On 20/09/12 22:27, Peter Hoban wrote:
Discovering the accuracy of a unit is easy. Find a convenient spot near
your house and with your GPS record its position. Come back next day
(or at least a few hours later) and do it again. Repeat daily until you
are sick of it and you will then have a good idea of how accurate any
particular observation is likely to be. No technical expertise required.
The question of absolute accuracy is complex. Survey marks mostly were
placed before the current modelling of the earth was developed. While
these may now have GDA coordinates (typically about 100 mm different
from WGS in Australia) there are complexities that arise (eg from
continental drift and the instability of the earth's axis of rotation)
which are significant variables. There are many assumptions in the
modelling.
WAAS also works in Europe and Japan. There is no likelihood of it being
implemented in Australia as our population density is too low. Switch
it off. If it is left switched on there is some risk that spurious
signals from other systems may degrade the accuracy of your device.
I've been using a Garmin GPSMAP 60CSx for a number of years. Normally
for logging GPS tracks by car for use in OSM, I use it in conjunction
with an external antenna (mounted above the driver's seat so it's closer
to the centre of the road).
With a good view of the sky, this GPS unit usually claims its accuracy
to be ± 3m, with one important exception. And that's when cornering.
If I superimpose track logs from several days in JOSM, I do see
incredible consistency in the tracks. The exception is in the corners,
where there's distinct variation.
The solution is to drive around corners more slowly (where safety
considerations permit). Then the GPS seems more inclined to accept
cornering as the reason for the deviation from going straight ahead
(rather than its interpreting the change in direction as resulting from
a noisy or degraded satellite signal instead).
I've struck another situation where the GPS reports significant
uncertainty about its position. That's when bushwalking with thick tree
cover and especially with cliffs or hills to one or both sides. It's
clear to me that the signal is weak, with echoes only making the
situation worse. Here the GPS might report an accuracy in the order of
±15m, and superimposed logs confirm this variation.
John
_______________________________________________
Talk-au mailing list
[email protected]
http://lists.openstreetmap.org/listinfo/talk-au
