[Qgis-user] Tablet vs GPS accuracy

[Michael.Dodd]

I agree with the points Richard has made here.  I mentioned in an earlier message on this topic that I'd done a lot of testing with smartphones vs consumer gps  see
http://www.open.ac.uk/blogs/otih/?p=466 for details on some of the tests.

Also tested consumer grade gps and a range of other surveying techniques with results and discussion shown here:
http://oro.open.ac.uk/30066/
________________________________________
From: Richard McDonnell [richard.mcdonnell at opw.ie]
Sent: Thursday, April 03, 2014 4:09 PM
To: Leo Kris Palao
Cc: QGIS Mailing List
Subject: Re: [Qgis-user] Tablet vs GPS accuracy

Hi,
It all boils down to the level of accuracy your job requires, Commercial or Survey Grade GNSS (Global Navigation Satellite System) now utilises NRTK (Network Real Time Kinematic) which can produce sub centimetre accuracies. We have found that the accuracies is often too good, with our surveys conflicting with Maps. We have seen +-40mm accuracies in the field, where a stake in the ground is surveyed and the kit can take us right back to the point days later, to within 40mm. it is worth noting that Survey grade GPS has a stated accuracy of sub meter without NRTK.
The next step down in accuracies would be GPS enabled Field Computers, again a dedicated kit but hand held form factor, the Trimble Geo-Explorer 7 is an example of one of these. They now also have the ability to utilise NRTK, bringing there accuracies to within 0.5m (or better).
The issue with phones and tablets is that, unlike dedicated systems, you have less control over how the coordinates are recorded. They don't take into account things like multipath (GPS signal bouncing off buildings, Ground etc.)  The number of satellites (the more the better) Most survey kits wont record a point with less than 5 satellites.
There is a way to get an idea of the accuracy for your proposed device. Take several readings over a day (7 or more) of 2 fixed points approx 50m apart,  every day, for a week. Then compare the results. you will have a spread of points, none of which are likely to be perfect, but will give you an idea of the accuracies of the device, the spread of the points being your expected error. A shorter test over a day would yield similar results but over a week, you would have a more comprehensive result. I have seen some tests, where accuracies with an i-Phone were stated as +-3m.
One other thing you should all note when using a phone or similar device to record your location, Use the native Latitude and Longitude to record the position, don't use the device to transform the coordinates to some other CRS. Use a dedicated CRS Transformation tool on a desktop/Laptop computer to convert to your desired CRS. Handhelds are not suitable for that purpose.
I hope this helps!!
Regards,

Richard.

On 03/04/2014 12:30, Leo Kris Palao wrote:

Hi QGIS Users,

I would like to inquire if there are some user that already assessed the positioning (location) accuracy of tablet vs commercial gps. We would like to use qgis android for fieldwork. We would also like to try qmap.

Thanks,
Leo



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