Switchback Mountain Survey

Switchback Mountain Survey

Oct 13, 2024

Eric, Branden, Daphne

Results: 8321.1 ft +/-0.1ft NGVD29

Exact same height as Mt St Helens to nearest inch

Now that I’m done surveying the five icecap or former icecap summits in WA, I’m focused on finishing an updated list of the WA Top 100 peaks. In the past there were huge gaps of Lidar coverage in Washington, and I’ve conducted over 25 ground surveys in these areas to get measurements for peaks on the list and contender peaks that might qualify for the list. My goal has been for each of these peaks to have a summit elevation measured by Lidar or differential GPS unit. I ‘d also like to get key cols measured with dGPS for peaks with prominence close to the 400ft cutoff.

In late summer 2024 the last of the Lidar datasets from 2022 was finally published online. This appeared to finish the project, with all peaks covered by Lidar or dGPS survey. But, interestingly, the new Lidar data resulted in a three-way tie for number 100! Lidar is only accurate to the nearest foot or so, and East Fury, St Helens, and Switchback Mtn were all measured at 8321ft when rounded to the nearest foot. I’d previously taken a dGPS survey of St Helens in 2023, but I knew that elevation could easily change this year. I’d found St Helens loses on average 0.3ft of elevation per year due to erosion. (The rate is surprisingly linear, with an R^2 value 0.96 for six data points between 1989 and 2023).

My next project was to settle this tie with dGPS measurements for each peak. A dGPS measurement could get the elevation down to the nearest inch, with ought to be able to settle the tie.

In late September I re-surveyed St Helens, and got an elevation unchanged from 2023 at 8321.1ft +/-0.1ft. This was interesting. Maybe the erosion rate is slowing down. (Though erosion down the steep north face is of course still happening, as I listened to for two hours as I took the measurement).

In early October I measured East Fury at 8321.5ft +/-0.1ft. East Fury is a former icecap summit and I determined the icecap has melted down about 30ft since 1980. It is now a rock summit.

Based on Lidar data, Switchback was 8320.9ft. But Lidar usually underestimates pointy peaks since the sample from the plane often doesn’t hit the true summit. Lidar also has a reported accuracy of +/-0.4ft in flat terrain, and higher in non-flat terrain. So it was still too close to call if it was higher or lower than East Fury.

Recently Trimble has let me use a DA2 unit, which is a more modern differential GPS. This is much lighter than the promark 220 I’m using, which is around 15 years old. The DA2 uses bluetooth so my phone can act as the data collection device instead of a dedicated unit.

I’d only tested the DA2 a few times in town, so I still felt best bringing both units along to be certain I’d get good data. I knew there was predicted aurora activity over the weekend, and this can have a bad affect on GPS data. But I decided to give the measurements a shot anyways in case the solar activity wasn’t too strong.

I would plan to mount both units next to each other on the summit. I’ve previously climbed Switchback a bunch of times and remembered bouldery terrain on the top. This sometimes poses challenges mounting my mini prism tripod so the antenna rod touches the exact summit. So I decided to bring my big Adirpro tripod with 6ft extendable legs. This way the tripod could get completely around the boulder summit so I could mount the antenna rod exactly where I wanted. I would bring my 2.0m antenna rod since this gives a better view of satellites near the horizon. With three people hauling gear and a shortish approach (at least, short compared to East Fury approach), it was an easy call to bring the extra equipment.

I would also bring my mini prsim tripod and 1ft antenna rod for the second GPS unit since I don’t have two big tripod setups.

Branden, Daphne and I left town early Sunday morning and drove to the Foggy Dew trailhead. We started hiking around 8am and made good time up the trail. The fall foliage was peaking, with great larch colors in the valleys below.

We hiked up Merchant Basin and reached the summit after a few hours and 7 miles hiking. I mounted the promark on the big antenna with the 2m rod and the DA2 on the mini tripod next to it.

We hung out for an hour admiring the view and logging data, and then as we were packing up a team of hikers that I happened to know reached the summit. We were happy to exchange nerd clusters, about the most sugary snack I’ve ever heard of that everyone appreciated.

The descent was quick, and we made it back to Seattle at a reasonable hour.

Back in town after 24 hours OPUS was giving me errors for both GPS data sets that nearby CORS sites had high noise in the data. This appeared to be related to the aurora. The general wisdom from other surveyors in online forums was to wait longer than the standard 24 hours and try again. I processed with CRS-PPP, a tool from the Canadian Government, and this gave me results but with too high errors to break the tie. In general PPP is not as accurate as the results from OPUS, which rely on nearby base stations for corrections.

After 48 hours I got a clean result from OPUS for the promark, though not for the DA2. I suspect the isssue may be that I mounted the DA2 underneath the large tripod, and this partially obstructed the satellite view. That, coupled with the solar activity, likely added too much noise to the DA2 measurement to process with OPUS.

So I am reporting the promark 220 OPUS results, which have low errors and low noise from nearby CORS sites.

The result is that Switchback Mountain is the exact same height as St Helens to the nearest inch at 8321.1ft +/-0.1ft NGVD29.

This means the three-way tie for number 100 is now broken (sort of). St Helens and Switchback are tied for number 101 and East Fury is number 100 at 8321.5ft. The Lidar/dGPS WA Top 100 list is now complete, meaning all peak elevations and prominences have been measured with Lidar or dGPS, and contender peaks have also been measured.

I will make a separate post detailing the new list and the differences between the Quad-based WA Top 100 list and the Lidar/dGPS-based WA Top 100 list.

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