Chugach 5,000' Peaks

The Chugach Mountains outside of Anchorage, Alaska, are a vast, unpopulated wilderness full of wildlife, glaciers, and more mountains than you can hike/climb in a lifetime. The portion of Chugach State Park closest to Anchorage boasts a handful of peaks above 5,000'. Specifically, the region south and west of Ship Creek and west of Indian Creek has a dozen (12) peaks that are identified on USGS topographic maps as having a summit more than 5,000' above sea level. Many people have identified these peaks as goals to summit in their hiking excursions. A now famous endurance challenge involves summiting all 12 peaks in a single outing (see: https://www.stockalpine.com/posts/chugach-front-linkup.html ). The routes (and there are several routes) have an elevation gain of ~20,000' and a distance of ~40 miles.

But are there really 12 peaks above 5,000'? Again, these are what the USGS topographic maps indicate. The maps were generated from stereo aerial photographs taken in the late 1950s and early 1960s, and were published in the early to mid 1960s.

The image above is a screenshot of the Anchorage A-7 USGS 1:63,360 topographic map (1960) that shows several of the peaks above 5,000'. It is important to note that the elevations were determined from photogrammetric estimates - not cadastral surveys. The operative work is "estimate". Few, if any, of the highest peaks in this region were actually surveyed. I have been to all of these summits (identified as 5,000'+) and do not recall seeing any survey monuments. The Bureau of Land Management's land survey website indicates no formal survey activities in the area except for section corner and witness points, and the Powerline utility easement.

How accurate are USGS elevation estimates for mountain peaks? My assessment is that they are pretty good, but not perfect. Recall that a few years ago the quite famous Mt. Marathon elevation of 3,022' was determined to be 2,974' when a surveyor decided to test the USGS topographic map elevation estimate. This is a discrepancy of 48'. Let me be clear, USGS topographic maps are the most amazing feat of mapping in the history of humanity. Every American should be in awe of this map set. Full stop. But they are not intended to be a perfect, infallible representation of the landscape. 

Can we do better than the USGS estimates? Yes. We could survey the summits. This is a costly and time consuming process. I imagine that like Mt. Marathon, perhaps someone might do it out of personal interest. What's the next best thing? Aerial or satellite systems that send pulses of energy to the ground and measure the return time. The most common systems are LIDAR (LIght Detection and Ranging) and InSAR (Synthetic Aperture Radar Interferometry). 

In 2010-2017, USGS flew the entire state of Alaska to generate a highly detailed elevation data (see: https://eros.usgs.gov/doi-remote-sensing-activities/2017/usgs/alaska-insar-elevation-data-status ). The gridded data set has 5m horizontal resolution, and from what I can tell has an estimated vertical accuracy of 1.5m. Using this data set, we can interpolate the elevation for all peaks in our area of interest. No longer are we limited to 1950s photogrammetric technology. 

So what does the InSAR data show? It shows that the 12 peaks are all indeed over 5,000' and that Mt. Williwaw is the highest and S. Suicide Peak is the lowest. But it also shows something interesting. It shows that Ptarmigan Peak is over 5,000'. Instead of 12 peaks over 5,000', the InSAR elevation data shows 13 peaks over 5,000'. 

The USGS map shows Ptarmigan Peak is 4,910'. However, when standing on the summit, it clearly is very close to the same elevation of S. Suicide Peak. Several people have reported that the GPS on their phone shows the summit to be well over 5,000', myself included. GPS is certainly more accurate in the horizontal direction than the vertical direction - but the > 5,000' GPS reading is a strong indicator that the topographic map value is far too low in elevation. 

Is the InSAR data to be trusted? An independent measure to cross reference would be nice. It turns out that we have a second data set for comparison. In 2015, the Municipality of Anchorage commissioned a LIDAR elevation mapping survey (also airborne). Unfortunately their area of study did not cover the 5,000' peaks in question; however, there are areas of overlap for many of the high peaks near Eagle River and Girdwood. A sample of peaks in the 4,000' to 5,500' range shows the 2015 LIDAR elevation data is within 3m of the 2010 InSAR elevation data. This is a very strong, independent verification of the InSAR accuracy. Even if the InSAR elevation for Ptarmigan Peak is 5m to 10m too high, it still is safely over 5,000'. 

The National Geodetic Survey has thousands of survey markers around the country that are used as control points for land surveys. In the part of the Chugach Mountains that we are discussing here, there are three monuments above 3,000' that were surveyed in the mid-1980s. One is at Near Point, one is at Rusty Point, and a third is on the McHugh Peak ridge line. Looking at the National Geodetic Survey data for these three sites and comparing it to the InSAR data, the National Geodetic Survey points are 3.8m higher in elevation than the InSAR. This may be a result of different vertical datums used by the National Geodetic Survey (NVD29) and the InSAR data (NVD88). In any event, the InSAR data is actually a little lower than the National Geodetic Survey data, confirming the 5,000' elevation of the peaks in question.

Finally, in 2021, the USGS published a new 1:24,000 topographic map series using the InSAR data (see map below). Those maps clearly show Ptarmigan Peak is over 5,000' (contours are 80'). Unfortunately they do not have Peak elevations noted like the previous maps did. 

Since the underlying data are provided, we can extract the highest grid cell value (5,052'). We expect the true peak elevations to be up to a few feet higher since the horizontal resolution of the underlying grid cells are 5x5 meters and the grid cell average value will not capture the highest elevation. This is why the nice rounded western summit of Ptarmigan Peak shows as 1 foot higher than the narrow, sharp, pointed eastern summit (true summit).

There is now overwhelming evidence that shows  Ptarmigan Peak is over 5,000' in elevation and therefore there are 13 peaks over 5,000' in this portion of the Chugach Mountains.

Postscript: On August 3, 2022, I hiked up the standard route of Ptarmigan with my drone in hand. I know, you are not supposed to have drones in the Park, but this was for science. Drones are very good at keeping a constant elevation. From the main summit, I calibrated the drone to the highest location and then flew it 360 meters to the summit immediately to the east. What I found is that the eastern summit is a 7.3 meters (24 feet) higher and is therefore the true summit of Ptarmigan Peak. In addition, my inReach and cell phone GPS both had elevations comfortably over 5,000'. For consistency, I will not edit the table or the map of the InSAR elevations since those reflect their true data source.

Google Earth File

Here is a Google Earth file that I made in GIS using the InSAR DEM and identifying peaks. It only covers the area south/west of Ship Creek and Indian Creek. I only left the peaks that have a page on the peakbagger.com site that are over 3,000' in elevation in InSAR. 

URL: http://www.61n150w.com/NamedPeaksPointsElevation.kmz