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GeoLab FAQ |
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How can I reset GeoLab's
printer when I get the "Printer selected is not valid"
message? |
When you get this message,
you must reset the printer path specifications used by
GeoLab. To do this, close GeoLab,
download
this small registry update file, unzip it, and
double-click the unzipped "GeoLabPrinter.reg" file. When
you run this file, the two registry entries that GeoLab
uses for the printer names will be cleared so that when
you next run GeoLab, the printer names will be set to the
default printer.
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How can I set up GeoLab
to use my own project directory as default directory? |
The trick here is to edit the properties
of a Window's shortcut to specify the default directory.
Such a shortcut is created in the GeoLab folder when you
install GeoLab, or you can create your own shortcut (one
for each project for example). To specify the default
directory, right-click on the shortcut and select
Properties. In the dialog that appears, click the Shortcut
tab, enter the desired directory path in the "Start in:"
edit box, and click OK. When you run GeoLab by
double-clicking on the shortcut, the default directory
will be set to the one you entered.
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GPS network simulations:
How do I generate the text records required for GPS
vectors? |
Using GeoLab you can generate a
coordinate difference observation group using the
Edit/Insert/Coordinate Obs command when you have a text
window open (ensure that you place your text cursor (the
blinking one) at the point in your text window where you
want the group inserted). You can select a number of
options in the dialog box presented to generate the type
of group you require. Once the group of records is
inserted into your text file, you can edit it using the
GeoLab Record Editor (Edit/Edit GeoLab Text Record menu
command).
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GPS network simulations:
How do I generate realistic covariance matrix data for GPS
vectors? |
There are, of course, many possible
realistic covariance matrices for GPS vectors. An example
coordinate difference observation group for GeoLab is as
follows:
3DD
DXYZ S001 S002 572.7240 -280.7980 -413.6370
CORR CT UPPR 32.50000
ELEM 1.0000000000000 -0.2032966000000 0.0045336000000
ELEM 1.0000000000000 -0.3644455000000
ELEM 1.0000000000000
ELEM 0.00056219690 0.00095490840 0.00115665900
This is from an actual network, and the standard
deviations may be realistic given that the CORR record
specifies a factor of 32.5 for the whole matrix. Normally,
the covariance matrix (or the correlation matrix and
standard deviations as used here) generated for GPS
vectors by post-processing software is usually very
optimistic and a scale factor is required to make it
realistic. Experience in processing networks of these
observations results in determining realistic scale
factors for the covariance matrices.
The standard deviations in the last ELEM record above
range from 0.5 mm to about 1.1 mm (obviously very
optimistic!). Using a standard deviation of 1.0 mm, the
corresponding variance is the square of 1.0, which is also
1.0. With the scale factor of 32.5, the resulting variance
is therefore 32.5, giving a standard deviation of about 6
mm. The only reliable way to know if this is still too
optimistic is by performing a number of network
adjustments using the actual observations.
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How do I perform EDM
baseline calibrations with GeoLab? |
An EDM (electronic distance measuring
instrument) calibration baseline usually consists of a
number of stations approximately in a straight line. The
baseline stations are at different spacing (distances
apart) so that both the EDM zero-correction and the scale
factor can be determined from a combination of distance
measurements on the baseline.
GeoLab has two auxiliary parameter types that can
represent the zero-correction and scale factor for the
calibration. These are the CONS (for the zero-correction)
and the SCAL (for the scale factor in PPM) parameter
types.
The calibration baseline is nothing but a special type of
network in which the distances between stations are
accurately known. These accurately known distances can be
entered into GeoLab as observations with the small
standard deviations. The coordinates of the baseline
stations can be entered with relatively larger standard
deviations (required to provide orientation for the
baseline network).
An example GeoLab input text file,
baseline.txt, has been prepared to illustrate in
detail how to calibrate your EDM on a baseline. Note that
you must rename this file with an IOB extension if you
wish to process it with GeoLab.
Note that GeoLab can also perform the calibration of your
EDM directly in the network that you have measured. Simply
assign the CONS and SCAL parameters to the EDM distance
measurements, and GeoLab will determine the corrections
when you perform the network adjustment. This may
sometimes be a better way to calibrate your instrument,
because the actual conditions in the area of the network
when you made the measurements may be quite different from
those on a calibration baseline, and the different
conditions may especially affect the scale of the distance
measurements.
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How do I perform a
resection with GeoLab? |
Many traditional computations can be
performed with GeoLab, including resections. A resection
is the determination of the coordinates of the instrument
station when the angles between three control stations are
measured. To perform a resection with GeoLab, you simply
create a GeoLab input text file with the coordinates of
the control stations defined and the angle measurements
given. GeoLab will do the rest when you process this file
with the Network/Process menu command.
Note that for the resection problem to have a solution,
the three controls points and the instrument point must
not all lie on the circumference of a circle. If you try
such a situation with GeoLab, you will receive a message
saying that the adjustment is singular (no solution).
An example GeoLab input text file,
resect.txt, has been prepared to illustrate in detail
how to perform a resection with GeoLab. Note that you must
rename this file with an IOB extension if you wish to
process it with GeoLab.
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How do I adjust a
traverse with GeoLab? |
One of the many traditional computations
that GeoLab performs is a traverse adjustment. To perform
a traverse adjustment with GeoLab, you simply create a
GeoLab input text file with the coordinates of the control
stations defined and the traverse measurements given.
GeoLab will do the rest when you process this file with
the Network/Process menu command.
A simple example GeoLab input text file,
traverse.txt, has been prepared
to illustrate how to perform a traverse adjustment with
GeoLab. Note that you must rename this file with an IOB
extension if you wish to process it with GeoLab.
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What coordinate system
does GeoLab use for adjustments? |
GeoLab performs adjustments and
simulations (pre-analyses) in a three-dimensional
ellipsoidal coordinate system. Because your measurements
(observations) are made in the spatially three-dimensional
world, this choice of a three-dimensional coordinate
system results in the most rigorous (no approximations),
accurate, and complete mathematical models (i.e. equations
for the measurements in terms of the coordinates of
stations) for the adjustment.
When adjustments are performed in a two-dimensional
coordinate system such as a map projection plane or the
surface of a reference ellipsoid, the observations have to
first be corrected so that they can be modeled in terms of
the two-dimensional coordinates. For example, a spatial
distance measurement must be 'reduced' to the surface of
the reference ellipsoid or map projection plane if those
coordinate systems are used for the adjustment. Such
reductions must be performed before the adjustment
computation begins, and they are normally made with
approximate formulae. Such approximations are usually
sufficiently accurate for most purposes, but for high
precision networks, or for networks of large extent, they
may needlessly cause distortions in the network.
If all corrections to observations are made with
sufficient accuracy, and we are not interested in adjusted
heights, there is no advantage of one adjustment system
over another. The only difference is how the height
information is incorporated: in a two-dimensional
adjustment we use the heights to reduce the measurements
to the two-dimensional coordinate system; and in a
three-dimensional adjustment the more complete and
rigorous mathematical models incorporate the height
information directly. The resulting adjusted coordinates
may be in a different coordinate system, but we can always
transform them to another coordinate system after the
adjustment is complete. If everything is done correctly,
the adjusted coordinates in either case should be the
same.
The main advantages in GeoLab using a three-dimensional
coordinate system for the adjustment are:
- All
observation types can be modeled directly without loss
of information;
- No
approximations are required.
There
are no advantages in using a two-dimensional system.
Even
though GeoLab uses the three-dimensional system to perform
the adjustment, it can still perform two-dimensional
adjustments (by fixing all heights), or even
one-dimensional adjustments (e.g. leveling networks) by
fixing all latitudes and longitudes. The three-dimensional
model is therefore more versatile than the two-dimensional
or one-dimensional models.
For details on the mathematics behind the GeoLab models,
please refer to the Geodetic Survey of Canada's Technical
Report No. 1, "Mathematical Models for use in the
Readjustment of the North American Geodetic Networks", by
Robin R. Steeves. To see a summary of these mathematical
models, have a look at our
article
on mathematical models.
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Why can't I move network
drawing (*.net) files to another directory? |
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When you draw a network with GeoLab (in a
network window) and save the drawing to a file, the file
created is a simple text file.
The content of a network drawing file is similar to the following: 2854.1
100.0
1.0
1.0
C:\geolab\baseline
The
first four lines are the drawing scales in the following
order: network, overlays and residuals, error ellipses,
and vertical error vectors. The last line is the path and
file name (without an extension) of the GeoLab adjustment
files.
Therefore, if you move your adjustment files to another
directory or computer, the last line may contain an
invalid path. In this case you could simply open the
network drawing file in a text editor and correct the path.
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