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I want to record the location of each grave site in a cemetery for overlaying on Google maps. My Android cell phone (HTC Thunderbolt) reports an accuracy of between 10 meters and 25 meters. I'd like to get accuracy of around 3 meters or better.

Would it increase my accuracy if I take a GPS reading from a precisely known survey marker in the cemetery first and use the difference between that reading and the actual longitude and latitude to correct the grave site readings?

What kind of accuracy could I expect to get with that method?

I estimate it would take about 3 hours to survey the cemetery. How consistent are the readings? Do I need to recheck the survey marker during that period? If so how often?

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    You'll want to read more about Differential Correction for GPS – RyanDalton Feb 4 '14 at 23:17
  • Don't trust the estimated accuracy reported by the device, I've done a large amount of benchmarking that shows this can be off substantially (using GPS Benchmark - play.google.com/store/apps/details?id=com.gpsbenchmark.android, full disclosure, my app). You might also want to try GPS Benchmark in the field with your survey marker ground truth plugged in, that way you'll instantly know how accurate your GPS while in the field. Also, before taking measurements do "Settings->Modify GPS Assist Data->Inject XTRA/Time data" to refresh your assistance data for best accuracy. – Sean Barbeau Feb 5 '14 at 2:53
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Not exactly what you are probably wanting to hear, but, you'd honestly probably be ahead, at least accuracy wise, to start at a known surveyed point and simply use relative distance measurements (example: 500 ft from survey marker 90 degrees - due east). I know some groups who have used a method such as that using a digital lazer range finder device (had built in compass as well, along with bluetooth to make recording data easy). They initially tried it using sub-meter professional GPS units, but they had too many trees in the way in their area to get the accuracy you need for grave sites even with those GPS units.

All that being said, your suggestion of using a survey point as a control would be better than nothing. If you are going to GPS each individual grave site like that though, I would make sure that you pay attention to the day you go out (minimize any bad weather or possible interference such as that) and I would make sure to not just take a single quick GPS reading, especially on the control point. One of the ways many professional GPS systems improve the accuracy of their readings is by not actually taking one reading per recorded point, but rather they may take anywhere from 10 to 30 or more readings (may take a couple of seconds to a couple of minutes depending on system and settings) and average the locations together to get a more accurate location that compensates for any momentary interference. There are also systems out there for professional GPS units/software that actually perform the same kind of correction you are talking about here, but using known regional survey control points that publish their correction measurements online, but I'm personally not aware of that really being usable with standard generic cell phones without a fair bit of specialized software... though if someone else knows of a good solution for this, please chime in.

Just my thoughts, hope they help.

  • Thanks - that helps. I'll try it out with a few sites, averaging the readings and comparing to the average survey marker readings and see what I get. If that doesn't work I'll look into buying a laser rangefinder. The goal is to get someone looking for a grave site close enough to find the marker. – George Feb 5 '14 at 0:09
  • Also, I don't know where you're from, so I don't know if this would be realistic for you or not as it would vary from community to community, but another thing you maybe could look into is maybe even checking with your local city &/or county GIS, Engineering, &/or public works offices. If your cemetery is a non-profit or run by a non-profit church or such as that, you could maybe see if the city/county would loan you some equipment. I know of some communities that partner with non-profits, mainly to get free labor for valuable community projects they couldn't do otherwise... just a thought. – John Feb 5 '14 at 15:48
  • We actually did ask Los Angeles County Public Works and also Parks and Recreation. Both said they had the equipment but would not loan it out. – George Feb 6 '14 at 21:23
  • Sorry about that, I guess that might me more of a rural community type thing... Sorry, wish I could be of more help. – John Feb 6 '14 at 22:15
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I have conducted research on this exact topic (cell phone GPS accuracy). The research is currently being reviewed by a peer reviewed journal, or I would share it with the community. I can tell you that cell phones do not have great accuracy. When we conducted our research (in different urban densities) we found that cell phone GPS accuracy had a mean relative accuracy of ~ >6 meters. We collected GPS locations from know geographic locations. Therefore, collecting GPS information from a survey point will not give you better accuracy, as the GPS in the phone is the issue. Part of our research was to compare cell phones to consumer grade GPS units (ie: Garmin units) and we found that newer Garmin GPS units (ie: Map76cs etc) would collect consistent accuracy close to 3 meters. Hope this helps.

  • Interesting - which journal, and what's the likely date of publication? – Simbamangu Feb 5 '14 at 14:30
  • It has been submitted to the CaGIS Journal. It is in the review process. My hopes are it will be finalized soon. – Ryan Garnett Feb 5 '14 at 15:31

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