Traditionally, Surveyors and Engineers have used a Total Station to collect the position and height data to create center lines, profiles and cross sections whilst designing a road.
Surveying a proposed road using GPS (GNSS) equipment and automated CAD design software requires a different methodology and approach. The traditional methods can be continued, however the results will be limited and resultant designs will be inferior.
Designing a road using Carlson Civil (automated CAD design software) utilizes a different set of rules in road design. A road is designed diditally relative to the natural topographical surface. The survey feeds a complete design solution; the road, the culverts, the channels and the overall effect of the design on the environment. A comprehensive set of customized reports are also available, documenting road design parameters and earth moving volumes (cut and fill).
The resulting road design (drawing suite) provides a complete data driven solution. On completion, all parameters set in the design can be refined and edited, at the press of the Process button, the design adopts the changes across every drawing..
At Planning level it is recommended that the impacts on the environment and drainage of the road design are considered…Google Earth is a good tool to use as it provides an aerial view of the location and path plus access to the elevation of the topographical surface (3-5m accuracy). Both Carlson SurvCE and Civil software provide export functions to Google Earth.
The road engineers plan the design based on this information working with plus budget constraints; optimization ids the key.
GPS technology allows the surveyor to collect a “mass” of data at high accuracy (6-10mm X, Y and 8-15mm Height) quickly and efficiently. It is important to discuss the limits and requirements of the project in advance.
Drainage hazards, potential for Land Slides, Flooding etc. This information is paramount to the success of the design solution. If surveyors are aware of every requirement at the planning stage the desktop designers will have a better chance of providing the best data for the construction of the road. Designing a road is now a composite of all professions working in unison.
NOTE: You can never collect TOO MUCH data in the field. It is a simple quick and easy process using GPS. As we consider the cost of surveying it is important to plan well and survey once. Then at completion, stake out the design, but learning the methodology used in the design software dictates the limits. Only with experience will (in)efficiencies become apparent.
Planning the Survey
It is important to establish survey control for the project in advance of the survey (see HOWTO Pre-planning a Survey). To achieve the best result we suggest the utilization of the Survey Planning software available online to select the best time to establish survey control.
This is when the satellite constellation is at the best. This will be a designated time each day within a 24 hour period, usually chosen between 7.00AM and 4.00PM.
GPS equipment provides an accurate position relative to a base point. There are four ways to locate and establish a base point for a survey.
- Permanent Marks: In most areas there will be Permanent marks available to use as reference for a survey. However in most instances these are few and far between and from experience are of low accuracy and too far from the survey area. If PM’s are available it is advisable to check the accuracy rating provided by NAMRIA.
- Averaging: Using this process produces a survey relative to a point accurate to 1-2 meters in Latitude, longitude and 3-5m in Height. Averaging is the process of selecting the Base point from the base setup in the data collection software – the GPS controller records 10-100 positions from the GPS and then averages the position. This is acceptable if we disregard the future use of our data. Eventually (in 3-5 years) all survey data will be imported to a GIS. If we use the average method the GIS will be limited in GIS accuracy by the method used to reference the road to the real world.
- Static Survey: (see HOWTO Static Survey). This method provides the most accurate position available today. This process utilizes long baseline data from monitored permanent GPS stations in proximity to the survey (Guam (US), Manila, Darwin (Australia), Hong Kong (China), Singapore) to establish a position within 20mm of accuracy on the earth surface.
- Utilizing NAMRIA (PageNet). This method utilizes the expertise and high accuracy provided by NAMRIA to survey. If this service in your area then the Static method will fulfill local needs.
A Geoid model should be installed in the data recorder software (SurvCE) to provide local computed gravity variations providing an accurate height relative to the Geoid. Height data can be further refined by referencing to a NAMRIA PM with an accurate height component.
It is good GPS survey practice to install a control point (base point) within the accuracy required in advance of the survey at a location near the mid point of the roads length. This process can take between 2 and 10 hours to perform. However, this is time well invested. Any survey team working on the road design and construction or tying this road design to an extension in the future will have the same high accuracy and will have reduced overhead in establishing control.
Until a reliable coverage of NAMRIA CORS Network (PageNet) is available across your area it is recommended that a Control Point be established every 1.0 km along a road way. This pint is best placed away from the roadway on public land. This allows surveyors, contractors or inspectors to work quickly and efficiently when laying out, constructing and checking the road design. A concrete plinth with a Number, date of installation and by whom should be noted on a stainless steel plate fixed into the concrete.
Using the Total Station method, data was collected at the 20m cross section stations along the center line of the road chosen by the surveyor. This is tedious and no longer necessary. If the center line is known or existing the surveyor will collect points regularly and randomly along this route where ever there are features or elevation variations. Distance is not important (maximum distance between points is 50m), however it is advisable to collect more center line data on bends and corners if the space for the road is limited (this allows designers to work within these constraints).
Where the road is flanked by legal boundary points (posts, fences etc.) an expedient method of locating the center line is to collect points along the legal boundaries then in either Carlson SurvCE or on the desktop using and Carlson Civil join the points between the boundaries, creating way points at the mid-point between the boundaries. Once the surveyor is satisfied with the position of the center line the surveying process is simple. Surveyors will then collect topographical data where ever there is a change in the topography (no closer than 20m apart) including the shape of the terrain at least 20 meters either side of the assumed center line…50 meters is recommended. The location of existing drains and culverts is also recommended plus any features that may influence the design of the road. traversing the center line and edge of the road way.
Where possible attach the Rover to a vehicle and collect data in Auto by Interval mode on the GPS controller. This reduces time and legs allowing a survey team to survey kilometers instead of hundreds of meters in a day.
In most surveys the limitations of UHF radio communications distances (1-2kms) will control the number of times the base will be projected forward. Install a base location pin no less than 150mm in length at each projected base point. Record the projected base point in Carlson SurvCE for future reference and retrieval.