Half Moon Survey

Half Moon Peak Survey

Sept 19, 2025

Eric and Nick

Results: 7992.1 ft +/-0.2ft NAVD88. Too short for WA Top 200 list by 0.7ft.

Last fall I finished a project to figure out an accurate list of the 100 highest peaks in WA using a combination of Lidar data analysis (assisted by Kyle Bretherton) and ground surveys (assisted by many other climbers). Results were published in the summer 2025 edition of the Evergreen State Surveyor put out by the Land Surveyors Association of Washington. A popular objective for climbers who have completed climbing the 100 highest peaks is to move on to the 200 highest peaks in WA. I decided to start another project to figure out the accurate list of these 200 peaks. The requirement to be on the list is a peak must have at least 400ft of prominence.

Lidar data has been made publicly available for all of the highest peaks in WA as of summer 2024. Kyle analyzed a bunch of data for a first pass at the WA Top 200 list, though a handful of edge-case peaks still existed. I’ve found based on my research that Lidar-derived summit heights for above-treeline peaks like this have error +/-1.7ft (95% confidence interval). This is based on 177 data points collected by me and Richard Hensley comparing differential GPS summit heights we measured to Lidar-derived summit heights. A scientific paper detailing this work is currently going through the peer-review process in Progress in Physical Geography.

A peak is an edge-case for inclusion if it is within +/-1.7ft of the elevation or prominence cutoff of the Lidar-derived list. For instance, a peak measured by Lidar to have 401ft of prominence is within the error bounds, since it might only have 399.3ft prominence. If peak number 201 is only 1ft shorter than peak 200 based on Lidar, they are within error bounds. Peaks with double summits within 3.4ft feet of the same elevation are also within error bounds (since each peak elevation could be in error by 1.7ft). These peaks all need to be surveyed with more accurate methods to resolve the uncertainty. Differential GNSS units can get 0.1ft accuracy and can resolve the edge cases.

There were around a dozen edge cases total, and I measured a few while working on determining the top 100 list. I measured a few edge cases this past spring once the snow melted (Shelokum, South Mirror Image, Wolftit, and a few others). A handful of edge cases still remain, and I’m hoping to finish them soon so the final WA Top 200 list can be known. One of these edge cases was Half Moon Peak.

Based on Lidar, Half Moon Peak had an elevation of 7991.5ft (NAVD88) and prominence of 242ft. The most updated Top 200 list based on my ground surveys and Lidar as of spring 2025 had Wallaby at number 200, with Lidar elevation of 7991.9ft (NAVD88) and prominence 535ft. At first glance of a list of peaks, it would appear the Half Moon had no chance of getting added to the Top 200, since even if it was higher than Wallaby it wouldn’t have enough prominence. But, Half Moon happens to be right next to Wallaby. If Half Moon was in fact taller than Wallaby, then their prominences would switch, with Half Moon getting ~535ft prominence and Wallaby getting ~242ft prominence. The Lidar-derived peak elevations were well-within the error bounds, being only 0.4ft apart. So they needed to be surveyed with dGNSS to resolve the edge case.

A Lidar elevation can easily be too high or too low, and it’s hard to predict in advance. If the summit is a sharp piece of bedrock, and the Lidar sample doesn’t hit the exact highest point, Lidar will under-measure the summit. If the peak has an artificial cairn as the highest point and Lidar happens to sample the cairn, it will over-measure the summit. Even if a summit is very flat and Lidar samples the exact summit, the number could be in error by+/-0.4ft. So it appeared to be a tossup whether Wallaby or Half Moon was on the list.

Half Moon is a technical peak, with the easiest route a 4-pitch 5.7 rock climb. It doesn’t get climbed too often, and the last report I could find was posted by geosean on cascasdeclimbers in 2019. I was hoping to climb it in early May, but snow conditions weren’t good. In Mid May another planned ascent didn’t work since it was too windy. I did manage to climb Wallaby in mid May. The summit had a big cornice, but I dug it off and was able to measure the summit with a Trimble DA2 GNSS receiver. I measured 7992.8ft +/-0.1ft. This was 0.9ft taller than the Lidar elevation, and was likely because the summit was a narrow fin of bedrock sticking up the Lidar did not sample. On that trip I took Abney level measurements from Wallaby to Half Moon, but the height difference was within the resolution of my Abney level, meaning that the peaks were within a few feet of the same height.

In mid September after I came back from summer international travels I again prioritized Half Moon. It was important to climb it before the autumn snows start, which might make the rock climbing sketchy. Nick and I left town early Friday morning and started hiking up from the WA Pass hairpin by 8am. We followed the climbers trail up towards Wallaby until the head of the valley, then hiked cross country up mostly open terrain to the base of the NW face of Half Moon. We then hiked up scree slopes to the notch below the north ridge by 10am.

Nick led all the pitches and I followed carrying the survey gear. For the first pitch we started just right of a big blocky arete in the notch and went up and left on a ramp to a slung horn anchor, about 100ft. Pitch 2 we went up and left to the skyline, then at a fixed piton went up and right using some hand jams to a flat ledge. This maxed out the rope at about 200ft and we built a gear anchor at the base of some steeper cracks. Pitch 3 we went straight up the cracks on the right side to a two-piton anchor at the edge of a ledge. We belayed on a slung boulder a bit farther along the ledge for a 90ft pitch.

Pitch 4 we walked the rope 200ft over to the big sandy ledge on the east side of the ridge to the end of the ridge below the big “echo hole” about 12ft up on the right (3rd class).

Pitch 5  we did an underclinging traverse left of the echo hole then up the exposed arete to a 2-piton anchor (100ft). From there we scrambled class 3 terrain to the summit by 1pm.

The summit was mostly flat, with one northern bit of bedrock highpoint, and a southern point with a cairn and small ammo-box register. I’d neglected to bring my Abney level, so we weren’t sure which point was highest. So we decided to measure both. I took a one-hour measurement on the north, then one-hour on the south at the highest bedrock. Nick took advantage of the flat summit to take a nap.

The summit register went back to 1968, one of the oldest I’ve seen in WA. It was in excellent shape, and I think the ammo boxes are some of the most protective registers I’ve seen. The peak seemed pretty popular in the past, with a half dozen ascents in 1984. But since then it averaged an ascent every 5 years or so. The most recent one was 2019. Hopefully this register doesn’t get stolen like I’ve heard is happening to many others this year in WA.

By 3pm we logged the data and headed down. We scrambled to the two-piton anchor, replaced the tat, and rapped down to the ledge near the echo hole. A single 60m rope would work here. We then walked over to the top of pitch 3, replaced the cord on the two-piton anchor, and did a single-rope rappel to the base of pitch 3. From there we scrambled down sandy ledges skiers left then skiers right to a hidden anchor of a slung horn and a nut. We again replaced the tat, then did a double-rope rappel to the slung horn at the top of pitch 1.

We replced the tat and did a double rope rappel straight down to the notch by 4:30pm. A single rope would also work there. We hiked back to the truck by 6pm and soon drove home.

I processed the data with TrimbleRTX. Results: North summit 7992.1ft +/-0.2ft (NAVD88), South summit with register 7991.0ft +/-0.2ft (NAVD88).

The north summit is the true summit, at 7992.1ft. This is 0.6ft taller than the Lidar elevation. Since Wallaby is 7992.8ft, this means Half Moon is acutally 0.7ft shorter than Wallaby. So Wallaby is still number 200 on the Top 200 list, and Half Moon does not have enough elevation or prominence to qualify for the list. Both peaks were under-measured by Lidar, as is typical for sharp peaks. The 95% confidence interval errors for each dGNSS measurment are +/-0.2ft. So in the worst case of errors, Half Moon could be 7992.3ft and Wallaby 7992.6ft. This would still have Wallaby 0.3ft taller than Half Moon. So Wallaby is taller than Half Moon with greater than 95% confidence. But it is really close.

I still have a few more edge cases to measure, so hopefully the WA Top 200 list will be finalized soon.

 

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