[Qgis-user] Tablet vs GPS accuracy

[Leo Kris Palao]

Hi Guys,

Thanks all for your valuable insights about my inquiry. I really appreciate
your comments. I can use your comments as reference on how we will also
evaluate GIS applications in Tablets.

Our application mainly falls on agriculture. We want to use the tablets for
surveying in Rural areas. And we want to use QGIS android so we can load
our shapefile and raster maps for validation. On the other hand, we will
also want to use tablets to take location of field corners of farmers field
to get the plot size (this is where accuracy is important).

I observe that when there is a wifi the location significantly increases in
tablets. But of course, wifi is not available in rural areas. So, if we
will use tablet we will just rely on available GPS signal, weather, and the
device.

Thank so much, and again I really appreciate the comments,
-Leo


On Fri, Apr 4, 2014 at 2:20 AM, Michael.Dodd <Michael.Dodd at open.ac.uk>wrote:

> 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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