Section 5: Survey Methods
Anchor: #i1011572Static Positioning
Static positioning typically uses a network or a multiple baseline approach for positioning. It may consist of multiple receivers, multiple baselines, multiple observational redundancies and multiple sessions. After processing the data to obtain baselines, a least squares adjustment of the results is required. This method provides the highest accuracy achievable and requires the longest observation times – from an hour to five hours or longer.
Static positioning is primarily used for ties to the NSRS when observing for TxDOT Level 1 and 2 surveys. However, this technique could be used for the other levels listed in Chapter 3, Table 3.4 of this manual using project control points.
Anchor: #i1011594FastStatic (Rapid Static) Positioning
The method of faststatic/rapid static positioning requires shorter occupation times than static positioning (i.e. 15 to 20+ minutes) and may use a radial baseline technique, network technique, or a combination of the two. Baseline lengths may not exceed ten (10) kilometers for L1 only receivers and twenty (20) kilometers for L1/L2 receivers.
Accuracy degrades at a predictable rate with this type of survey; therefore, longer baselines may be used when design survey quality is not needed. Please refer to the manufacturer’s specifications for minimum occupation times, number of satellites observed, and minimum amount of cycle slip free data collected for this type of data collection method.
FastStatic requires a least squares adjustment or other multiple baseline statistical analysis capable of producing a weighted mean average of the observations. More than one base station will be used to provide redundancy for each vector.
FastStatic or rapid static techniques could be used for observing Levels 3 & 4 listed in Chapter 3, Table 3.4 of this manual. It provides baselines that do not exceed the maximum distances stated above in the first paragraph of this subsection.
Anchor: #i1011623Post-Processed Kinematic (PPK) Positioning
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 faststatic/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-processed kinematic positioning may be used for Level 4 listed in Chapter 3, Table 3.4 of this manual.
Anchor: #i1011641Real-time Kinematic (RTK) Positioning
Real-time kinematic positioning is similar to a PPK or 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.
Real-time kinematic (RTK) positioning may be used for Levels 3 and 4 listed in Chapter 3, (Table 3.4 TxDOT Level of Survey Accuracy) of this manual. Level 3 surveys require that a second observation be made after losing and regaining initialization. Additionally, Level 3 surveys require that a second base station be set up for the purpose of creating a second baseline. Most GPS units will allow the averaging or adjustment of the two or more baselines while remaining at the point.
Level 4 surveys; however, will accept the single radial baseline solution (see Chapter 4, Table 4.1, Instrumentation Requirements). The surveyor must also follow the TxDOT redundancy requirements (see Section 6 of this chapter, RTK for Topographical Surveys) and the equipment manufacturer’s 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 ten (10) kilometers from the base station. There may be circumstances where this maximum range may be extended. Permission to extend this range will be extended at the TxDOT project surveyor’s discretion.
Radio transmission and cell phones are the most common way of providing the communication link between the base and rover.