One other issue with using GPS in a tree canopy is multipath. That is the
signal you receive has been reflected and is not directly from the space
craft. This induces an error since it takes longer to get to you than it
would on the direct path. This can resut in errors on the aaverage of a
meter per nanosecond usnig any of the the GPS processing algorithms.
Multipath can typically induce errors on the oder of several 100 meters.
Thes same type thing can happen in a canyon.
The fuzziness is not induced by the USG it results because the accuracys of
the satellite position and velocity state vectors supplied to the
commercial and the timing signal available to the commercial user are not
as good as those supplied to DOD users. This can be somewhat over come by
using long term processing of multiple satllite data using at mulitple
locations, i.e. differential GPS.
For most purposes GPS is sufficient for accuracies to mark a trail.
However, in heavly wooded areas or canyons, GPS may not be very useful.
Truman
Truman Prevatt
Sarasota, FL