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General Field Methods
When working in difficult conditions, field operators must pay careful
attention to the receiver tracking. Most receivers will provide some
audible feedback to the operator, with different sounds signifying
different events. Most receivers will play a sound (a beep) when there is a
successful position fix – when the receiver is silent, no data is being
collected. Others are silent when data is being successfully captured, and
sound a warning when no data is captured. Field operators may prefer
one or the other, but it is essential that the field operator know when
data is being successfully collected.
Static Point Features
Under heavy forest canopy, static point features can experience quite a
bit of error. This is because the signals from the satellites are affected by
the tree trunks, branches, and foliage. Static points are particularly
susceptible to large errors because the relationship (geometry) between
the satellites being used, the GPS antenna, and the trees and other
objects affecting the signal does not change very fast. On line features,
that relationship is constantly changing as the antenna moves and the
errors are much easier to deal with
For point features, the best way to increase accuracy is to stay longer.
Over time, the errors will average out and the resulting point will be more
accurate. However, recent testing indicates that point occupation times
must be 5 minutes or more for the errors to effectively average out.
Another way to increase point accuracy may be to change certain receiver
settings. If the receiver allows users to select a Dilution of Precision
(DOP) cut-off value, this might be lowered. Some receivers have other
settings such as maximum accuracy or Signal-Noise Ratio (SNR)
Naturally, however, there is a trade-off between accuracy and
productivity. Any of the above measures will certainly decrease the
productivity, to the point where it is not possible to work around the
trees. Obviously, taking longer at points is going to mean that the survey
takes longer. Changing receiver settings often means that the receiver
does not get signals at all under forest canopy.
One method, which may work, for increasing point accuracy is to move
the antenna around during the occupation. This seems counter-intuitive
since the idea of a static occupation is that the antenna is not moving.
Some movement may help to change that geometry between the
satellites, antenna, and trees and foliage. Very limited testing has been
inconclusive and more testing is planned. Until then, it cannot hurt to
move the antenna around a bit within reason and considering the
accuracy specification - 1 metre or so should not cause problems, and it
may help. Regardless, in difficult conditions, it is often necessary to
move the antenna somewhat as signals are lost and re-acquired.
There are some points for which accuracy is very important. Usually
these will be points which have no associated line (i.e. are not points
along a dynamic line survey), and that have some special significance.
For example, legal survey monuments (property corners) or Points of
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Commencement for a supplementary traverse. In these cases, the
occupation time should be increased, or perhaps a more open area found
and a point offset performed.
Dynamic Line Features
Line features tend to show random errors in the individual position fixes
making up the line. Because of this, it is essential to collect much more
data than would be strictly necessary to define the line’s location and
shape. When working in marginal conditions, it is very important to
collect as many positions as possible. In difficult conditions, field
operators should log data as fast as possible (usually one second), even if
that means that fixes are less than a metre apart.
After differential correction, a mapping/GIS operator will generate a
smooth line (interpreted line) using the position fixes as a guide. This
line is usually drawn over top of the GPS data, rather than selectively
deleting position fixes deemed to be in error (a very time-consuming
process). The fundamental rule for creating this interpreted line is that
more data is better, as it provides the operator with more information to
base decisions on. More data will ensure a more reliable and accurate
final line, and it does not require any more time at the mapping stage.
The biggest problem with surveying dynamic lines in difficult conditions
is that gaps in satellite coverage mean gaps in the line. Field operators
must pay very special attention to these gaps and ensure that they do not
become too large. Sometimes it is necessary to walk back and forth over
a certain section to ensure there are enough position fixes that the line
can be defined at the mapping stage. Moving the antenna across the line
(left and right of the stream centreline) often helps. Since the receiver
has 1 metre accuracy at the best of times, a metre left or right of the
centreline will not degrade the accuracy of the final product at all.
For dynamic line features, it is a good idea for the field operator to
visualize the map being made. Each successful position fix can be
thought of as a dot, and field operators can keep a virtual map in their
minds as they are surveying in difficult locations. Field operators should
think about the gaps in the line, and should ensure that there are enough
fixes to define bends and meanders in the stream.
5.7 Planning and GPS Processing
GPS receivers almost always come with software, which performs several GPS-
specific tasks. Usually, these tasks include: pre-mission planning, data and
project management, receiver download, reference station search and download,
differential correction, and exporting to GIS or mapping software (such as ESRI’s
In some situations, it is possible to obtain real-time corrections, that is,
observations are corrected in the field and the positions stored in the GPS
receiver are corrected before downloading to the computer. Real-time
corrections have certain advantages and limitations and there are many factors
to be considered before relying on real-time surveys.
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This section considers some of the functions typical of planning and processing
of GPS products. Where appropriate, practical advice is included.
5.7.1 Pre-Mission Planning
Mission planning shows the optimum times to do field work and shows
when it will not be productive to use GPS (especially under forest canopy).
This is fairly simple to compute from geometry (knowing the user’s
position, the satellite locations, and the proposed time).
However, users must remember that mission planning is only a
theoretical tool and does not predict real-world survey conditions. Forest
cover, local terrain, or satellite outages all mean that coverage predicted
by mission planning software is optimistic at best.
Mission planning software requires three inputs: the user’s location, the
intended date and time of the survey, and a current almanac.
Interpreting the results requires common sense and experience.
The user’s location can easily be obtained from topographic maps, or
even road maps. The user location only needs to be accurate to a few
hundred kilometres (about 2 degrees of latitude or longitude in BC). For
surveys on the ground, the user’s elevation is not important.
It is important that the local time offset from UTC (the time zone) is
accounted for in the mission planning. Most mission planning programs
assume that planning is being done on UTC (six to eight hours ahead of
local BC time). There will be a provision in the software to allow users to
specify a local time zone.
GPS satellites work on a sidereal day, rather than a solar day.
Consequently, mission planning times are based on a sidereal day.
Although the time will be correct within a few minutes for the specific
day, the times of concern (high DoP, few satellites) will advance by four
minutes per day, or approximately 30 minutes per week. If a mission
plan is done for a Wednesday, times will be 8 minutes earlier on the
Monday and 8 minutes later on the Friday. After one week (a 28 minute
difference), it is probably wise to generate a new mission plan.
The operators of the GPS satellites often take satellites off-line for
maintenance or to change their orbits. Periodically, satellites will be
decommissioned and new ones will be launched and made available.
Maintenance usually affects one or two satellites per week, so it is
important that users consider scheduled maintenance in the mission
Users should ensure that they have a current almanac, which describes
the orbits and status of the satellites. For future planning (for example if
a satellite is scheduled for maintenance), users should also know about
forecast outages. For current almanacs and for information about the
status of GPS satellites, the official website for GPS information should be
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Often manufacturers and/or vendors will make this information available
on their own web sites (including perhaps almanac files in proprietary
When working under heavy forest cover or in steep terrain, mission
planning becomes more important than ever. Section 3.6.4. provides
some practical advice for mission planning for work under difficult
5.7.2 GPS Reference Stations
GPS surveys done for mapping purposes use a technique called
differential GPS. Observations made at a known reference station are
used to correct some errors, which will be noticed at both stations.
Without differential correction, the expected accuracy of GPS positions is
about 20 metres.
Normally, GPS mapping surveys are done as post-processed surveys.
During the survey, GPS data is stored on the field receiver, and on a
reference receiver. After the survey is done, data from both receivers is
downloaded to a computer and the positions are differentially corrected
after the survey.
Another method of differential correction is real-time (RT) differential
GPS. Instead of storing the corrections at the reference station, they are
broadcast as soon as possible to users in a local area. If the users have
the appropriate equipment, their GPS receivers can correct positions in
real-time and display and store corrected positions in the field, rather
than waiting until later to get corrected positions.
Post-Processing Reference Data
There are a number of reference stations in or near to British Columbia
which provide reference station data for correcting GPS surveys. Typically
these stations are available through Internet access (ftp or web-based).
Most manufacturers provide some form of automatic Internet search and
download in their processing software and in many cases, users are given
a choice of reference stations appropriate for stream mapping.
In British Columbia, there are two main providers of post-processing
reference data: Terrapro GPS Surveys Ltd., and Geographic Data BC (an
agency of the provincial government). Other private companies have
reference stations, but do not provide public access to the data.
Both GDBC and Terrapro provide reference data on a subscription (user-
pay) basis, usually based on an hourly, monthly, or yearly agreement. For
stream mapping purposes, there is no practical difference in the quality
of the data so users should make decisions based mostly on convenience
or local concerns.
There are also a number of reference stations in British Columbia,
operated by the Geological Survey of Canada for which access is free
although not always convenient. In the US, government agencies such as
the National Geodetic Survey, Coast Guard and the Department of
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Transportation have established a network of reference stations known as
CORS (Continuously Operating Reference Stations). Access to this data is
free and there are several stations appropriate for stream surveying in
Terrapro GPS Surveys
Coast Guard Beacons
Geographic Data BC
User-Pay Reference Stations
Free-Access Reference Stations
Other Private Stations (no public access)
Other Public Reference Stations
Figure 5.17 GPS reference stations in and around British Columbia, Canada.
ɷ Geographic Data BC (subscription based):
ɷ Terrapro GPS Surveys Ltd (subscription based):
ɷ Geological Survey of Canada: http://www.nrcan.gc.ca/gsc
ɷ US CORS system: http://www.ngs.noaa.gov/CORS
Real-Time GPS Surveying
In the field, there are often advantages to having real-time corrections
available. If operators must find features which are very difficult to
identify visually (e.g. buried treasure), then real-time GPS may be the only
way to find them. Depending on the receiver, users may be able to
review their data in the field as a limited quality control (QC) tool. Some
systems allow users to upload spatial data (e.g. orthophotos or existing
stream data) and display it as a background to the field data capture –
these systems usually require a real-time GPS system.
In the office, the benefits are marginal at best. Having the data corrected
in real-time may save some time in the processing stage, but with most
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