Big Craggy Double Summit Survey

Big Craggy Double Summit Survey

Eric Gilbertson

October 22, 2024

Results

West Summit 0.2ft  taller than East Summit
Big Craggy West Summit – 8473.3 ft +/-0.1ft NGVD29
Big Craggy East Summit – 8473.1 ft +/-0.1ft NGVD29

In mid October I finished my last dGPS survey to determine exactly which peaks are on the Lidar/dGP Washington Top 100 list. There only remained one survey left to do, and that was to resolve the last double-summit on the list. The goal of my project is to determine exactly which 100 peaks are the highest in Washington and also where the true summit of each peak is. (Note: this list follows the traditional rule that peaks must have 400ft of prominence to qualify).

There are surprisingly quite a few peaks on the Top 100 list that have multiple locations of similar height in the summit area. Up until now I’d resolved all of these double summit cases with an Abney level, theodolite, Lidar data, or dGPS measurement. These double summit peaks are Cardinal, Katsuk, Sherpa, Enchantment, Greenwood, Buckner, Buck, and Copper. In each of these cases two highpoints lie within 1000ft or so horizontal and it is not obvious to the naked eye which is taller. They are generally within a few ft to a few inches difference in height. In most cases one summit is significantly more difficult than the other. Thus, it is imporant to peakbaggers to know which location is the true summit.

Big Craggy has two summits separated by 420ft horizontal distance, but the terrain in between them is easy class 2 walking. I suspect most peakbaggers in the past have just tagged both to be sure. But, for completeness of the project, I still wanted to survey for certain which one is the true highpoint.

These peaks are covered by Lidar data, but were measured within a few inches of the same height. This is well within the error bounds of Lidar data in mountainous terrain. Thus, based on all existing measurements, it was unclear which peak was higher.

I decided to do a dGPS survey using two dGPS units to increase confidence in results. I usually get errors down to 0.1ft with one-hour measurements. So as long as the peaks were 0.2ft or more different in heights, I should be able to resolve the difference without having overlapping error bounds. I would bring the Promark 220 from Seattle University and the new DA2 from Trimble. This would serve as a good test to compare the accuracy of the devices. I would measure each summit with each device (for four total measurements). This ought to increase confidence in results.

I knew the summit rocks could be difficult to mount my mini prism tripods over to measure the exact highest point. So I planned to haul up two large 2m tripods with 2m antenna rods. With these setups I could likely get the tripod legs to spread around any summit rocks so the tip of the antenna rod would exactly touch the highest point of rock. The tradeoff, though, is these tripods are much heavier than the mini versions. Luckily Big Craggy is not too far a hike in, though, so I was ok hauling in the extra gear.

If the highpoints were within less than 0.2ft of height, however, I’d have to try more heroic means of measuring to resolve the true highpoint (like maybe take 4 hr measurements instead of 1 hr, or haul up my 20 arcsecond theodolite).

This time of year in October is nearing the end of mountain survey season in Washington, which is marked when deep snows blanket the summits. I’d intentionally saved Big Craggy as my last peak to survey, since it is located east of the crest in one of the driest mountain areas of the state.

The weekend was bad weather but Tuesday looked sunny. I decided to do a super early survey pre-dawn Tuesday so I could work the rest of the day.

I drove out east Monday evening after work and camped at the Copper Glance trailhead. I was moving at 4:30am Saturday.

I followed the trail to around 6,000ft, then cut into the woods and followed the standard SE ridge route up Big Craggy. I’d already climbed Big Craggy many times before so was very familiar with the route.

I topped out at sunrise as planned, and soon mounted the Promark on the big tripod on the east summit. I was very happy to have lugged up the extra weight, since I was able to mount the tip of the antenna rod exactly on the highest point of rock, as verified with the abney level.

I took out my 10 arcminute 5x Abney level and pointed it at the west summit. I measured the West summit was within 10 arcminutes angular difference, which was within the resolution of the device. This meant, at that distance, the peaks were less than 1ft height difference, as I expected from the Lidar data. So, the Abney level was not sufficient to tell which was taller.

I then brought my other tripod to the west summit. I mounted the DA2 so the antenna rod again exactly touched the highest rock. The DA2 uses my phone as the computer of the device to save weight. So I let my phone sit logging data. I then took a bunch of pictures with my backup phone.

I returned to the East summit after an hour, then packed up the equipment and brought it over to the west. I swapped tripods then, so the Promark started logging data on the West summit and the DA2 started logging data on the East summit. Clouds started rolling in then, but I still had excellent views of freshly snow-covered mountains to the west, and golden larches down in the Copper Glance Creek valley below. Luckily for me Big Craggy was not snow covered.

As I was waiting around another climber appeared on the summit and we chatted a bit. He continued on to West Craggy. I had considered also measuring West Craggy, since it would be kind of nice to eventually get a dGPS measurement for each of the 100 highest peaks. But that’s a much bigger project and I didn’t really have time for the extra measurement that day unfortunately.

I soon packed up my gear at the end of the measurements and headed down. I made good progress back to the truck, and was able to get back to internet in Winthrop in time to get my work in for the day.

The next day I processed all the data with OPUS. The Promark gave East summit 8473.1ft +/-0.1ft and the West summit 8473.3ft +/-0.1ft. The DA2 gave East summit 8473.2ft +/-0.1ft and the West summit 8473.4ft +/-0.2ft. So each device measured the West summit 0.2ft taller than the East.

If all measurements are included, this means the range of measurements are East summit is between 8473.0ft – 8473.3ft. The West Summit is between 8473.2ft – 8473.6ft.

The error bars slightly overlap if the DA2 measurements are included. However, not all measurements are equally accurate, so it’s not really fair to lump them all together. If only the measurements of the Promark are used, which have the lowest error, then the error bars meet but do not overlap. These are the 95% confidence intervals, so I would be comfortable concluding with high probability that the west summit is higher than the east.

I would report the final results as those of the Promark. This means the West summit is 0.2ft taller than the east summit. So the elevation of Big Craggy is 8473.3ft +/-0.1ft.

 

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