GPS RTK Surveying
Overview
Real-time
kinematic (RTK) positioning is similar to a total station radial survey. RTK
does not require post processing of the data to obtain a position solution. This
allows for real-time surveying in the field and allows the surveyor to check
the quality of measurements without having to process the data.
kinematic (RTK) positioning is similar to a total station radial survey. RTK
does not require post processing of the data to obtain a position solution. This
allows for real-time surveying in the field and allows the surveyor to check
the quality of measurements without having to process the data.
RTK
positioning may be used for Level 3 and 4 surveys as mentioned in Section 5 of
this chapter. Level 3 surveys require that a second base station be set up for
the purpose of creating a second baseline. Trimble units (and most others) will
allow the averaging or adjustment of the two or more baselines while still
standing at the point. Level 4 surveys will accept the single radial baseline
solution. The surveyor must also follow the manufacturers prescribed methods.
positioning may be used for Level 3 and 4 surveys as mentioned in Section 5 of
this chapter. Level 3 surveys require that a second base station be set up for
the purpose of creating a second baseline. Trimble units (and most others) will
allow the averaging or adjustment of the two or more baselines while still
standing at the point. Level 4 surveys will accept the single radial baseline
solution. The surveyor must also follow the manufacturers prescribed methods.
Real-time
surveying technology may utilize single or dual-frequency (L1/L2) techniques
for initialization, but the subsequent RTK survey is accomplished using only
the L1 carrier phase frequency. Therefore, all RTK surveys are currently
subject to the limitations of the L1 frequency which is 10 kilometers from the
base station. In order to maintain a 2 cm level of accuracy, distances are
usually considerably less than this but there may be circumstances where this
maximum range may be extended.
surveying technology may utilize single or dual-frequency (L1/L2) techniques
for initialization, but the subsequent RTK survey is accomplished using only
the L1 carrier phase frequency. Therefore, all RTK surveys are currently
subject to the limitations of the L1 frequency which is 10 kilometers from the
base station. In order to maintain a 2 cm level of accuracy, distances are
usually considerably less than this but there may be circumstances where this
maximum range may be extended.
Planning
As with
static GPS surveying, mission planning is an important step in performing a RTK
survey. There are times of the day when the numbers of satellites available
will vary. The positions of the satellites at various times of the day are also
a factor. Planning your work around these times greatly increases productivity
and the quality of your results. Most, if not all, software packages include a
utility allowing suers to predict satellite coverage.
static GPS surveying, mission planning is an important step in performing a RTK
survey. There are times of the day when the numbers of satellites available
will vary. The positions of the satellites at various times of the day are also
a factor. Planning your work around these times greatly increases productivity
and the quality of your results. Most, if not all, software packages include a
utility allowing suers to predict satellite coverage.
The
Trimble planning utility supported by Technology Services Division (TSD) is
simply called “Planning” and offers charts, graphs, and sky plots to aid in
determining the best times for GPS reception and data quality. Number of
satellites and PDOP are the most important indicators. A recent almanac
(approximately 12 minutes of broadcast ephemeris data collected within the last
couple of days) is needed. The surveyor can collect this or download it from
several sites on the internet. The Trimble site, www.trimble.com, uses a file extension of .ssf on their
daily almanac files.
Trimble planning utility supported by Technology Services Division (TSD) is
simply called “Planning” and offers charts, graphs, and sky plots to aid in
determining the best times for GPS reception and data quality. Number of
satellites and PDOP are the most important indicators. A recent almanac
(approximately 12 minutes of broadcast ephemeris data collected within the last
couple of days) is needed. The surveyor can collect this or download it from
several sites on the internet. The Trimble site, www.trimble.com, uses a file extension of .ssf on their
daily almanac files.
The
selection of the base station sites will also affect the success of the RTK
observations. Users who select a poor base stations site will likely have
problems throughout the entire survey. Select a site with good sky visibility
down to 10 or 15 degrees from the horizon. Be aware of high power transmitters
such as microwave, TV stations, military installations, high voltage
transmission power lines, etc.
selection of the base station sites will also affect the success of the RTK
observations. Users who select a poor base stations site will likely have
problems throughout the entire survey. Select a site with good sky visibility
down to 10 or 15 degrees from the horizon. Be aware of high power transmitters
such as microwave, TV stations, military installations, high voltage
transmission power lines, etc.
Multi-path
may be caused by radio wave reflective objects such as trees, buildings, large
signboards, chain link fences, etc. Because of the orbits of the satellites,
obstacles to the north of the antenna setup are not as detrimental to
reception.
may be caused by radio wave reflective objects such as trees, buildings, large
signboards, chain link fences, etc. Because of the orbits of the satellites,
obstacles to the north of the antenna setup are not as detrimental to
reception.
It is
worth the effort to get the base stations in optimum locations. A problem at
the base is a problem at all rovers. A problem at one rover is only a problem
at that one rover.
worth the effort to get the base stations in optimum locations. A problem at
the base is a problem at all rovers. A problem at one rover is only a problem
at that one rover.
If
possible, users should take part in the selection of any project control points
in the beginning stages of a project. This is to insure that the points can be
surveyed with GPS and well spaced for project coverage of real-time kinematic
(RTK), if GPS is likely to be used. Of course, the primary project control
points selected should always be GPS friendly.
possible, users should take part in the selection of any project control points
in the beginning stages of a project. This is to insure that the points can be
surveyed with GPS and well spaced for project coverage of real-time kinematic
(RTK), if GPS is likely to be used. Of course, the primary project control
points selected should always be GPS friendly.
Preparing the
Data Collector
It is
important to have the Feature Code Library or a feature table loaded into the
data collector before going to the field. A list of available existing control
points that may be available should also be included to prevent having to type
in long coordinate values. On the Trimble TSC and ACU data collectors the
control file can simply be an ASCII file with the format: point name, northing,
easting, elevation, feature and feature code. The extension can be .txt or it
can be a .csv file.
important to have the Feature Code Library or a feature table loaded into the
data collector before going to the field. A list of available existing control
points that may be available should also be included to prevent having to type
in long coordinate values. On the Trimble TSC and ACU data collectors the
control file can simply be an ASCII file with the format: point name, northing,
easting, elevation, feature and feature code. The extension can be .txt or it
can be a .csv file.
The “job”
can be created in the office and exported to the data collector or the user can
enter the job parameters in the field. Some of the settings involved are datum,
projection, scale, and scale factor and measurement units. The Trimble data
collection software is called, “Survey Controller.”
can be created in the office and exported to the data collector or the user can
enter the job parameters in the field. Some of the settings involved are datum,
projection, scale, and scale factor and measurement units. The Trimble data
collection software is called, “Survey Controller.”
A survey
“style” is used as a template to repeat settings for a particular type of
real-time kinetic (RTK) survey. The style contains dozens of settings to
include all the base and rover radio settings, receiver settings, data and
accuracy parameters, etc. The software comes with several styles, but the user
will eventually want to create their own to accommodate their particular brand
of radios, or cell phones. The user’s styles can be created in the
“configuration” icon and will be saved under the Trimble Data folder on the
hard drive of the data collector.
“style” is used as a template to repeat settings for a particular type of
real-time kinetic (RTK) survey. The style contains dozens of settings to
include all the base and rover radio settings, receiver settings, data and
accuracy parameters, etc. The software comes with several styles, but the user
will eventually want to create their own to accommodate their particular brand
of radios, or cell phones. The user’s styles can be created in the
“configuration” icon and will be saved under the Trimble Data folder on the
hard drive of the data collector.
Setting Up the
Base Station
Set the
base station at one second collection rate and at 13 degrees elevation mask. It
is advisable to use a fixed height tripod (usually 2-meters). It is possible to
do the entire survey without ever having to make an H.I. measurement – a
considerable advantage over the conventional survey.
base station at one second collection rate and at 13 degrees elevation mask. It
is advisable to use a fixed height tripod (usually 2-meters). It is possible to
do the entire survey without ever having to make an H.I. measurement – a
considerable advantage over the conventional survey.
Obviously,
it will be of little value to have data based on a coordinate system or datum
that is unusable. However, control local in NAD 83 datum may sometimes be
unavailable within range of the RTK system. Perhaps control points have been
destroyed or maybe no control was extended beyond the several project control
points on a large project. In these instances, a proper GPS methods can be used
to bring control points to areas suitable for RTK base stations.
it will be of little value to have data based on a coordinate system or datum
that is unusable. However, control local in NAD 83 datum may sometimes be
unavailable within range of the RTK system. Perhaps control points have been
destroyed or maybe no control was extended beyond the several project control
points on a large project. In these instances, a proper GPS methods can be used
to bring control points to areas suitable for RTK base stations.
There is
also the option of starting an RTK survey with an autonomous position for the
base. While collecting the RTK data at the rover, have the base station log raw
observables for post-processed GPS. After post-processing the data to establish
correct coordinates, users can apply the corrected coordinates to edit the base
station positions and shift the collected RTK data to its proper positions.
Trimble data collectors have a selection for this purpose, which is called a
“here” option.
also the option of starting an RTK survey with an autonomous position for the
base. While collecting the RTK data at the rover, have the base station log raw
observables for post-processed GPS. After post-processing the data to establish
correct coordinates, users can apply the corrected coordinates to edit the base
station positions and shift the collected RTK data to its proper positions.
Trimble data collectors have a selection for this purpose, which is called a
“here” option.
A similar
option is to start your base station with an autonomous (here) position but
then observe control points and calibrate or localize (manufactures use
different terms) to shift the data to the existing control values.
option is to start your base station with an autonomous (here) position but
then observe control points and calibrate or localize (manufactures use
different terms) to shift the data to the existing control values.
There are
drawbacks to using these two approaches. The problems arise from the inaccuracy
of the position of the base stations. For each ten meters of error in the base
position, introduce an additional 1ppm (1mm per kilometer) error in our
baselines. (This rule holds true with all GPS surveying techniques users might
choose.)
drawbacks to using these two approaches. The problems arise from the inaccuracy
of the position of the base stations. For each ten meters of error in the base
position, introduce an additional 1ppm (1mm per kilometer) error in our
baselines. (This rule holds true with all GPS surveying techniques users might
choose.)
Even
after the base station data is post-processed and the coordinates are
determined and shifted, error will remain in the baselines if the 10 meter
fault is present. The finding of an autonomous position of more than 10 meters
error, especially if baselines of more than a few kilometers were used, may
mean a redo of the survey. For this reason, it is strongly recommended that
base stations be known control points.
after the base station data is post-processed and the coordinates are
determined and shifted, error will remain in the baselines if the 10 meter
fault is present. The finding of an autonomous position of more than 10 meters
error, especially if baselines of more than a few kilometers were used, may
mean a redo of the survey. For this reason, it is strongly recommended that
base stations be known control points.
If it is
not possible to use a control point, there are different approaches that can be
applied to assure that inaccurate base station coordinates are held to a
minimum.
not possible to use a control point, there are different approaches that can be
applied to assure that inaccurate base station coordinates are held to a
minimum.
1.
Use the Wide Area Augmentation System (WAAS) corrected signal
(still only accurate to about 7 meters).
Use the Wide Area Augmentation System (WAAS) corrected signal
(still only accurate to about 7 meters).
2.
Use equipment that allows for averaging of code derived autonomous
positions.
Use equipment that allows for averaging of code derived autonomous
positions.
3.
Predetermine position with code equipment using a correction
service.
Predetermine position with code equipment using a correction
service.
Using an
unknown position for the base station in the methods described above is a poor
substitute for the practice of occupying a known position.
unknown position for the base station in the methods described above is a poor
substitute for the practice of occupying a known position.
The Rover
Set the
rover receiver at one (1) second collection rate and 15 degrees elevation mask.
The rover rod should always be a fixed height. Unlike conventional surveying
from a total station, line of sight is not needed – there is no need to raise
or lower the rod height. Usually a 2-meter rod is used. Not having to make this
measurement eliminates one more chance of error.
rover receiver at one (1) second collection rate and 15 degrees elevation mask.
The rover rod should always be a fixed height. Unlike conventional surveying
from a total station, line of sight is not needed – there is no need to raise
or lower the rod height. Usually a 2-meter rod is used. Not having to make this
measurement eliminates one more chance of error.
Before
getting too far away from the base station, check the radio (or cell phone)
link to the rover.
getting too far away from the base station, check the radio (or cell phone)
link to the rover.
The first
thing that must be done upon “starting the survey” on the data collector is to
initialize the system (resolve the integer ambiguity). Several methods that are
acceptable when preformed properly.
thing that must be done upon “starting the survey” on the data collector is to
initialize the system (resolve the integer ambiguity). Several methods that are
acceptable when preformed properly.
·
Known point initialization – this is the fastest and safest way to
initialize. Where possible and practical this should be the method of choice.
The firmware uses the three-dimensional deltas of the relative WGS84 positions
of the base stations and the known point occupied by the rover as an aid in
solving the integer ambiguities.
Known point initialization – this is the fastest and safest way to
initialize. Where possible and practical this should be the method of choice.
The firmware uses the three-dimensional deltas of the relative WGS84 positions
of the base stations and the known point occupied by the rover as an aid in
solving the integer ambiguities.
·
New point initialization – this is a technique that is usually
used on equipment that does not have the ability to solve the integer ambiguity
on the fly (OTF).
New point initialization – this is a technique that is usually
used on equipment that does not have the ability to solve the integer ambiguity
on the fly (OTF).
·
On-The-Fly (OTF) initialization – this is the most common
technique used by most equipment today. There must be care taken when using
this method. The possibility of an incorrect initialization may be remote but
remains a possibility.
On-The-Fly (OTF) initialization – this is the most common
technique used by most equipment today. There must be care taken when using
this method. The possibility of an incorrect initialization may be remote but
remains a possibility.
To avoid
the possibility of an undetected incorrect initialization use one of the
following methods to check the system.
the possibility of an undetected incorrect initialization use one of the
following methods to check the system.
After the
OTF initialization, observe a point, this can be a temporary mark or a point in
the survey. Discard the first OTF initialization and OTF re-initialize by
moving more than forty (40) feet away from the point to be used as check. After
the new OTF initialization has been accomplished return to the point being used
as a check and re-shoot. Compare the first and second shots to within an
acceptable tolerance. If the points check, proceed with data collection with
the confidence of surveying with a correct initialization.
OTF initialization, observe a point, this can be a temporary mark or a point in
the survey. Discard the first OTF initialization and OTF re-initialize by
moving more than forty (40) feet away from the point to be used as check. After
the new OTF initialization has been accomplished return to the point being used
as a check and re-shoot. Compare the first and second shots to within an
acceptable tolerance. If the points check, proceed with data collection with
the confidence of surveying with a correct initialization.
If the
error between the two points is beyond the expected error one or both of the
OTF initializations used for a check are incorrect. The user must change the
location by a difference of more than two (2) feet of H.I. or more likely move
more than forty (40) feet away in a different direction. This will usually
provide enough information to identify the OTF initialization that is
incorrect. Once the problem is solved, users can begin the survey. This
procedure must be repeated with any loss of initialization.
error between the two points is beyond the expected error one or both of the
OTF initializations used for a check are incorrect. The user must change the
location by a difference of more than two (2) feet of H.I. or more likely move
more than forty (40) feet away in a different direction. This will usually
provide enough information to identify the OTF initialization that is
incorrect. Once the problem is solved, users can begin the survey. This
procedure must be repeated with any loss of initialization.
As with
any surveying techniques the user would want to check a known point in the
survey before beginning work, apply the same logic to your RTK survey.
any surveying techniques the user would want to check a known point in the
survey before beginning work, apply the same logic to your RTK survey.
The user
is now ready to observe points. The amount of time of occupation will vary
depending on conditions such as obstruction, multi-path, noise, etc. The user
may have to resort to:
is now ready to observe points. The amount of time of occupation will vary
depending on conditions such as obstruction, multi-path, noise, etc. The user
may have to resort to:
·
increasing occupation time to a couple of minutes at one second
epochs
increasing occupation time to a couple of minutes at one second
epochs
·
a more stable setup (use of a tripod or bipod)
a more stable setup (use of a tripod or bipod)
·
use of a ground plane when in a multi-path environment
use of a ground plane when in a multi-path environment
The use
of the most recent list of TxDOT feature codes is mandatory. As of November
2003, the “txdot2k” is the most recent. This TxDOT list is available in Trimble
format as “txdot2k.fcl” and in CAiCE ™ format as “txdot2k.ftb.” Topographic
data should be collected in a manner similar to a conventional topographic
survey in that the rover operator(s) must be aware of the fact that they are
collecting chains of connected points to create break lines that will not be
crossed in the creation of the TIN file.
of the most recent list of TxDOT feature codes is mandatory. As of November
2003, the “txdot2k” is the most recent. This TxDOT list is available in Trimble
format as “txdot2k.fcl” and in CAiCE ™ format as “txdot2k.ftb.” Topographic
data should be collected in a manner similar to a conventional topographic
survey in that the rover operator(s) must be aware of the fact that they are
collecting chains of connected points to create break lines that will not be
crossed in the creation of the TIN file.
The
selection of feature codes for “as-builts” and various features will also
determine what points will or will not be included in a DTM. TxDOT’s Technology
Services Division (TSD) provides a class on Survey Data Management System® (SDMS)
for data collection that would be helpful in understanding the procedures for
collecting data with RTK for topo work.
selection of feature codes for “as-builts” and various features will also
determine what points will or will not be included in a DTM. TxDOT’s Technology
Services Division (TSD) provides a class on Survey Data Management System® (SDMS)
for data collection that would be helpful in understanding the procedures for
collecting data with RTK for topo work.
There are
two screens associated with each measurement on the Trimble data collector: the
“measure” screen and the “attribute” screen. The initial (default) screen is
the “measure” screen, which will allow the user to key in the feature code.
Before pressing “measure” however, open the “attribute” to answer the prompts
such as FG: or GM: or whatever else appears, then make the measurement.
two screens associated with each measurement on the Trimble data collector: the
“measure” screen and the “attribute” screen. The initial (default) screen is
the “measure” screen, which will allow the user to key in the feature code.
Before pressing “measure” however, open the “attribute” to answer the prompts
such as FG: or GM: or whatever else appears, then make the measurement.
If the
user has not checked the “prompt for attributes” box this will not appear.
Users can continue shooting points without going back to the attributes screen,
until the situation changes when a new figure number, geometry setting, or
whatever else the user might want to change is needed.
user has not checked the “prompt for attributes” box this will not appear.
Users can continue shooting points without going back to the attributes screen,
until the situation changes when a new figure number, geometry setting, or
whatever else the user might want to change is needed.
A good
rule of thumb is to reoccupy about 25% of all points requiring the accuracy of
a Level 3 survey after a new initialization or about 10% of the points in a
topo survey.
rule of thumb is to reoccupy about 25% of all points requiring the accuracy of
a Level 3 survey after a new initialization or about 10% of the points in a
topo survey.
Upon
successful completion of the user’s observations, the user will now have a
radial survey. Users must move the base station to a second control point and
repeat the process for surveys that will not allow single baseline solutions
(Level 3).
successful completion of the user’s observations, the user will now have a
radial survey. Users must move the base station to a second control point and
repeat the process for surveys that will not allow single baseline solutions
(Level 3).
If radial
lines are permitted, such as on a topographical survey or wing panel locations,
it is still a good idea to occupy a second base station if another control
point is nearby to randomly check a few of the points already established.
lines are permitted, such as on a topographical survey or wing panel locations,
it is still a good idea to occupy a second base station if another control
point is nearby to randomly check a few of the points already established.
Post Processing
An
alternative method of performing a kinematic survey is to collect the data and
process it at a later time. This does not require the use of a communications
link (i.e. radio or cell phone) and can be combined with RTK to perform infill
when the link is temporarily down. Post processed kinematic survey methods
provide the surveyor with a technique for high production measurements and can
be used in areas with minimal obstructions of the satellites.
alternative method of performing a kinematic survey is to collect the data and
process it at a later time. This does not require the use of a communications
link (i.e. radio or cell phone) and can be combined with RTK to perform infill
when the link is temporarily down. Post processed kinematic survey methods
provide the surveyor with a technique for high production measurements and can
be used in areas with minimal obstructions of the satellites.
PPK uses
significantly reduced observation times (i.e. 0.5 to 3 minutes, usually 10-30
seconds per point) compared to static or fast-static/rapid-static observations.
This method requires a least squares adjustment or other multiple baseline
statistical analysis capable of producing a weighted mean average of the
observations. Post processing will allow kinematic surveying to be used for
some Level 3 surveys.
significantly reduced observation times (i.e. 0.5 to 3 minutes, usually 10-30
seconds per point) compared to static or fast-static/rapid-static observations.
This method requires a least squares adjustment or other multiple baseline
statistical analysis capable of producing a weighted mean average of the
observations. Post processing will allow kinematic surveying to be used for
some Level 3 surveys.
Integrating
Conventional Measurements
To be
added in a later edition.
added in a later edition.
Using Networked
RTK (VRS)
Networked
RTK is a new variation of RTK data collection. Rather than setting up a base
station on the project, a number of permanent and continuously operating base
stations are set up at about a 30 – 40 mile spacing, providing that
augmentation to the basic position is determined at the rover.
RTK is a new variation of RTK data collection. Rather than setting up a base
station on the project, a number of permanent and continuously operating base
stations are set up at about a 30 – 40 mile spacing, providing that
augmentation to the basic position is determined at the rover.
These
stations send GPS data into a central computer that streams the correction data
to the Internet. The data can then be accessed by way of a cell phone/cell
modem at the rover receiver. The data collector then uses this information to
provide real time solutions with the same speed and accuracy as Base Station
RTK but without the complication of setting up a receiver and radio on a local
control point.
stations send GPS data into a central computer that streams the correction data
to the Internet. The data can then be accessed by way of a cell phone/cell
modem at the rover receiver. The data collector then uses this information to
provide real time solutions with the same speed and accuracy as Base Station
RTK but without the complication of setting up a receiver and radio on a local
control point.
This
system yields the same accuracies as the normally accepted three (3) miles of a
standard radio-linked base station and rover. TxDOT has installed a number of
these RTK networks. Coverage is growing across the state but presently covers
the major metropolitan areas and some of the rural districts. Refer to Figure
3-14 in this chapter.
system yields the same accuracies as the normally accepted three (3) miles of a
standard radio-linked base station and rover. TxDOT has installed a number of
these RTK networks. Coverage is growing across the state but presently covers
the major metropolitan areas and some of the rural districts. Refer to Figure
3-14 in this chapter.
The name
virtual reference station (VRS) was coined for this method because a “virtual”
base station point is determined by the computer from the network of base
stations. The virtual base station is never more than 3 miles from the rover
and is automatically redefined when the rover goes beyond that preset distance.
virtual reference station (VRS) was coined for this method because a “virtual”
base station point is determined by the computer from the network of base
stations. The virtual base station is never more than 3 miles from the rover
and is automatically redefined when the rover goes beyond that preset distance.
The TxDOT
RTK Network uses the VRS technology. A virtual base point near the project is
computed by the central computer. The user operating a rover unit dials in to
the TxDOT IP address for connection to the system. The data-ready cell
phone/modem must then be physically carried by the user to maintain constant
communication between the rover and the Internet. For information about this and
other features that vary from place to place and time to time, contact your
district survey coordinator, who in turn, may contact the administrators of the
system at TSD.
RTK Network uses the VRS technology. A virtual base point near the project is
computed by the central computer. The user operating a rover unit dials in to
the TxDOT IP address for connection to the system. The data-ready cell
phone/modem must then be physically carried by the user to maintain constant
communication between the rover and the Internet. For information about this and
other features that vary from place to place and time to time, contact your
district survey coordinator, who in turn, may contact the administrators of the
system at TSD.
Specific
cell phone services and connection information should be obtained from the local
cell phone service provider.The same procedures and precautions as outlined for
Base Station RTK should be followed using the TxDOT RTK networks. The
difference is simply that users are not working from a base station set up by
the user for a particular project. Users will not need to occupy the known
station with a GPS receiver transmitting correction data to the rover(s). The
work will be accomplished from a network of GPS base receivers.In the case of a
Level 3 point, where users would normally occupy a point more than once and
from two or more base stations; three to six RRP’s are already being used in
the coordinate calculation using the networks. The point should still be
occupied twice at different times of the day.
cell phone services and connection information should be obtained from the local
cell phone service provider.The same procedures and precautions as outlined for
Base Station RTK should be followed using the TxDOT RTK networks. The
difference is simply that users are not working from a base station set up by
the user for a particular project. Users will not need to occupy the known
station with a GPS receiver transmitting correction data to the rover(s). The
work will be accomplished from a network of GPS base receivers.In the case of a
Level 3 point, where users would normally occupy a point more than once and
from two or more base stations; three to six RRP’s are already being used in
the coordinate calculation using the networks. The point should still be
occupied twice at different times of the day.
There is
an option in Survey Controller to do an “Observed Point,” which will
automatically collect for a specified amount of time; usually three minutes.
This gives the mark a special status in the priority of stations in the TGO
program.
an option in Survey Controller to do an “Observed Point,” which will
automatically collect for a specified amount of time; usually three minutes.
This gives the mark a special status in the priority of stations in the TGO
program.
The TxDOT
RTK Network is based on the National Spatial Reference System, which means that
all coordinates are in the NAD83 datum and accuracy and compatibility should
not be a problem. This however, can work against users when all previous work
was done on local coordinates or the area of previous control may carry local
biases.
RTK Network is based on the National Spatial Reference System, which means that
all coordinates are in the NAD83 datum and accuracy and compatibility should
not be a problem. This however, can work against users when all previous work
was done on local coordinates or the area of previous control may carry local
biases.
To
overcome the clash of coordinate values, the process of “calibrating” to the
existing control is used. This was not used as extensively via the base station
method where the control point coordinates were the start of subsequent GPS
work.
overcome the clash of coordinate values, the process of “calibrating” to the
existing control is used. This was not used as extensively via the base station
method where the control point coordinates were the start of subsequent GPS
work.
Most RTK
network surveys should be done after a calibration to existing control. Even if
the horizontal component doesn’t require a calibration, consider performing the
vertical calibration. GPS solutions require the aid of a geoid model for
elevations. In several areas around the state, the geoid model has a difference
of more than a tenth of a foot from known elevations. If any known bench marks
exist in the area; calibrate to them.
network surveys should be done after a calibration to existing control. Even if
the horizontal component doesn’t require a calibration, consider performing the
vertical calibration. GPS solutions require the aid of a geoid model for
elevations. In several areas around the state, the geoid model has a difference
of more than a tenth of a foot from known elevations. If any known bench marks
exist in the area; calibrate to them.
In TxDOT
districts with RTK networks, users should apply for a password through their
district survey coordinator. This password will allow users to access the
system through the Internet.
districts with RTK networks, users should apply for a password through their
district survey coordinator. This password will allow users to access the
system through the Internet.
Consultants
with active contracts are allowed to apply for a password through the district
survey coordinator but are limited to just TxDOT work with the TxDOT system.
The use of a private real time network for TxDOT work by consultants is up to
the discretion of the district survey coordinator.
with active contracts are allowed to apply for a password through the district
survey coordinator but are limited to just TxDOT work with the TxDOT system.
The use of a private real time network for TxDOT work by consultants is up to
the discretion of the district survey coordinator.
The End Product
When topo
data is collected with real-time kinematic (RTK), the output format most often
will need to be in Survey Data Management System® (SDMS) format. The
TxDOT supported TGO software exports directly to the SDMS® format. There must
be point connectivity (break lines) and the standard TxDOT feature codes will
insure this. The file is available at district offices in Trimble format for
Trimble TSC-1, TSC-e and ACU data collectors. As mentioned above, the file name
is “txdot2k.fcl”. Conventional data collected on the Trimble data collector
using Survey Controller can be included in the same .dc (job file).
data is collected with real-time kinematic (RTK), the output format most often
will need to be in Survey Data Management System® (SDMS) format. The
TxDOT supported TGO software exports directly to the SDMS® format. There must
be point connectivity (break lines) and the standard TxDOT feature codes will
insure this. The file is available at district offices in Trimble format for
Trimble TSC-1, TSC-e and ACU data collectors. As mentioned above, the file name
is “txdot2k.fcl”. Conventional data collected on the Trimble data collector
using Survey Controller can be included in the same .dc (job file).
The .dc
(job file) can be downloaded to the PC with the Trimble Data Transfer program
or using Microsoft ActiveSync with a USB connection. By importing into Trimble
TGO, the user can see the work graphically and do some editing if necessary
before exporting the final product as an ASCII or SDMS.cal file.
(job file) can be downloaded to the PC with the Trimble Data Transfer program
or using Microsoft ActiveSync with a USB connection. By importing into Trimble
TGO, the user can see the work graphically and do some editing if necessary
before exporting the final product as an ASCII or SDMS.cal file.
The use
of LandXML format is being investigated as an alternative standard of transfer.
of LandXML format is being investigated as an alternative standard of transfer.
If an
ASCII file of final coordinates is needed, the most often requested format is:
name, northing, easting, elevation, feature code.
ASCII file of final coordinates is needed, the most often requested format is:
name, northing, easting, elevation, feature code.
NOTE: that all data passing hands should include notes on datum,
projection, geoid model, and date of survey. Coordinates should be designated
as state plane coordinates or surface adjusted coordinates with
an accompanying CAF or SAF factor.
projection, geoid model, and date of survey. Coordinates should be designated
as state plane coordinates or surface adjusted coordinates with
an accompanying CAF or SAF factor.
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