Section 3: Planning
Anchor: #i1011230Overview
Planning is probably the most important part of the performance of a control survey utilizing GPS survey measurement techniques. Proper planning will give one added confidence that quality data will be collected. Regardless of the level of the survey, the items listed below should be addressed before the field data collection process begins.
Anchor: #i1011242Reconnaissance
Prior to the commencement of any TxDOT survey, all significant aspects of the project should be understood so that the project can be performed effectively and efficiently. Based on the TxDOT level of survey to be performed, go to Chapter 4, Table 4.1 and Chapter 5, Table 5.1 and review the specifications for the project.
Perform a reconnaissance survey of the site to:
- determine the location and sky visibility of existing and new control stations
- pick the locations for new stations making sure satellites can be recorded in a minimum of three quadrants
- look at logistics of project and determine transportation required
- gain permission to access station(s) on private land
- if applicable, the surveyor should notify law enforcement of their activities; record sky visibility chart data and access requirements for all stations
- look for any objects that could be sources for radio interference
- look for any multipath conditions that may affect data collection.
Monumentation for New Stations
All monumentation for new Level 1 points are to be in accordance with the following NGS publications.
Concrete Marks, from NGS Operations Handbook and Manual of Geodetic Triangulation, S.P. 247
Setting a Survey Disk in Bedrock or a Structure from NOAA Manual NOS, NGS 1, Geodetic Bench Marks
Setting a NGS 3-D Monument Based on Revised NGS 3-Dimensional (3-D) Rod Mark [Draft Version] by: Curtis L. Smith, National Geodetic Survey, July, 1996
It is recommended that new Level 2 points also follow these construction specifications, but the TxDOT surveyor in charge may call for less stringent requirements.
Anchor: #i1011364Naming Convention for Level 1 and Level 2 Monuments
The recommended naming convention for Level 1 and Level 2 monuments is as follows:
Example: 1580032
|
Digits |
Indication |
|---|---|
|
158 |
|
|
0032 |
|
Some districts use variations of this by including a prefix or suffix.
Figure 6-2 and Figure 6-3 are sample data sheets for documenting the monuments likely to be used in the future. There must be a data sheet for all Level 1 and Level 2 monuments. Districts may use their own data sheet form, but it must contain all the horizontal and vertical geodetic data of this sample data sheets. An RPLS signature and seal is recommended for data sheets for Level 1 and Level 2 GPS monuments.
Figure 6-1. List of the standard county designator codes used by TxDOT.
Anchor: #i1011406Control Point Data Sheet Form
Figure 6-2. TxDOT Control Point Data Sheet.
Anchor: #i1011420GPS Control Point
Figure 6-3. Sample project specifications control point.
Anchor: #i1011434Satellite Health and Availability
Only healthy satellites should be observed during the course of data collection. The satellite health situation can be checked by accessing the latest GPS status message from the USCG web site at http://www.navcen.uscg.gov/. This status message can also determine if there were problems after the data collection period is over.
There are times of the day when the numbers of satellites available will vary. Especially with real-time kinematic (RTK) positioning planning a work around for these times greatly increases productivity and the quality of results. Most, if not all, GPS software packages include a utility allowing the user to predict satellite coverage. A minimum of five (5) satellites are to be logged for any GPS work. In order to project satellite availability, the software will require a recent ephemeris file.
One internet site for obtaining this file is: http://www.trimble.com/planningsoftware_ts.asp?Nav=Collection-8425
Anchor: #i1011463Sky Visibility
Prior to data collection, the surveyor should look at each station to determine the extent, if any, of sky visibility obstructions greater than ten (10) degrees above the horizon. This survey should include obstructions in all four (4) quadrants of the sky.
If there are obstructions, the most desirable place for those obstructions to be located is northward of the station to be surveyed because of the design of the satellite constellation. If there is an obstruction in that area, it could still be a source of multipath at the GPS antenna. Therefore, the obstruction should be located.
Anchor: #i1011480Satellite Geometry
The geometric quality of a constellation of satellites is measured by Position Dilution of Precision (PDOP). It is also measured by Geometric Dilution of Precision (GDOP). The difference between PDOP and GDOP is that GDOP considers time, where PDOP only considers geometry.
The user should be aware of the manufacturer’s recommendations of maximum DOP values for the various types of surveys the user will perform. The vertical component of the GPS position is the most likely component to lack in quality if the DOP values are high. Therefore, if performing a vertical control survey, collect data with conservative DOP values.
One way to ensure that quality data are collected for the vertical is to collect satellite data that includes at least one satellite that is tracked greater than seventy (70) degrees above the horizon. However, a VDOP of less than 4.0 is all that is required. A PDOP of over 6.0 should probably be considered to be too great for usable data, making a PDOP of over 7.0 is unacceptable. Static data during periods of high DOP values should be deleted. Performance of RTK is more demanding and should not be done at PDOP values of 4 or greater.
Anchor: #i1011512Space Weather Considerations
A highly active ionosphere can have more severe implications for GPS observations. Magnetic storms and solar radiation storms will affect the signal-to-noise ratio (SNR) and may cause initialization problems with real-time kinematic (RTK) positioning and noisy data in static observations.
One measure of space weather activity is the scale developed by NOAA. Their Web site address, www.sec.noaa.gov/NOAAscales/, will predict activity on a scale of 1 to 5 with anything above 1 (one) becoming a hindrance to quality data collection.
The Costello Geomagnetic Index charts are found at http://www.sec.noaa.gov/rpc/costello/ Kp index of five (5) or more may cause problems and GPS surveying should not be done at six (6) or above. RTK should not be done at a Kp index of five (5) or greater.