Wallaby Peak Survey

Wallaby Peak Survey

On the summit

May 18, 2025

Eric and Hogan

Results: 7992.8ft +/-0.1ft NAVD88 (Lidar was in error by 0.9ft)

There’s currently a three-way tie for number 200 on the Washington Top 200 list based on Lidar and my previous surveys. Lidar can have errors up to a few feet on sharp peaks, but my differential GPS unit can get errors down to 0.1ft or lower. The three peaks are all within 1ft based on Lidar, so that means it’s not known which is actually on the Top 200 list (a somewhat popular list in washington, especially for Bulgers completers).

The route

I just got a new flexible-leg tripod I wanted to test out. This allows me to mount the GPS antenna on a sharp boulder instead of mounting it next to the summit and using a tape measure to measure up to the summit to correct the result afterwards. It’s also much lighter than my other 1ft antenna rod mini tripod, so good for techincal mountaineering surveying trips.

Hogan and I started up from the Washington Pass hairpin at 7:30am Sunday morning and hiked up continuous icy snow to the Kangaroo Pass, then cramponed up the southwest ridge to Wallaby. There was unfortunately a 4ft tall cornice on the summit, but we cleared it off with our ice axes and mounted the DA2 on the highest rock. We took a cold one hour measurement as we were blasted with wind and light snow showers. The partly-sunny forecast was not materializing.

Hiking up

We hiked back down, decided to bail on our second objective because of the wind and snow, then snowshoed out by mid afternoon.

I processed the data with TrimbleRTX and found an elevation 7992.8ft +/-0.1ft NAVD88 vertical datum. This is 0.9ft taller than the Lidar-derived elevation (7991.9ft). This is likely because Lidar measurements are only taken at horizontal spacing 3-6ft, so miss sharp summits. On top of that, each measurement has a nominal error +/-0.3ft. So that can add up to errors of 1-2ft typically on sharp summits.

© 2025, egilbert@alum.mit.edu. All rights reserved.