San Juan Sightlines
A Line of Sight Analysis of Mt Constitution
In the northwestern corner of Washington state stands Mount Constitution, the tallest mountain in the San Juan Islands. Once serving as a fire lookout, its panoramic views now draw in visitors hoping to see the neighboring islands, nearby towns, distant mountains, and even bits of Canada that have been advertised by the San Juan Islands Visitors Bureau. But how accurate are those claims?
This analysis was conducted to assess how much a visitor could realistically see from the summit of Mount Constitution under ideal conditions. Although not intended to support tourism development or infrastructure planning, it can provide context for historians, given the site’s prior use as a fire observation post, emphasizing its importance as a vantage point.
DEM Data: SRTM 1 arc-second void-filled elevation models from USGS Earth Explorer.
ArcGIS Pro for data preparation, spatial analysis, and visualization.
To validate the visibility claims from the summit of Mount Constitution, a systematic GIS-based workflow was employed:
Using the USGS Earth Explorer website, high-resolution 1 arc-second Digital Elevation Model (DEM) data from the Shuttle Radar Topography Mission (SRTM) was downloaded. This dataset, released in 2014, provides consistent elevation data with a 30-meter resolution across both the United States and Canada, ensuring uniform coverage for the region of interest.
Multiple DEM files covering the northwestern United States and southwestern British Columbia were combined into a single seamless raster using ArcGIS’s Mosaic to New Raster tool.
During the mosaic process, the pixel bit depth was carefully configured to match the 16-bit depth of the original SRTM files. This preserved elevation detail and ensured consistent symbology across the entire region.
A point feature layer was created to represent the observer’s location at the center of Mount Constitution’s lookout tower.
The observer height was set at 15 meters, derived from the 45-foot height of the observation platform (with a slight overestimation in favor of the advertised claims).
The Viewshed tool in ArcGIS Pro was used with the geodesic model enabled to account for Earth’s curvature.
A field called OFFSETA was added to the observer point layer to capture the eye-level height for the analysis.
Default tool parameters were applied for other settings, and obstructions such as the lookout tower’s shack were excluded to further favor the advertiser’s claims.
Despite some limitations, the marketing claims of the San Juan Islands Visitors Bureau regarding the views from Mount Constitution are largely accurate, highlighted by the following insights:
There's a maximum of 65 miles of visibility before terrain is obscured behind the Earth's curvature. This places landmarks such as the town of Bellingham (17 miles away) and Mount Baker (47 miles away) well within the visible area. The Vancouver skyline (42 miles away) significantly shorter than Mount Baker, would also likely be discernible in favorable weather conditions.
While broad views of the San Juan Islands are available, substantial portions of the islands, including most of Orcas Island, are obscured by Mount Constitution’s own terrain. Notable obstructions include:
After completing the analysis, I made a trip to visit Mount Constitution to verify my results first-hand. While some of the predicted highlights were indeed visible, I discovered two limitations that could be remedied in a second iteration of this project.
An elevation based sightline only takes the physical terrain into account. Tree cover and other foliage are not considered. Tall evergreens cover a majority of the mountain, including areas close by my observation point, and block a significant portion of the view. Including Landsat data would identify the treeline on the summit and allow me to remove those areas from the final product, creating a more realistic depiction of visibility.
While I visited Mount Constitution on a beautiful sunny day, the Pacific Northwest is known for its inclement weather. Rain, low lying fog, cloud cover, and general haze will drastically limit visibility from the summit. Identifying typical weather patterns and incorporating average conditions would provide additional insights for visitors and stakeholders alike.
