Mt Rainier Elevation Survey

Mt Rainier Elevation Survey

Aug 27-28, 2024

Eric Gilbertson and Josh Spitzberg

Summary of Results (NGVD29 Datum):

Columbia Crest has melted 21.8 ft since 1998, revealing a new summit location on the SW Rim

Columbia Crest: 14,389.2 ft +/- 0.1 ft
(lat/lon 46.852950, -121.760572)

SW Rim: 14,399.6 ft +/- 0.1ft
(lat/lon 46.851731, -121.760396)

Introduction

Mt Rainier is the tallest peak in Washington, the most topographically prominent peak in the contiguous US, and the most heavily glaciated peak in the contiguous US. Until recently, it was one of the few peaks in the contiguous US with a permanent icecap on the summit (the others are Eldorado, Colfax, and Liberty Cap, a subpeak of Rainier, all in WA). The peak is very significant in Washington – it is easily visible from Seattle on a clear day, and its picture is even on the state license plate and the state quarter. Some people refer to it simply as “The Mountain.” [1a]

The elevation of such a significant peak is important, and surveyors have been measuring the height of Mt Rainier since the mid 1800s (see Table 1). Early measurements using barometers brought to the summit were prone to high errors, but later triangulation measurements were more accurate. The triangulation method involves pointing a theodolite at the summit from a location of known elevation and position. An angle is measured to the summit and, using the angle and trigonometry, the summit elevation can be calculated. Measurements can be taken from multiple locations, and the results averaged to give a final summit elevation.

It is important that the measurement of a peak like Rainier be taken at the appropriate time of year. For a peak with a permanent icecap on the summit, the accepted elevation is the elevation of the icecap at the lowest snow time of year. This is generally late summer, when the seasonal snow has melted off the most and autumn snows have not yet started accumulating. Measuring at this time of year ensures seasonal snow does not count towards the summit elevation.

The summit area of Mt Rainier has a crater rim that melts out to rock every summer, but there has historically been a permanent dome of ice on the west edge of the rim. This ice dome is referred to as Columbia Crest, and has historically been the highest point of the peak. Thus, the elevation of the highest point of ice on Columbia Crest in late summer has historically been considered the elevation of Mt Rainier.

The triangulation method was employed in the summer of 1914 and again in 1956 by the USGS (United States Geological Survey) to measure the elevation of Columbia Crest [1]. The 1956 survey measured the elevation 14,410ft, and this is the elevation printed on the quads (official USGS topographic maps). This is also the elevation used by Mt Rainier National Park [6]. (Note: this elevation is in the NGVD29 vertical datum. I will be reporting all elevations in this datum so they can be fairly compared. A datum is essentially what surveyors use to define mean sea level extended across land. See appendix for results in NAVD88 datum).

Triangulation can be subject to small errors in the measured angles and the distances between the theodolite and the summit. This can lead to small errors in the final result.

Starting in 1988 several surveys have been performed using GPS (Global Positioning System) technology to increase accuracy. This involves bringing a GPS unit to the summit and collecting data from many satellites to measure the summit elevation. In July 1988 surveyors from the Land Surveyors Association of Washington (LSAW) mounted a GPS on the top of Columbia Crest and measured an elevation of 14,411.1ft [1]. They also measured the SW Rim. On August 27, 1998 [1] surveyors from LSAW conducted more GPS surveys, finding essentially the same summit elevation for Columbia Crest, 14,411.0ft. On July 23, 2010 [2] LSAW surveyors returned and measured the USGS monument near the summit and the SW Rim, though they did not measure Columbia Crest.

The GPS units used in these more recent surveys are 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), 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 [3].

There was one other summit measurement taken in 2007 [4] by a a different method, LiDAR (Light Detection and Ranging). This involves a plane flying over the summit and measuring the time for a signal to bounce off the summit back to the plane. The plane’s position is known very accurately, and the time for the signal to return can be used to calculate the elevation of the summit. This measured a summit elevation of 14,405.6ft. Lidar has higher errors than using a differential GPS. Errors in flat terrain can be as low as +/-0.4ft [5], but signals can miss some features in terrain that’s not flat, so errors in mountains can be a bit higher. The gold standard for elevation measurements with the least error is differential GPS measurement.

Until now the 14,411ft measurement has been the officially accepted elevation of Rainier. (Though, Rainier National Park still uses the 14,410ft number from the 1956 survey) [6].

Starting in 2023, however, I started hearing from friends that guide climbs of Rainier for RMI (Rainier Mountaineering Institute) that it appeared that Columbia Crest was melting down significantly. They said it now didn’t even look like it was the highest point in the summit area. A rocky point on the southwest edge of the rim appeared higher. Rainier climbing guides go to the summit many times over the summer and return every summer, so they are uniquely qualified to make these observations.

I’d personally climbed Rainier four times by 2023, and this matched my observations. In August 2007 I climbed Rainier and Columbia Crest clearly looked like the highest point. However, in February 2015 I climbed again and I couldn’t tell if the SW rim or Columbia Crest was higher, so I went to both locations. I climbed again in May 2018 and May 2019, and again couldn’t tell for certain which was higher, so visited both locations.

I’ve recently built up skills in surveying peaks, and by 2023 I was equiped to actually measure the current elevation of Rainier and determine if, in fact, Columbia Crest was melting down. Since 2022 I’ve been working on a project to survey the 100 highest peaks in Washington to find up-to-date elevations. I use a 20-arcsecond mechanical theodolite, 10-arcminute 5x and 1x sight levels, and a survey-grade differential GPS unit (Spectra Promark 220 with Ashtech antenna). I’m a teaching professor in mechanical engineering at Seattle University and the Civil Engineering department allows me to borrow surveying equipment for this project. Using the differential GPS I’ve usually taken 1-hour long measurements on summits and gotten errors down to +/-0.1 ft.

I also survey peaks internationally to determine the elevations and locations of country highpoints. I’ve discovered and surveyed new country highpoints of Uzbekistan [7], Saudi Arabia [8], Togo, Guinea-Bissau, and Gambia.

2023 Survey Attempt

In the summer of 2023, while I was in Uzbekistan climbing and surveying the highpoint, I first heard about the melting down of Columbia Crest on Rainier from friends. That was a particularly warm summer in Washington, and by mid August the climbing routes on Raininer had melted out so much that all guiding companies ceased guiding. Snow bridges across crevasses melted out much more than normal. Guided groups generally put laders across crevasses on the most popular route, Disappointment Cleaver (DC), but by mid August the crevasses were too wide even for the ladders, so the ladders were pulled. This was very unusual. I’d never heard of the routes melting out so much.

I really wanted to survey the elevation, but I unfortunately didn’t get back to Seattle until early September. In a normal year climbers would still be summitting then, and it would be the perfect time to survey the elevation at the lowest snow time of year. But, according to the Rainier climbing blog (maintained by climbing rangers for the park), there was no known viable route to the summit. However, one of my friends had managed to reach the summit via the DC route a few days after the ladders got pulled. So I was hopeful maybe I could find a way following his route.

Nick and I brought my survey equipment up on September 8 to attempt the DC route. We were the only ones attempting it, and it was very icy and melted out. We managed to climb to 12,800ft, but then got stopped by a huge crevasse spanning nearly the entire face. The snow bridge my friend had used had melted out. We couldn’t find a way across, so we had to retreat. It appeared there was no safe route to the summit since the glaciers had melted down so much.

I did manage to get one measurement from that summer, though. One of my friends sent me a picture taken on July 23 from Columbia Crest looking towards the SW Rim. I used the GeoPix photographic analysis surveying software written by Edward Earl [9] to measure the relative height of the SW Rim. This involves identifying known peaks in the background of the image, then using the known locations of Columbia Crest, the SW Rim, and the background peaks. I measured the SW Rim was 8.0ft +/-0.5ft taller than Columbia Crest. The 2007 Lidar survey included the elevation of the SW Rim (which is rock so wouldn’t have changed), so I could subtract 8.0ft from that and get a measurement of Columbia Crest. This yielded a measurement of 14,390.7ft. The error would be approximately the sum of the Lidar error (+/-0.4ft) plus the photographic analysis error (+/-0.5ft). So the error would be approximately +/-0.9ft.

This measurement was consistent with the guides’ observations. Columbia Crest had melted down significantly (20ft) and the highest point had changed to be the SW rim. In order to get a result accurate enough to be accepted as official, though, I really needed to bring a differential GPS unit up and survey both Columbia Crest and the SW rim. That, unfortunately, would have to wait basically a full year until it was again late summer. I hoped there would be a safe route to the summit in late summer 2024 to try again for the measurement.

2024 Survey

I’m usually out of the country most of the summers on mountaineering expeditions, and in summer 2024 I was on an expedition to make the first SE-NW traverse of Greenland. I planned to be home by Aug 1 so I would hopefully have time to survey Rainier during late summer before the route might melt out.

However, the Greenland expedition took a bit longer than expected.  Finally, by August 22 I made it back to Seattle. Luckily, according to the Rainier climbing blog and my friends observations, the DC route was still in safe conditions. Apparently this August in Washington has been a bit cooler than normal, which is good for snow bridges staying intact.

I took a few rest days, but didn’t want to wait too long in case the route melted out more. I wanted to climb the route with the highest chance of success for reaching the summit, and that was the DC route.

Josh was in town visiting from Boston, and we decided to go for the next summit window, which appeared to be Wednesday August 28. That day the winds were forecast to calm a bit (to 20-30mph) and it was supposed to be sunny. Freezing levels had been low all the previous week and were expected to stay below the summit that day, so the route would likely be safe. My classes at Seattle University don’t start until mid September, so I was available weekdays. This date would be very similar to the date of the 1998 LSAW survey, which was Aug 27. So data comparison should be fair.

We planned to go for a two-day climb to help with acclimation. I’ve done single-push ascents of Rainier without issue, but on those trips I generally didn’t linger long on the summit. For the surveying trip we’d need to spend at least 2-3 hours up there, which I know can be a bit riskier if unacclimated. The surveying plan was to take one-hour measurements on each of Columbia Crest, the SW Rim, and the USGS monument (if I could find it). I would also take sight level measurements between Columbia Crest and the SW Rim as a backup measurement to determine the relative height.

On Monday I took the dGPS outside to take a test measurement to verify everything was in working order. I mounted it on my mini tripod and turned it on, but after an hour it still was not able to acquire satellites. I had last taken measurements with it on Mt Fuji, in Japan, and sometimes if I bring it overseas to a location far from the last location measured, it takes a long time to start collecting data.

I was worried I might not get it working in time, though. Luckily there was a backup unit at Seattle Univeristy. I drove in, picked up the other unit, then took a 30-minute test measurement outside. It worked fine and was able to acquire plenty of satellites. So finally I was good to go.

Monday evening at 7pm I reserved a timed-entry permit for the park. We drove through the entrance gate Tuesday at 11am and then at Paradise we checked in at the Wilderness Information Center. We got a permit for Camp Muir for that night, though the rangers said the shelter might be full. So we reluctantly packed a tent just in case.

We made good time hiking up, and there was a lot more snow than I remembered. In Septemer 2023 I was able to hike completely on rock and dirt to Camp Muir except for a short 20ft section of snow. This time nearly half of the ascent was snow covered. We reached the shelter after about 3 hours and luckily there was space for us inside. The ascent had been socked in the clouds with occasional snow showers. It was apparently unseasonably cold for late August, with freezing levels down to 6500ft, just above Paradise.

At 5pm a climbing ranger came in and described the conditions on the upper mountain. He’d just been up DC Monday and said it was in good shape. There were a few ladders across crevasses and they were solid. There was a guided group camped at Ingraham Flats going up, another at Camp Muir, and there were three independent teams going up plus one solo climber.

Many groups start between 10pm-1am from Camp Muir so they can tag the summit at sunrise and get down before it gets too hot and snow bridges get weaker. Our calculation was a bit different, though. We knew the wind was supposed to be decreasing over the day and it would start off unseasonably cold at sunrise but gradually warm up to near freezing at the summit. I was nervous high wind might knock the antenna over and make it very cold waiting around for the measurement. The temperature wasn’t forecast to be getting warm enough to jeopardize the safety of the route, though. So we decided to sleep in a bit.

We were up and moving by 3:45am. We traversed the Cowlitz Glacier, shortened the rope to hike over Cathedral Gap, then crossed Ingraham flats to the base of Disappointment Cleaver. We had to cross two ladders at the upper Ingraham flats, and these were no problem. We then shortened the rope again and hiked up close to each other in crampons up the cleaver. At the top we were hit by strong wind and caught up to a guided group and a few independent groups resting.

We passed them and continued on

to the upper Ingraham glacier. Luckily there were no groups immediatly above us, so we wouldn’t have to worry about bottlenecking. I’d heard that earlier in the summer teams would have to wait an hour or more at the ladder sections on the upper route, but we could avoid that.

We zig zagged up the face, crossing a snow bridge around 12,800ft at the exact place Nick and I had bailed last September. It was way easier now with that snow bridge. Past that we climbed one single ladder, then traversed right below the “wave” feature. This is a huge crevasse with an overhanging ice wall at the top that would be very challenging to ice climb. The wall drops to about 15 ft at the shortest, and here the guides/rangers have placed a double ladder.

The ladder reminded me a bit of the Khumbu Icefall on Everest, but less extreme. There they have over a dozen ladder sections, sometimes triple ladders. Above the double ladder the route wove through more crevasses until we got near the rim. A solo climber ran down, then a team of three descended.

By that elevation the wind started picking up as expected. We reached the southeast rim around 8am, took a short break on the leeward side, then crossed the crater rim to Columbia Crest. There was a bit of shelter on the southwest side, and we stopped there to prepare the survey equipment.

The summit was covered in a few inches of fresh powder from the Tuesday snow event, but the highest point was a patch of dirty ice, clearly not the fresh snow. The wind was 20-30mph and gusty, making this one of the more challenging surveys I’ve done. I quickly opened up the pelican case, screwed the antenna onto the antenna rod and mounted it on the tripod. I planted the tripod securely in the snow, connected the cable to the GPS, and started logging data.

I then ran down out of the wind to warm up a bit. When my fingers regained sensation I went back up with my sight levels and took angular inclination readings up to the SW Rim highpoint, then returned to the sheltered area. Josh took out our emergency sleeping bag and pad and crawled inside to wait out the one hour measurment. I then ran over to register rock and found the summit register. This was the first time out of my five summits I’d actually found it. Previous times it was always buried in the snow.

I signed us in, then looked around for the USGS monument at the coordinates indicated on the quad. The monument is a big metal rod that was pounded into the ground with a USGS marker on the top. The 2010 LSAW team surveyed the monument and I wanted to get another measurement there for comparison. The 2010 report showed a picture of a distinctive rock next to the monument, and I located that rock. However, there was no monument there. It appears to have gotten stolen. There was no use taking another measurement if there was no monument there, since I could not be certain I was in the exact same location as the previous measurement, so at least that saved us an extra hour on the summit.

This problem of missing USGS monuments appears to be common on Rainier. The 1998 LSAW survey team reported not being able to locate the monument. It’s unclear from their report if they installed a new one or not. The 2010 team reported the monument had been removed from the ground and vandalized, and they had to pound it back in the ground.

After the 1 hour timer ended I ventured back up to Columbia Crest, logged the data, then packed up. I then hurried over to the SW Rim highpoint. With my sight level I verified that the highest point was a large rock laying on the dirt on the north edge of the rim. I mounted the tripod directly on top of the rock, and used a few smaller rocks to stabilize the legs. The wind was even stronger there, but the tripod held solid.

I started logging data, then used my sight levels to measure angular declinations down to Columbia Crest. I also verified that the monument location was shorter than the SW Rim. The SW Rim was definitely the highest point on Mt Rainier.

To stay warm I ran back and forth between the monument, Columbia Crest, and the SW Rim a few times, then ate some snacks at the sheltered area. Running at 14,000ft+ definitely got me breathing hard. We noticed the big 6-person guided group reached the southeast rim, but they didn’t come over to the summit. I’ve heard this is common for guided trips up Rainier, and I’m kind of confused why they get so high up but don’t continue the short distance across the rim to the actual summit.

Finally the one hour clock ended, and we both went to the SW Rim and packed up. The wind never let up, and I was relieved the tripod held without blowing over. We hiked back over to the SE rim, roped up, and started down the mountain around 10:30am.

The descent went smoothly, and luckily all other groups had already gotten below the ladder sections, so there was no bottlenecking. It also hadn’t gotten too warm yet, and the fresh coating of powder snow from Tuesday stayed cold and never glopped up. There were no particularly worrisome snow bridge crossings.

We soon reached the top of Disappointment Cleaver at the same time as a guided group around 12pm. We made good time down the cleaver, then retraced our route back to Camp Muir by 1:30pm. By then the Muir snowfield was nice and soft in the afternoon sun, making for excellent boot glissading down. We reached the visitor center just in time to turn in our permit to the rangers, and were soon driving home.

Discussion

After 24 hours I processed the measurements using OPUS, then converted to NGVD29 vertical datum using NCAT [10]. I found Columbia Crest is 14,389.2 ft +/-0.1ft and the SW Rim is 14,399.6 ft +/-0.1ft. This means Columbia Crest has melted down 21.8ft since the 1998 LSAW measurement, and the SW Rim is now the true highpoint of Rainier at 14,399.6ft. These values are consistent with my sight level measurements, which were that the SW Rim is 11.0ft +/-1.3ft taller than Columbia Crest. The SW Rim measurements from 2024 are similar to but slightly shorter than the previous dGPS and Lidar measurements of that point.

I separately processed the data with PPP processing [15], giving the same results. The data was independently processed with WSRN [16] processing using VERTCON [17] to convert to NGVD29, and also got the same results.

When I got home I learned that just within the past few weeks a new Lidar dataset had been published from a measurement flight in 2022 [11] (data is often not publicized immediately, and this one hadn’t been published when I checked a few weeks earlier). I analyzed this data using QGIS surveying software [12] and it measured Columbia Crest at 14,392.3ft and the SW Rim at 14,398.7ft. This means Columbia Crest has melted down 3.1ft since 2022.

Interestingly, the height for the SW Rim from the 2022 Lidar measurement is 0.9ft lower than I measured. This difference is exactly the height I measured with a tape measure of the summit rock above the surrounding dirt (0.9ft). This likely means the 2022 Lidar pass measured the dirt but missed the rock. This is not unusual, as Lidar data is taken only every 3-6ft horizontal spacing, and can thus miss a rock smaller than that width.

The 2007 Lidar data measured the SW Rim as 14,399.5 ft, which is within 0.1ft of the dGPS measurement. Because the dGPS measurement has an error of +/-0.1ft and the Lidar measurement has a nominal error of +/-0.4ft (in flat terrain), then these two measurements are consistent and within the error bounds of each other.

To understand how the elevation of Columbia Crest has changed over time, Table 1 shows all elevation measurements of Columbia Crest and the SW Rim from 1841 to present, including the exact date and measurement error when known. Table 1 shows that the early measurements in the 1800s were prone to high errors, but the measurements got significantly more accurate by 1914 and 1956.

Figure 1 is a plot of the elevations of Columbia Crest and the SW Rim over time for the period since the 1988 measurement, the first measurement with errors less than 1.0ft.

The plot shows how Columbia Crest has been losing elevation at an increasing rate since 1998. Before 1998 the elevation stayed relatively constant. Between 1998 and 2007 Columbia Crest lost on average about 0.7 ft per year. This rate increased to 0.9 ft/year between 2007-2022, then 1.6 ft/year between 2022-2023, then 1.5 ft/year between 2023-2024. The rate of elevation loss during the past two years is the same within the error bounds of the measurements, so it is unclear if the rate is increasing, decreasing, or staying constant.

The SW Rim has been measured between 14,401.5 ft (1988) and 14,399.6 ft (2024). It’s unclear if the drop in height was from snow melt, erosion, or some other reason. The 2024 measurement was on rock.

This plot shows that in approximately 2014 the SW Rim overtook Columbia Crest as the highest point on Mt Rainier.

To visualize the extent of melting of Columbia Crest, Figure 2 shows the Lidar point cloud data for the western crater rim from 2007. Figure 3 shows Lidar data from 2022. The upper region is Columbia Crest and the lower region is the SW rim highpoint. Pixel colors represent elevation, ranging from 14,390ft (light blue) to over 14,400ft (dark red). In the 2007 image Columbia Crest is clearly larger and taller than in the 2022 image. It is taller than the SW rim in the 2007 image and shorter in the 2022 image.

Figure 4 shows the view from June 27, 2009 looking from the SW Rim towards Columbia Crest [13]. Figure 5 shows the same view from 2024. Columbia Crest is clearly much larger and taller in 2009 vs 2024.

Conclusion

The Columbia Crest icecap that used to be the highpoint of Mt Rainier has melted down 21.8 ft since 1998. Columbia Crest is no longer highest point on Rainier, and instead the SW Rim is the highest point at 14,399.6ft.

Appendix

Note that in this report I have reported all elevations in the NGVD29 vertical datum so that historical measurements can be fairly compared to more recent measurements. This is consistent with the LSAW reports from the 1988, 1998, and 2010 surveys where all data was converted to NGVD29. A vertical datum is how surveyors define a zero elevation, essentially mean sea level extended across land. The NGVD29 datum was created in 1929. An updated datum, NAVD88, was created in 1988. Elevations from one datum cannot be directly compared to elevations in a different datum. OPUS gives raw output in NAVD88 datum and I used the NCAT tool [10] to convert to NGVD29.

For reference, the raw output from OPUS was:

Columbia Crest: orthometric height 4387.835m +/-0.028m (14,395.8ft +/-0.1ft) NAVD88 computed using Geoid18

SW Rim: orthometric height 4391.000m +/-0.032m (14,406.2ft +/-0.1ft) NAVD88 computed using Geoid18

Acknowledgements

Funding was provided by the American Alpine Club, with equipment provided by Seattle University. Katie Stanchak advised on methodology and equipment and data analysis. Lily S helped with device calibration. Baxter helped with logistics planning. Dustin W. provided summit pictures for photographic analysis. Kyle B helped with Lidar processing. Rainier climbing rangers and guides kept the DC route open so we could make it up to the summit.

References

1a. Barcott, B. . “The Mountain is Out”, April 27, 1999, Western Washington University

1. Signani, L., “The Height of Accuracy,” July 19, 2000, Point of Beginning, https://archive.ph/IVhw#selection-1113.5-1113.26 

2. Schrock, G. “Rainier – The Unforgiving Mountain,” Jan 27, 2011, The American Surveyor, https://amerisurv.com/2011/01/27/rainier-the-unforgiving-mountain/  

3. Online Positioning User Service (OPUS), NOAA, https://geodesy.noaa.gov/OPUS/

4. USGS Lidar Explorer Map https://apps.nationalmap.gov/downloader/

5. USGS, “What is Lidar and Where Can I Download It?”, https://www.usgs.gov/faqs/what-lidar-data-and-where-can-i-download-it 

6. Mount Rainier National Park, https://www.nps.gov/mora/index.htm 

7. Gilbertson, E., “Alpomish, First Ascent and New Country Highpoint,” 2024, The American Alpine Journal, https://publications.americanalpineclub.org/articles/13201216908

8. “Surprise – Al Sawda is not the highest peak in Saudi Arabia,” Anas Al-Yousef, Okaz, https://www.okaz.com.sa/last-stop/na/1667177 , Aug 29, 2018

9. Earl, E., 2017, “GeoPix Photo Analysis,” available for download at https://github.com/ericgilbertson1/PhotoAnalysis/tree/main

10. NGS Coordinate Conversion and Transformation Tool (NCAT), https://www.ngs.noaa.gov/NCAT/

11. Washington Lidar Portal https://lidarportal.dnr.wa.gov/

12. QGIS.org. QGIS Geographic Information System. QGIS Association. http://www.qgis.org

13. Rainier summit view June 27, 2009, @arhuber, https://www.youtube.com/watch?v=qDQ6jqAs2h0

14. Matamoros, F., “Taking the measure of a mountain,” The News Tribune, Nov 16, 2006

15. Precise Point Positiong processing, https://webapp.csrs-scrs.nrcan-rncan.gc.ca/geod/tools-outils/ppp.php?locale=en&_gl=1*1as3oua*_ga*NTQ2NTE3OTMwLjE3MTQ2NzgzMjU.*_ga_C2N57Y7DX5*MTcyNjA4NjU3My42LjEuMTcyNjA4NjY1OS4wLjAuMA..

16. Washington State Reference Network, http://www.wsrn.org/

17. VERTCON, North American Vertical Datum Conversion, https://www.ngs.noaa.gov/TOOLS/Vertcon/vertcon.html

 

 

 

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