Mt Davis, PA State Highpoint Survey

Mount Davis, PA State Highpoint Survey

Sept 9, 2024, Eric Gilbertson

Results:
Mt Davis traditional summit is still the state highpoint

Traditional summit: 3213.3 ft +/- 0.2 ft
Davis North: 3210.0 ft +/- 0.1 ft
Davis South: 3209.8 ft +/- 0.1 ft

(NGVD29 datum, same as the quad uses)

The location of the highest point in Pennsylvania was recently called into question when data from 2019 Lidar measurements were analyzed. The state highpoint has traditionally been considered Mt Davis, based on the USGS quads, with an elevation of 3213 ft (NGVD29 datum – I will be using this datum throughout the report to be consistent with the quads). This elevation was measured with traditional theodolite-based surveys.

More recently, the USGS has been surveying using a new technique, Lidar. This involves a plane flying over an area and bouncing signals off the ground. The plane’s location is known very accurately with GPS, and by measuring the time for the signal to bounce off the ground the ground elevation can be measured. Measurements are taken approximately every 3-6ft horizontal spacing.

Lidar has a nominal accuracy of +/-0.4ft in flat open ground for points sampled. However, dense tree cover can affect accuracy, and the tops of pointy boulders are not always directly sampled. Thus, errors can be higher in these situations.

Lidar measurements were conducted for the Mt Davis area in 2006 and in 2019. Based on the 2019 Lidar data, it appeared there were two locations near the traditional Mt Davis summit that were higher than Mt Davis. I will refer to these unnamed points as Davis North and Davis South.

I downloaded this data from the National Map Downloader site

(https://apps.nationalmap.gov/downloader/) and analyzed the point cloud laz file using QGIS, a free surveying software package. The data showed the summit boulder was 3212.8ft, the northern candidate was 3213.9ft, and the southern candidate was 3213.8ft. This meant there was a chance either the northern or southern candidates were the true highpoint of Pennsylvania.

However, I was skeptical of this conclusion. I expected the measurement of the summit boulder was an underestimate, as is usually the case with Lidar measurements of a pointy summit. The boulder is only a few feet wide at the top, so it was very unlikely the Lidar measurment sampled the exact pointy top. Most likely it surveyed a lower point on the boulder.

The northern and southern candidates were in dense brush, according to reports from fellow peakbaggers who bushwhacked to them. It’s possible to filter out points in QGIS based on ground returns or non-ground returns. However, I’m not aware of systematic studies confirming the accuracy of this filtering. If any points were miss-filtered, it’s possible for the elevation to be elevation of vegetation and not ground. Thus I expected these two candidates had been overestimated by Lidar.

With all three locations within 1ft of each other based on Lidar, it appeared to be too close to call which was actually the highpoint. The only way to know for sure in this situation would be to be able to measure each location more accurately, ideally to the nearest inch. This would mean bringing a survey-grade GPS unit to each summit.

This type of unit is much more accurate than handheld consumer-grade GPS units, like those found in a phone. Consumer-grade units can have very high vertical errors, up to +/-50ft, as a result of effects like atmospheric distortion, multipath errors, and a limited number of available satellites. Survey-grade GPS units, called differential GPS units (dGPS), can get errors down to +/-0.1ft or better. They generally have access to more satellites, have external antennas to help with multipath errors, and are capable of correcting for atmospheric distortions using base stations located around the state. Data usually needs to be collected over a long period of time (like ~1 hour or more), then post processed at least 24 hours later. Nowadays the US government provides a publicly-available software tool, OPUS (Online Positining User Service) to process the data.

I have access to one of these units, so I decided to settle the controversy and figure out the true highpoint location of Pennsylvania.

I live in Seattle but was visiting family on the east coast, and I happen to have family in southwest Pennsylvania. So I decided to sneak in a survey on the way. This survey would require some additional equipment than what I was used to using when I survey peaks in Washington. I packed some loppers and a small hand saw to clear brush at the bushwhack candidates. The GPS antenna needs a clear view of the sky for the best measurement.

The summit boulder appeared to be a challenge to survey. It was likely too big to mount my large 6ft tripod around so I could balance the antenna rod on top. And it looked too pointy to balance my mini tripod on top.

So I machined custom gorilla-pod-style legs and modified my tripod to have these attached. This way it could sort of hug the top of the boulder. But when I tested at home it didn’t work as well as expected. So instead I decided I could use my 6ft tall tripod and perhaps attach sticks to the ends to extend the legs.

I landed in Pittsburgh at 9pm, and learned that TSA had decided to remove the metal tips of my tripod legs. My tripod was in checked luggage, so I don’t see why they cared. Luckily they had missed one, though, and I could still use this for my antenna rod end tip. I would have to make do with the messed up tripod legs though.

I rented a car and arrived at the highpoint parking lot shortly before midnight. My plan was to take a 6hr measurement of the summit boulder at night so nobody would disturb it. Then I would move on to the other candidates.

It took some finagling but after gorilla-taping a few sticks to the tripod legs I was able to securely balance the antenna exactly on the summit. I started logging data just after midnight. The boulder had about 2 inches of concrete on top where the USGS monument was mounted, so I noted that I’d need to subtract that amount from my measured value.

I laid out my bivy sack next to the boulder and soon went to sleep. At 5am I was woken up by the dGPS beeping that it was low on battery. Luckily I was close enough to hear. I quickly logged the data and turned off the device. I had two spare batteries for the next measurements. Then I dismantled the tripod and packed up.

I then drove the short ways north to Camp Buckey Road and found a nice pullout a bit north of the northern candidate. I put the sleeping bag out in the back of the car and went to sleep for a few more hours.

By 9am I packed back up and headed into the woods. I’d heard from Chris G. that the recommended approach for the northern candidate to minimize bushwhacking trouble was to start from the west and then head south to hit it.

I started out in easy open woods, then when I got within about 0.1 mile of the top I encountered dense rhododendron. Pushing through was difficult with my big tripod and loppers strapped to my full pack. A few times it made most sense to crawl, but I eventually reached the capstone rocks next to the twin pines described in trip reports.

There were three rock features near the boulder. The east one had some branches lopped off, then there was one to the north and one to the northwest. I knew Lidar would not be accurate enough to determine which was tallest, but I had brought a 5x 10-arcminute Nokia sight level for this purpose. The problem, though, was I couldn’t see from one rock to the other, even though they were only about 20ft apart from each other.

That’s where the loppers and hand saw came in. I spent the next 2 hours removing rhododendron and bushes enough so I could see from one boulder to the other. I measured that the northwest boulder was tallest by a few inches. Unfortunately that was the most difficult one to mount a tripod on. The highpoint was on the edge of the rock outcrop that was surrounded by 10ft dropoffs.

There was another rock outcrop nearby, so I managed to get one tripod leg fully extended touching that outcrop and the other two on the main outcrop. I couldn’t get the antenna rod tip exactly on the highpoint, but I got it nearby and used my tape measure to measure the offset I’d need to add to the final measurement.

I took a 2 hour measurement, then headed back down, reaching my car by 2pm. I then drove back to the Mt Davis parking lot, refilled my water, and started toward the southern candidate.

I walked to the boulder, then hiked down the trail to the south a short ways, then entered the trees. The bushwhacking was very dense and a different experience than before. Instead of crawling under rhododendrons I was now pushing my way through a wall of dense brush, including some thorns. It was again not easy with the big pack with things sticking out.

After about 20 minutes of thrashing I reached the appropriate coordinates and found a rock poking out of the ground. I spent another 30 minutes lopping out brush from around the rock to get a clear view of the sky, then mounted my big tripod. I got the antenna sticking up over 7ft off the ground, which really helped with acquiring satellites.

I took another 2 hour measurement, then packed up and headed out. I had enough time to visit my Aunt in Greensburg for dinner.

After 24 hours I processed the data. I usually process with OPUS, but it  was giving me error messages related to the nearby base stations for the traditional summit measurement. So I used my backup method, PPP processing (precise point positioning). This can have slightly higher errors but does not rely on base stations.

I was able to successfully process the northern and southern candidates with OPUS to get the smallest errors possible for those. My final results were

Traditional summit: 3213.3 ft +/- 0.2 ft
Davis North: 3210.0 ft +/- 0.1 ft
Davis South: 3209.8 ft +/- 0.1 ft

This means the traditional summit is still the state highpoint, and the northern and southern candidates are each about 3ft lower than the boulder.

Interestingly, this is consistent with my hypothesis about the lidar data. Lidar underestimated the boulder height and overestimated the height of the peaks in dense brush. This is good to keep in mind for Lidar measurements. They are not necessarily accurate to the nearest inch even if the precision of the data is to the nearest inch. Dense brush and non-flat terrain can lead to significantly higher Lidar errors, in this case errors close to 4ft!. The gold standard for elevation is always a dGPS ground measurement.

 

 

 

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