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line. Sharp bends in the line, and mismatches where lines should join are
other indicators of significant systematic errors in line features.
Common Errors Due to Operator Interpretation
The editing and interpretation stage is perhaps the most important step
in a GPS mapping survey. It is at this stage that data quality is assessed,
and that errors are identified and corrected. If the data is not good
enough to meet project specifications, it should not make it past this
The interpretation stage is where final decisions about the quality of the
data are made. After this step, the product becomes a map which is
generated using GPS data, but does not usually contain any actual GPS
data - the edited and interpreted lines and points have no quality
information associated with them. The Standard GPS Quality Assurance
procedures described in section 8 compares the generated lines and
points on the map with the original GPS data, but these procedures would
only be applied on a portion of the entire project.
The interpretation is usually done by mapping operators with a good
knowledge base of the nature of errors in GPS positions. However, not all
operators have the required training and experience, and some may make
errors which, although they do not affect the quality of the GPS data, they
certainly affect the quality of the final map.
Because of the possibility of interpretation errors (actual problems with
the linework or not properly correcting errors in the GPS data), the QA
procedures consider both the GPS data quality and the quality of the
Common interpretation errors include:
ɷ Consistent mis-interpretation of data noise. This is not
common, but can cause significant systematic errors,
especially in area calculations. This error becomes
immediately obvious during QA as the interpreted line does
not make much sense given the underlying GPS data.
ɷ Mis-identifying errors in field procedure as boundary features.
For example, field operators may inadvertently collect GPS
data when they are wandering about looking for the beginning
of a ribboned line to be surveyed or a cruise strip to be tied
(or perhaps even looking for a good log for a bathroom
break!). To experienced operators, these field errors often
look like anomalies in the feature, but some mapping
operators will assume they are part of the feature.
ɷ Following noisy GPS data too closely. This is a common
mistake of inexperienced operators who are trying to be
conscientious, and who sometimes do not realise that natural
features are not so convoluted. This can be avoided by using
some sort of scale reference to constantly remind the operator
the distances involved - for example the 10 metre diameter
circles included in many of the examples in this section.
Although this is not strictly an error, it does make the
interpreted line look strange to most observers, and can cause
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errors in such derived quantities line length (which would
report as being longer than it really is).
ɷ Ignoring large gaps in the data. Especially in difficult
conditions, some gaps in the data will be unavoidable. If the
gaps are greater than 25 metres, mapping operators should
ensure that line features are straight before drawing straight
lines across the gaps. The temptation may be to just sweep
the gaps under the rug, however, most interpretation errors
can be minimised by having others, especially field personnel
and more experienced mapping operators, review the data
before submitting it.
5.8.5 Integrating Conventional Survey Data
There may be many instances where conventional survey data must be
integrated with GPS data. It may be that a fill-in survey (see 5.6.2.) is
required where terrain and forest cover conditions mean that GPS is very
difficult. In some cases, GPS positions might be used only to control
surveys done with compass and tight-chain. This is especially true for
stream mapping scenarios, especially where expensive GPS equipment is
not always available, and where deep-forested gullies are often
5.9 GPS Data Processing
5.9.1 Handling GPS Data Using Pathfinder Office And
This document is intended as a set of suggestions on how to most
effectively collect stream mapping data with Trimble receivers into
ArcView. It presumes that users are familiar with both software
packages, especially ArcView (since GIS software typically require much
more training than simple GPS softwares such as Pathfinder Office).
Both the features (points and lines) and the individual position fixes
should be exported. Just exporting the features, which is the default
mode of Pathfinder Office (and most other GPS packages) is not sufficient
as operators cannot identify gaps in the data, systematic errors in point
To export both features and fixes in Pathfinder Office, two separate
export operations, with two different export configurations, must be run.
1) Setting up the Export Options
To create the export setups, first use the
button to create a
new export setup with a descriptive name such as “ArcView Features”.
The following options will probably be the system default – if not, change
them so that
Export All Features
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Note: Under the
tab, the user can add such useful information as
average PDOP for features, number of fixes in the feature, or standard deviation
for point features. These will become attributes in the ArcView table, and can be
queried or displayed in ArcView.
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tab allows the user to filter out positions which do not
meet certain standards – for example if some “2-D” (less than four
satellite) data was collected in the field which needs to be filtered out
before export. Of special note here is that, with Selective Availability
(S/A) off since May 2000, it is often difficult to tell corrected from
uncorrected data. Even though uncorrected data might look good or
smooth, even to an experienced user, the positions can be out by over 10
metres for long periods. Users should always ensure that the
option is not chosen.
Filter By Precision
option is not really an indicator of data
quality, especially under forest canopy. It seems to correspond almost
exactly to the HDOP at the reference station (using 95% confidence),
tab should be used to ensure that the data
exported can be brought into ArcView in the proper location. If the GPS
data does not fit with base information such as existing maps or
orthophotos (or even other GPS data), this is almost always the reason.
In most cases, the UTM map projection (in the appropriate zone) is used.
In some instances – especially when using GIS data from the BC Ministry
of Environment, the
BC Albers Projection
must be used. This is not
included in Pathfinder Office’s list of coordinate systems. If the BC Albers
projection is necessary, use the Coordinate System Manager (
oordinate System Manager
). Create a new group if necessary
with a name such as “Custom” (
Add Coordinate System Group
and then add the BC Albers projection to the new group (
lbers Equal Area Conic…
). The Projection screen
should look like this:
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Note: that other options such as
are set up here
also. The datum transformations and geoid models provided with
Pathfinder Office are approximate only, and not accurate enough to meet
the 5 metre accuracy requirement. Unless they have been thoroughly
tested in the local area, these conversions should never be used. NAD 83
(or WGS 84) and
No Geoid Model
are the only acceptable choices here. If
other datums or orthometric heights are required, more accurate
transformations (such as available from Geomatics Canada) should be
Next, create a second setup to export the GPS position fixes only. Create
a new setup with a name such as “ArcView Fixes”. If the “ArcView
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Features” setup above is currently chosen, the system will create a copy
of that – the only change will be to rename the setup and to change the
“Type of Data To Export” from
, as shown
2) Exporting Data
To export data to ArcView, the export routine must be run twice using
the two setups created above. The setup is chosen through the list box.
The export routine will create a series of files for each feature type, and
for the fixes. There will be a shapefile *.shp, an index file *.shx, and a
database table *.dbf. These files will have a prefix relating to the name
of the feature, for example “Point_Generic” or “Stream Centreline”. The
file for the point fixes will always have the prefix “Posnpnt”.
Pathfinder Office will export these files to the folder indicated under
Output Folder. If there are files with the same name there, they will be
overwritten. This may be a problem if, for example, GPS files are
exported daily throughout a project – the last day’s data will be
continuously overwritten. Users should develop some system to manage
these files, either by renaming files, creating separate directories for each
export operation, or merging files.
Note: that, when the export routine is run the second time to obtain the
fixes, a warning message about overwriting files will usually be
generated. If the above problem is properly handled, this applies only to
an information file and the warning can be safely ignored.
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3) Importing into ArcView
Since the data is exported in many different shapefiles (one for each
feature type and one for the position fixes), they must all be loaded into
ArcView. The important thing is to ensure that the position fixes are
shown, and in a way that data quality can easily be assessed.
To import data into ArcView, use
. Multiple themes
can be chosen by using the SHIFT key and the mouse.
The features will be shown by default symbols. These should be changed
to make the display easier to understand – especially the fixes and line
features. Double-clicking on the theme in the legend will bring up a
symbol editing menu. Although users may find other symbols preferable,
the following have been found useful:
cross or "X"
Note: themes can be displayed based on their attributes as well. This can
be very useful in displaying position fixes based on, for example, PDOP
(as a quality measure). It can also be used to show individual line
features during the data interpretation phase (discussed below). The
following screen capture shows a typical survey area using the above
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4) Setting up for Data Interpretation
The data interpretation stage is the most important stage of all. This is
where the errors in the GPS data (usually due to the effects of forest
canopy) are assessed. If the data is not good enough, sections can be re-
If the data meets specifications, the errors must be smoothed to resolve
the remaining random and systematic errors. If this interpretation of the
remaining errors is not done, further display and analysis is not possible.
In the example above, canopy effects cause much back and forth in the
line features – any query such as “how many metres of riffle habitat are
there on the stream” would obviously be very far out. Also, the point
feature (circle symbol) towards the bottom of the screen has obvious
systematic errors and must be brought back to the line.
During this stage, it is essential that the operator is always aware of
distances on the screen. One way to do this is to place circles of a certain
diameter (e.g. the accuracy specification) along the line. Using circles,
the orientation of the fixes is not important, as the scale is obvious in any
direction. This technique helps ensure that operators are not
generalising too much, nor are they zooming in too much and spending
time on errors, which are much less than the accuracy specification.
Circles are placed as graphic objects, using the circle tool. This, however,
is quite cumbersome, as the circle must be placed interactively. The
following Avenue script can be used to create a tool which will place a 5
metre diameter circle (2.5 metre radius) at a location the user clicks. Feel
free to enhance the script.
Documents you may be interested
Documents you may be interested