Topographic Surveying: Methods and Techniques
Direct and Reverse Intersection in Topographic Surveying
In surveying, points B2 and B1 perform a visual on points we aim to survey, and then on the next point on the itinerary, B3. We continue this way until the final point in the case of a framed route, or the point of departure in the case of a closed traverse. A section is the distance between two vertices of the itinerary. A section is not considered the distance between the base and references.
Direct Intersection: This consists of determining the coordinates of a point with angular observations from other points of known coordinates. The point for which I want to calculate the coordinates may be inaccessible. The intersections can be simple or compound; in the first case, the data are taken strictly as necessary. The methodology for the observation is to park the apparatus at point A and make angular observations of points C and B. Then, the device is parked at point B, and angular observations are made to points A and C. These angular observations, to increase accuracy and eliminate systematic errors, can be performed in a direct circle and a reverse circle.
To calculate the angle, use the resolution and the difference in readings. Since we know the coordinates of points A and B, we can calculate the distance between these points and therefore have sufficient data to solve the triangle and get the coordinates of C.
Reverse Intersection: This consists of determining the coordinates of a point by making observations, from that point, to at least three points of known coordinates. For the calculation of a simple inverse intersection, the Pothenot method is used. The methodology for the observation is to park the unit on the unknown point coordinates and make angular observations at three points with known coordinates.
Altimeter Methods in Surveying
Concept of Leveling
Leveling is used to determine:
- The difference in height between two or more points.
- The height of a series of dots on a map compared to drawing a dimensioned drawing, or to draw the section of land in the event that the raised dots are aligned.
- Stake out points of horizontal surfaces such as floors, foundations, or flooring, or points on the slope of roads or sanitation.
To perform the lift to level, you need the following equipment:
- An instrument capable of establishing a visual or a horizontal plane, such as a level.
- A level rod.
- A field notebook, called the leveling pad, which records data, performs error checking locking in the field, and draws the sketch of the survey.
- A tape measure to put points on the plane that served as the basis for the survey.
Methods of Grading
Depending on the instrument used to obtain the difference between two points, there are three different grading methods:
- Leveling by horizontal visual: geometric leveling.
- Leveling steep sights: trigonometric leveling.
- Barometric leveling.
Leveling by Horizontal Visual or Geometric Leveling
To perform this method, a topographic instrument is used; this instrument is capable of providing horizontal visuals. To find the difference between two points, the instrument is situated in the middle of both, the sights are placed on the first, and the reading is noted. Then, the sight is placed on the other point, and the reading is noted. The gap between the two points is the difference in readings obtained. We set the level at the midpoint of the level to ensure that the possible error that could be committed to taking readings from the instrument bias is compensated and free from error.
Leveling by Inclined Visuals or Trigonometric Leveling
Also called trigonometric leveling. The difference between two points is obtained by observing the zenith angle of sight that goes from one point to another and measuring the geometric distance between them. The gap or vertical distance between two points is the product of the geometric distance by the cosine of the zenith angle. If you know the short distance between points, the gap is the product of the reduced distance from the zenith angle cotangent. This method is typically used for leveling over long distances. Currently, mostly used are total stations.