Another GPS check

I was missing one activity for the first excursion I'm working on for the Geology LabBook so I decided to knock it out on a recent visit to Littleton, Colorado. It was actually in the opposite direction but was near the bus terminal for the route 66 to Littleton so I figured I would take the full tour and see where the bus went, so I hiked up to the Arapahoe at Village Center light rail station to cross I-25 for a short walk to the nearby NGS survey marker.

If you want to know where you are, a survey marker is your best reference. That's, in fact, what it's there for (for more details on survey markers, see the blog for February 8, 2021, Survey Quest). All I needed was to lay my smartphone on the plaque and use the GPS tool of my Physics Toolbox Suite to get my bearings. Here's a copy of my results.

Going to the National Geodetic Survey website, I collected the data for this survey marker.

https://geodesy.noaa.gov/NGSDataExplorer/

39° 36' 25.29718"(N) 104° 53' 29.26822"(W) 

Ellip Ht. 1730.432 (meters) 6/27/12 adjusted

Epoch 2010.00

Ortho Ht. 1747.6 (meters) 5634 (feet) GPS OBS

In the US the National Geodetic Survey uses the best data available from multiple precision survey methods to determine geographic measures for sites across the country (many other countries have similar services). If you want to see how they came up with data at a particular site, the url listed above will tell you.

On April 18th, 2022 at 12:34 PM (according to the time stamp on the screenshot shown above) my smartphone accessed the transmissions from 22 satellites to triangulate (I guess with 22 satellites that would be eikosiduolate) my global coordinates and elevation. It was off by 0.14398 seconds of latitude to the south, 0.01582 seconds of longitude to the east, 5.432 meters too low in elevation. According to the program I wrote that calculates the distance between two global coordinates, my GPS was  off by 0.08 meters. 

I know...at one place I say the error is 8 centimeters and at another, 3.22 meters. Which is it?

Remember, I hiked up to the survey marker for a standard reference for local measures of position and altitude. My assumption that the survey data is exact is faulty. All measurements have error, but I feel justified in thinking that the error in the survey data is negligible. And my app being only 8 cm off makes me really happy. Of course, on another day, it might be 3 meters too high or 2 meters too low. How do I know?

The app said so. When it says the error is ±3.22 meters, it means that the true value might be anywhere within 3.22 meters to either side of the reported value.  The reported global coordinates may be as far as 3.22 meters away from the survey marker. 8 cm is well inside that area. Actually, 3.22 meters is pretty good for satellites 20,000 kilometers away.

3.22 meters is the result of the app taking several.measures and calculating the spread.

It was a nice day. I strolled back to the bus terminal and boarded 66 for Littleton. It's a pretty little town with interesting little shops like the Savory Spice Shop, which had a spectacular smoked black pepper mix, and Penzey's Spices, where I found a  Bird's Eye pepper I had been looking for. And Zoey's Place pet market had a cow ear for Vincent.

Then I caught bus 66 back up the hill (Littleton is on the river) to home.