Mada za sehemu hiiSimple Survey And Map MakingMada 5
Prismatic compass survey is a method under topographical surveying in which the directions of survey lines are measured using a prismatic compass and the lengths of the lines are measured with a tape or chain. The primary goal is to establish a controlled network of connected points, known as a traverse.
Alternatively, it is a surveying technique that determines the position of objects in the field by measuring the bearing angle between magnetic north and the line of sight to the object. This method is commonly applied in constructing traverses that form either an open or closed polygon.
- Facilitates rapid mapping of areas for topographical details.
The compass allows for quick data collection, making it ideal for preliminary surveys or areas where time is limited. - Measures angular bearings between objects with accuracy.
By determining relative directions, the surveyor can identify spatial relationships and layout. - Helps establish traverses through a network of stations.
Stations are connected to form a framework which can be plotted to represent the survey area accurately. - Enables location of field objects using intersection or resection methods.
These methods help determine unknown positions based on angles from known reference points. - Determines the magnetic north of the survey area.
Knowing magnetic north is critical for aligning maps and ensuring correct orientation during mapping.
- Prismatic Compass — A circular instrument with a magnetic needle and a graduated compass card marked in degrees (0° to 360° clockwise). The compass allows the user to determine the bearing of a line or object by aligning the sight with the target and reading the angle through the prism. It helps identify directions with respect to magnetic north.
- Ranging Pole — A tall, slender pole, usually painted in red and white bands, used to mark the positions of survey stations. These help in aligning and maintaining straight survey lines during fieldwork.
- Arrows — Small metal or steel pins used to mark temporary points on the ground while measuring distances with a tape or chain. They help keep track of the number of chain or tape lengths measured.
- Tape Measure or Chain — Used for taking linear distance measurements between survey stations. The tape is typically made of steel or fiber, while the chain consists of linked metal segments. Both serve to determine the length of survey lines.
- Field Notebook and Pencil — A small durable notebook used to record observations, bearings, distances, and other data collected during the survey. The pencil allows for easy writing and correction in the field.
- Conduct a preliminary survey to identify and mark main stations.
During this stage, select prominent points for main stations and mark them using ranging poles or existing natural features such as trees. These form the basic framework for the survey. - Lay out and mark survey lines as straight and long as possible.
The accuracy of the survey improves with longer and unobstructed survey lines. Clear marking helps in sighting and angular measurement. - Label and mark all main stations alphabetically (e.g., A, B, C, D, etc.).
Proper labeling ensures a clear reference system for recording angles and distances. - Measure angles between stations using a prismatic compass and record the forward and backward bearings.
Bearings are taken from each station to the next (e.g., A to B, B to C). The forward bearing (FB) and the backward bearing (BB) for each line are noted in the field book. - Measure linear distances between stations using a chain or tape and record them.
Accurate distance measurement is essential for plotting the traverse and calculating positions. - Take offsets and tie measurements to nearby features during traversing.
These additional measurements help in locating significant landmarks or objects relative to the survey line, adding detail to the map. - Record all data systematically in the field notebook.
Entries should include station names, bearings, distances, and offset details, forming the basis for drawing the final plan.
- Forward and backward bearings differ by exactly 180°.
This rule ensures consistency when checking or correcting angle measurements. - If the forward bearing (FB) is greater than 180°, subtract 180° to find the backward bearing (BB).
Example: If FB = 230°, then BB = 230° - 180° = 50°. - If the forward bearing is less than 180°, add 180° to find the backward bearing.
Example: If FB = 75°, then BB = 75° + 180° = 255°.
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