*GPS accuracy at surveyor monuments * I went out and found some surveyor monuments (benchmarks) to give my GPS devices the acid test. My wife Gayle helped me find two monuments near the local golf course. The Club2 monument was an excellent brass monument on a concrete mound, the other was down in a ditch, looked damaged, and probably had been moved by mowing equipment. More on finding monuments below. These provide highly accurate positions to test your GPS device.
Here are some new accuracy values to go by, as measured on my Bad Elf GPS Pro (GPS satellites only), Windows vitalASC 2-in-1 tablet with GNSS sensor, and iPhone 6 with GNSS (but wifi and LTE receivers off). It was a clear day with a good view of the sky, and I got 10 of 13 or 14 satellites locked in on the Bad Elf. Here are the differences in meters from the actual NGS location of the Club2 monument: *Bad Elf: 2.85 m from true location* (average of 4 fixes, "±3.3 m" on the best reading according to Bad Elf app) *Windows tablet: 2.64 m* from true location (1 fix) *iPhone 6: 2.53 m* from true location (1 fix, but "±5 m" according to Compass 55 app) All in all, all three devices performed well under near ideal conditions, and got less than ±3 m error. The iPhone score may have been good luck; if I had taken 4 fixes it might have been worse. In my recent article I gave my iPhone 6 and iPad mini-2 only ±10 m accuracy, but that was based on fixes in my back yard under worse conditions than this test. But I think owners of iPhone 5S and later can be pretty confident of their GPS locations on a good day, probably ±3-5 m. Under poor conditions I think the Bad Elf will perform better than iPhone/iPad or my Windows tablet, and the Bad Elf app gives you more information to measure accuracy. I found information on surveyor monuments on my local quad at the National Geodetic Survey at https://www.ngs.noaa.gov/ Pick these menus: Data & imagery/Survey Mark Datasheets. There you can search several ways, but if you know the USGS quad you live on, search for that quad name, confirm which state it is in, and download the xml format, which will open in Excel as .xlsx. You can also start with county and so on. In the spreadsheet you can explore the data fields (columns) and find monuments that have a good or better "LAST_COND". Many are along highway ditches and get mowed over, moved, or lost. The "wkt_geom" field gives you the decimal lat/long to 17 decimal places from their GIS, and the accuracy may be within millimeters. *William R. (Bill) Elliott* *speodes...@gmail.com <speodes...@gmail.com>* 573-291-5093 cell
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