[Qgis-user] Tablet vs GPS accuracy

[Richard McDonnell]

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
>
>
>
> _______________________________________________
> Qgis-user mailing list
> Qgis-user at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/qgis-user


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