PURPOSE OF TOPOGRAPHIC SURVEY
- Collecting additional data on topography, project area and detailed survey to serve the preparation of basic design documents, construction cost estimates and total investment.
- Providing topographical and hydrological survey documents to serve for preparing basic design drawings for construction.
- Collecting documents to accurately determine the construction volume and total construction investment.
METHOD OF TOPOGRAPHIC SURVEY
Topographic survey includes: Establishing a primary horizontal control network (primary traverse network) and levelling network (secondary traverse network) for large scale mapping, feasible research report and technical construction design.
Primary traverse network
Preliminary traverse network is measured by GPS technology, connected with 02 points of National coordinates. The establishment of preliminary traverse network is carried out in 3 steps:
Step 1: Constructing primary traverse point. Primary traverse point is made of concrete, central of point is made of porcelain. Points are buried in a stable geological area, far from the construciton area and convenient for low-level traverse netwwork development and detailed measurement. When selecting points location, it must satisfy the following technical requirements:
- Ensuring the view around the point is not obscured and number of satellites are evenly distributed in the sky around the point.
- Unaffected by electric broadcast stations (Away from broadcast station ≥ 500m) disturbing satellite signals affecting the calculation results.
- Choose a well-ventilated place, away from tall buildings and saltwater lakes, to avoid reflections from other objects.
Step 2: Measuring primary traverse network. Primary traverse network is measured by static GPS method, the receivers receive continuously satellite signal for more than 1 hour in favorable conditions according to the calculated schedule. The receiver is set to automatic recording mode, records 1 measured value every 05 seconds to memory.
Step 3: Measuring class IV levelling network. Starting from two class III or higher National altitude points near the area, using model Leica LS10 machine made in Swiss (or equivalent precision machine) to traverse the altitude primary horizontal control point with accuracy of class IV levelling.
Secondary traverse network
Secondary traverse network is developed based on primary traverse network. Secondary traverse network is also carried out in 3 steps:
Step 1: Constructing secondary traverse network. Secondary traverse point is made of concrete, central of point is made of screws. The points are buried in places with stable structure.
Step 2: Measuring secondary traverse network. Measuring secondary traverse network based on primary traverse network.
Step 3: Measuring technical levelling network. From existed class IV levelling points in the area, the technical levelling points shall be set up. Measuring technical levelling network by high geometry method from the middle in one direction.
Based on primary and secondary traverse network, using electronic total station to measure detailed terrain in the area and adjacent areas.
The detailed topographic mapping is conducted in accordance with the mapping process. Electronic total station is used for field drawing is manufactured by Leica or machine with equivalent accuracy. Measured datas such as the coordinate and elevation of required points are calculated, automatically recorded into the memory of the machine and then transmitted to the personal computer using specialized software.
Measured data is tranfered from electronic total station into computer by specialized software in the form of XYH column and processed by geodetic software. Topographic map clearly shows other terrain features.
- TCVN 9398:2012 “Surveying in construction – General requirement”.
- TCVN 9401:2012 “Technical of measuring and Processing GPS data in engineering survey”.