of NSHTs.

The FS’ scenery management process is dictated by “Forest Service Manual 2300—Recreation, Wilderness, and Related Resource Management—Chapter 2380—Landscape Management” (FS, 2003) and “Landscape Aesthetics—A Handbook for Scenery Management” (FS, 1995), which comprise the FS’ scenery management system. The FS separates trail segments into units referred to as “scenic character units” that are grouped based on similar ecological factors. Overall, the FS follows the BLM guidelines laid out for the IAM of NSHTs.

The NPS refers to “Documenting America’s Scenic Treasures—The National Park Service Visual Resource Inventory” (Sullivan and Meyer, 2016) for NSHT inventories, which uses the park unit as the designated area of importance and only requires inventory efforts when a development proposal occurs. This process is different from the BLM and FS given that the NPS is not a multiple-use agency.

Trail administrators across the NPS, BLM, and FS may conduct a viewshed analysis related to individual National Environmental Policy Act of 1969 (42 U.S.C. 4321 et seq.) projects they are involved with to assist in determining the effects of proposed actions. These viewshed analyses differ in methodology given the different availability of NSHT data, DEM data, and preferences and analysis specifications chosen by the trail administrator.

Methods Used in Bureau of Land Management Inventory, Assessment, and Monitoring Reports to Delineate Inventory Analysis Units

A viewshed analysis uses a DEM to distinguish the land area that is visible from a specific point or set of points. For example, a person on top of a mountain would have a much larger viewshed than a person in a valley, and the exact extent to which a person can see can be determined from these DEMs. To conduct a viewshed analysis, specific parameters must be defined, such as the observer offset, surface offset, and radius. The observer offset (also referred to as “offset A”) is the height or level of the observer, whereas the surface offset (also referred to as “offset B”) is the height of the target being observed. The observer offset most often tends to be around the average eye level for adults (BLM, 2020b). The surface offset is usually set to 0 m (0 ft) to represent what someone could see on the surface of the Earth, but it can be set higher to analyze the effect of adding something to the landscape, such as a transmission line or wind turbine. The radius of a viewshed analysis is the chosen maximum distance that the viewshed is confined to. For example, a viewshed could span 10 mi, but if the radius was set to be smaller—for example, 5 mi—then the returned polygon for the viewshed would only span as many as 5 mi out from the trail.

Staff and contractors from the BLM who were involved in prior inventories observed that there were parts of the IAU delineation process that were counterintuitive and would be difficult to generalize across locations. Specifically, the trail segmentation required a level of inventory (field observation) to categorize the presence of high-potential route segments and similar trail conditions, trail characteristics, and ownership. Additionally, conversations with staff revealed that the process of segmenting a trail is highly localized and depends on the characteristics of each individual landscape; therefore, trail segmentation should occur once an inventory has started and the interdisciplinary team is assembled. Not having an initial IAU was troublesome for understanding the scope of the inventory effort. During the IAM Evolution meeting, staff from the BLM noted that refining the IAU after a viewshed analysis would be hard to accomplish without being physically present on the trail or having staff with on-the-ground experience. Therefore, the process for creating a final IAU should be flexible and allow for input and refinement by the interdisciplinary team.

In 2023, four IAM reports involving NSHTs from the BLM were published with IAUs delineated using different viewshed specifications, which are shown in table 2. Conversations with BLM staff involved in these IAM processes revealed that the initial size of the radius in the viewshed analysis typically starts large (for example, 30 mi) and then is adjusted to the size that is the most intuitive for the landscape and area. For example, the size of the radius for Cascade-Siskiyou National Monument was 2 mi, whereas the size of the radius for the CDNST in New Mexico was 30 mi because the views are less obstructed. As shown in table 2, the parameters for the viewshed analysis differ across reports, and each of the reports uses a different radius. Of the two reports that specify offsets, different offsets were used as well.

Viewshed Analysis for Draft Inventory Analysis Units

The viewshed analysis parameter recommendations can be found in table 3. According to a visibility study of wind turbines in Wyoming and Colorado, the conservative suggested limit of the viewshed analysis radius is 30 mi (Sullivan and others, 2012). This research indicates that in optimal conditions, the human eye can see wind turbines as far as 36 mi away. However, a casual observer was only likely to notice turbines within 26 mi, indicating that 30 mi is a reasonable middle metric for visibility (Sullivan and others, 2012). Other studies noted similar visibility distances; for instance, a solar facility was visible from 29 mi when lit (Sullivan and Abplanalp, 2017), power towers were easily visible beyond 20 mi (Sullivan and others, 2013), and offshore wind turbines were visible beyond 26 mi (Sullivan and Abplanalp, 2013). These studies indicate that projects being proposed and developed, even if miles off a trail, have the potential to affect the nature and purposes of a NSHT and its Federal protection components. Additionally, conversations with BLM staff and partners indicated that 30 mi is a reasonable radius to use to create draft IAUs, with the understanding that the draft IAU would be refined to encompass only what is locally sufficient for monitoring and protecting the landscape elements that contribute to the nature and purpose of the trail.

It is suggested that offset A, or the observer offset, be set at 1.52 m (5 feet [ft]) to account for the average eye-level height of adults in the United States (BLM, 2023a). This recommendation is consistent with volume 2 of BLM Technical Reference 6280-1. However, volume 2 of BLM Technical Reference 6280-1 does not provide guidance on offset B (BLM, 2023c). Offset B, or the surface offset, should be set to 0 m (0 ft) because the draft IAU should be based on the person-level viewshed rather than any specific development or projects (which would have a higher offset).

A DEM should be used as an input to the viewshed analysis rather than a digital surface model (DSM) because the DEM will result in a more conservative viewshed. A DSM consists of the elevation of the environment in addition to natural and built features of the landscape (for example, forest canopy, buildings), whereas a DEM only includes the environment elevation and so will assume more can be seen from the trail than a DSM. Volume 2 of BLM Technical Reference 6280-1 recommends using a 10-m DEM, and we concur with this recommendation because it balances a higher resolution result with the computation time necessary. However, a 10-m DEM will still have a large computation time if run over large spatial scales, so a lower resolution DEM may be necessary in the interest of computation time. The U.S. Geological Survey National Map Downloader (https://apps.nationalmap.gov/downloader/) maintains 10-m DEMs for the entire United States.

When creating the polygon that encompasses the viewshed analysis to serve as the draft IAU, the Aggregate Polygon tool in ArcGIS (Esri, 2023) can be used to streamline and standardize this process. The recommended parameters for the Aggregate Polygon tool can also be found in table 3. Specifically, the aggregation distance, minimum area, and minimum hole size are set to 10 mi to mimic if the boundary were hand drawn around the viewshed analysis results, excluding no areas. The aggregate distance parameter specifies the maximum distance that polygons can be apart from one another and be combined. Minimum area and minimum hole size are the minimum sizes of polygons and polygon holes to be retained. Using the Aggregate Polygon tool saves time when creating the polygons (compared to drawing by hand) and allows for increased consistency across all trails. For purposes of cross-jurisdictional coordination, resulting draft IAUs should not be edited to exclude any non-BLM lands, whether public or private (this same approach is used by the FS for viewshed analyses in scenic character units). The result of the viewshed analysis should remain as is, given that the viewshed does not differ depending on the land being public or private. Viewsheds should also not be limited by jurisdictional or State boundaries. When a trail involves partnerships with Canada or Mexico, it may be appropriate to allow the viewshed boundaries and draft IAUs to extend into those countries. As trail segments are inventoried for management, the draft IAU will be refined to a final IAU that consists of only the land area necessary to support the nature and purpose of each designated trail. This IAU will continue to be monitored and assessed according to policy.

It is important to note for NHTs specifically that this viewshed only encapsulates what can be seen from the trail and that further efforts are needed to document associated heritage resources of NHTs. Notably, high-potential sites may be geographically separate from the designated alignment and therefore may be outside of the viewshed, which is another consideration for using a 30-mi radius to define the draft IAU. Even with that radius, there may be locations outside the perimeter of the draft IAU to include when finalizing the IAU because there may be elements that are important to the trail that are not within view from the trail. Further, draft IAUs do not represent a land allocation or special designation—they are informational in nature and intended to trigger an inventory and analysis for projects proposed within the boundaries.

Viewshed Analysis Steps

The following steps describe the suggested method for how the viewshed analysis could be performed using ArcGIS Pro version 3.1.2 (Esri, 2023), although other GIS programs could be used to perform the viewshed analysis. An example of this procedure is provided in the “Example—Continental Divide National Scenic Trail in New Mexico” section of this report.

Order of Operations

Draft IAUs can be created on all NSHTs before inventories are conducted. A proposed project that intersects the draft IAU boundaries would trigger the need for an inventory of the trail resources before the managing agency can authorize the project. Once an inventory is triggered, the interdisciplinary team should segment the trails following the guidelines outlined in volumes 1 and 2 of BLM Technical Reference 6280-1 (BLM, 2020a, b). The refinement of the draft IAU would occur during the initial inventory effort based on fieldwork and understanding where the trail-related and contributing landscape elements exist. Although it is somewhat common in reporting of IAM efforts to include both the initial viewshed analysis and the resulting finalized IAUs, the process of moving from the initial allocation of space to the final IAU is not typically documented. Thus, these reports tend to leave a lot of steps unreported and are therefore not reproducible. Starting an inventory effort with a consistently delineated draft IAU introduces more transparency. Between the draft IAU delineation and refinement, the interdisciplinary team may decide to include an area beyond the original viewshed extent or exclude areas that do not contain anything significant to the trail. For the greatest possible transparency in these processes, there should be an official documentation of the decisions that are made to refine the draft IAU to the final IAU and a description of the decision process. Refinement of the IAU will allow for the use of NHT heritage resource data that more accurately reflect the location of the NHT as compared to the designated alignment. Trail administrators and land managers must be in agreement regarding heritage resource data used in place of the designated alignment for analysis purposes.

Participants in the BLM IAM Evolution meeting agreed that having draft IAUs completed for every NSHT was desirable. Generally, the perspective was that draft IAUs would be helpful in speeding up the inventory process. Specifically, this preplanning action would result in better IAM products compared to responding only to proposed actions. The draft IAUs would serve as a preliminary boundary inside of which a proposed action would result in a full IAM process. This creation of draft IAUs for every NSHT is the next step in the effort of documenting and inventorying trails.

Example—Continental Divide National Scenic Trail in New Mexico

The recommendations described in table 3 are visualized by using the New Mexico portion of the CDNST as an example. The CDNST is a NST that consists of 3,100 mi between the borders of Alberta, Canada, and Chihuahua, Mexico. The CDNST follows the Continental Divide along the Rocky Mountains and crosses Montana, Idaho, Wyoming, Colorado, and New Mexico. Although most often used for hiking, the CDNST also is used by mountain bikers and horseback riders (Hill and Fothergill, 2022). Because the CDNST passes through 25 national forests, 21 wilderness areas, and 3 national parks, it is managed by the BLM, FS, and NPS (FS, undated). The CDNST is not one unbroken trail, and as of 2023, there are still gaps in the trail (Continental Divide Trail Coalition, undated).

Figure 1 shows the CDNST in New Mexico and depicts the viewshed analysis results from using the parameters in table 3 and the resulting polygon that represents the draft IAU using the parameters in table 3 to delineate the polygon. Along with the best practices in this section for parameters, all areas that can be seen from the trail are included in the draft IAU. That is, there are no areas from the viewshed analysis excluded for any reason. Figure 2 shows the CDNST in New Mexico with the draft IAU created according to the best practices and steps discussed in this section. This example draft IAU would be used to inform the area surrounding the CDNST that would trigger an inventory if a proposal were within the designated area on the map. The final IAU would be edited after review from the interdisciplinary team during the localized IAM process.