I have been rendering shaded relief topographic maps using both USGS DEM and OSM data for hiking. I had noticed that while the county boundary (along with that between Pierce and Yakima) had aligned quite well with the divide, there were other places where the boundary did not.

Furthermore, I had noticed some anomalies such as the boundary passing through some small lakes near the Cascade Crest, and in other cases, lakes whose drainage streams passed over the boundary (such was the case with a lake near Dutch Miller Gap). I had assumed that these were at best rough approximations of the boundary and at worst errors.

Within the last few months, one-meter resolution DEM data of King County was made available by the USGS at https://apps.nationalmap.gov/downloader/, which formed the basis of the adjustments I made.

I performed analysis on the new DEM data, particularly analyzing drainage flow in the vicinity of the Cascade crest in order to find a better placement for the boundary. This included finding more precise locations for peaks and saddle points. I did skip over a small area around Snoqualmie Pass, as I noticed that the manmade infrastructure showed up in the DEM data and made my analysis complicated there.

As for Dutch Miller Gap, I simply neglected to update the position of the node. However, I did find that the topographical saddle was in fact to the southeast of what is indicated by OSM currently, and the realignment of the boundary reflects this. I assume there are other saddle points and peaks that I missed.

I had inferred that USGS-provided DEM data is very well aligned owing to the observation I made that the GPS track I generated of the PCT in the area of Chinook Pass aligned *very well* with segments of the trail that were visible in a shaded-relief rendering of DEM data of the area, which is also one-meter resolution.

I would like to offer a comparison:

Before: https://imgur.com/5kib4iO

After: https://imgur.com/eYXLlZX

Do note that the change to West Fork Snoqualmie River is someone else’s change.

~Brian