A map is equally crucial for forming meaning between disparate distances as charting those distances. Have you ever considered how much our maps affect how we conduct business, professional, and personal lives? This is not just a comment on the iniquitousness of GPS devices. In the construction business, nothing gets done without the input of topographical surveyors.
A topographic surveyor wears reflective vests and peers into the distance through a telescope-like instrument. Most construction businesses could not operate without data collected by topographic surveys. According to law, surveys of topography determine the distances between properties and their boundaries.
The process of determining the relative locations of points (places) on the earth’s surface by comparing horizontal distances, elevation differences, and directions. A topographic map shows the location of places (observable features); it serves as a base map.
Which instrument is used in the topographical survey?
The main instrument used in topographical surveys is the Total Station, which is a high-precision electronic surveying instrument.
Total Station combines the traditional angular measurement of a theodolite with electronic distance measurement technology, which enables the instrument to provide precise measurements of both horizontal and vertical angles and distances.
It is an electronic instrument that captures measurements digitally and can be linked to a computer for data processing and analysis. The data collected by Total Station is highly accurate and can be used to create detailed maps and models of the land, including contour maps, cross-sections, and three-dimensional models.
In addition to Total Station, other instruments that may be used in topographical surveys include GPS receivers, laser levels, and drone-based systems. However, Total Station remains the most widely used and accurate instrument for topographical surveys due to its high level of precision and versatility.
Uses of topographic surveying in geography
- Creating topographic maps.
- Constructing cross-sectional profiles of topographic features.
- To establish a basis for accurately locating location points.
General principles of topographic surveying
- Determining potable error requires choosing a scale in advance.
- To work from the most accurate to the least precise method.
- It is recommended to orient each survey according to true north.
- Surveying always begins with establishing horizontal and vertical control: The distance, direction, and difference in elevation between key fixed points.
- Plan a survey that includes checks on the accuracy, such as redundant points, the pacing of measured distances, surveying between fixed points, etc.
Methods of topographic surveying
Method 1: Horizontal distance
- Tachymetry: An optical measurement of space using a telescope and a stadia rod (one stadium is about 607 feet).
- Slope distance is measured with a tape and reduced to horizontal distance using the slope gradient’s cosine.
Method 2: Difference in elevation
The use of a level telescope and an audiometer or Vertical angles and slope distances (height is the product of the space and sine of the curve). Leveling is more accurate since elevation differences are measured rather than calculated. Foresight and a backlight are taken at each position of the automatic level: on the traverse, a foresight to the station before the group, and on the traverse, a backlight to the station after the story. Therefore, the stadium rod occupies two stations on the survey, one before the level and one after. Elevation differences between successive stations are determined by the backlight from the stadia rod and the foresight from the same rod. In each level position, the foresight and backlight lengths should be similar since an accurate sighting depends on the distance from the level.
Level operators should ensure that the level is set up midway between two stations (note: the distance that can be spotted decreases as the slope increases because the stadia rod will disappear above or below the level line of sight). The lengths of backlight and foresight can be determined by either pacing the rod person or measuring the distance between the upper and lower crosshairs (tachymetry).
Method 3: By Direction
- Measurement of the horizontal angle with a compass.
- Venire scales are used for precision measurements.
- Using a reference line or meridians to express direction.
- A true meridian is a north-south axis.
- A magnetic meridian is a line parallel to the earth’s magnetic lines of force.
- An assumed meridian is an arbitrary line.
- The horizontal angle measured by a compass.
- Vernier scales are used for precise measurements.
- Direction is expressed about a reference line or meridian.
What are the types of horizontal angles?
- The bearing angle is measured about a meridian using a quadrant and acute tip, e.g., 37oE, 62oE, 20oW.
- Azimuth is the angle measured from the north branch, e.g., 45o (northeast) and 180o (south).
- The angle of deflection: Between a line and its prolongation, it is a right or left angle depending on whether the new line is on the right (clockwise) or left (counterclockwise) of the preceding line.
- An interior angle is an angle inside a closed polygon.
What are the types of Traverses?
- In azimuth surveys: The profile is always parallel to contours (i.e., maximum slope angle); standard for slope profile surveys.
- Closed traverse: Starts and ends at a known control point; correction for closure errors.
- Closed-loop traverse: Starts and ends at the same station; permits estimation of closure error and use of interior and deflection angles.
- Open traverse: Surveying from a known position to an unknown point; no computational error correction; all measurements must be repeated to detect errors.
Error and the soil’s shape are the two fundamental problems of topographic surveying.
The shape of the earth (the geoid) is a consideration only in geodetic surveying, where over long distances, flat surfaces are not level, plumb lines are not parallel, and the sum of the angles in a triangle is more significant than 180o; thus detailed surveys over large areas employ the principles of geodesy (the mathematical properties of an ellipsoid that emulates the earth); however with most surveys, including virtually all topographic surveying in geography, the departure of horizontal lines from level and plane angles from spherical angles are negligible and can be ignored.
In-plane surveying, the earth’s surface is viewed as an equilateral triangle; parallel lines are regarded as level, similar angles as plane angles, and plumb lines as parallel within the survey.
All the natural features and elevations of a property are identified and depicted by a topographic survey. The 3-dimensional map shows all the natural and artificial elements and improvements of a 3-dimensional property. The map reveals their location, size, height, and any elevation changes.
A topographical land survey measures and identifies the exact location and specifications of natural and artificial features on a piece of land. After the survey is completed, a detailed plan includes human-made features like boundaries, neighboring buildings, walkways, etc.
Topographical surveys don’t always take exactly one, three, or five days to complete, so overtime is expected within the industry to keep jobs from running into the next day.