Key Concepts and Methods in Topography
Sketch: A rapid drawing made without precision or details, representing the main or most significant lines and dimensions of an area or object.
Limit of Visual Perfection: The eye has a limit beyond which two points, to be seen together, must be separated. It is the minimum distance that the eye can see two separate points. The visual limit is 0.2 mm; if two points are separated by less than this value, they will be seen as a single point.
Scale: A map or plan should bear a relation of resemblance to reality; therefore, we use the scale. It is a constant proportional ratio of the distance between two points on the map, divided by the distance of these two points in reality. The most common scales in topography are 1/100, 1/200, 1/500, 1/1000, 1/5000, 1/10000, 1/20000, and 1/50000.
Topographic Map: A representation, usually partial, of the relief of the land surface at a defined scale.
Geoid: The body of nearly spherical shape, but with a slight flattening at the poles, defined by the equipotential surface of the gravitational field of the Earth that matches the average level of the sea. The geoid is generally considered to be theoretically determined geodesically for planet Earth.
Ellipsoid: A closed curved surface whose three main orthogonal sections are elliptical; that is, those caused by planes containing two Cartesian axes. A mathematical figure that resembles the Earth.
Topographic Level: This will be ready when its vertical axis is in place.
Kilometric Error: The error that occurs in one kilometer.
Contour: Projection on the horizontal reference plane of the intersection of the land with successive equidistant planes parallel to the plane of comparison.
Altimetry and Flatness
Altimetry is the branch of topography that studies the set of methods and procedures to identify and represent the height, also called “elevation,” of each point on a reference plane. Altimetry is achieved with terrain representation (contour maps, profiles, etc.).
Flatness is the part of topography that studies the set of methods and procedures aimed at achieving representation of all interesting details of the land on a flat surface (plane geometry), regardless of their relief.
Topographic Methods
- Radiation: Allows obtaining heights and distances of several points taken from the same station.
- Leveling Closed: An indirect measurement system for obtaining heights and distances of details. It has the capacity to check for errors in measurements by the possibility of compensating for these errors.
- Polygonal Open: Consists of a series of lines, but these do not return to the starting point or close a point with equal or greater accuracy. This is to be avoided because it offers no means of checking for errors and mistakes.
Types of Errors
- Systematic Error: Under fixed working conditions in the field, these are constant and of the same sign and are therefore cumulative. For example, in measurements of angles, bad equipment or graduation drag in transit, bad tape, or state error by temperature.
- Accidental Error: These occur indifferently in one direction or another and therefore might be positive or negative. For example, in measurements of angles, graduation readings, offset from the visual signal in measurements of distances, etc. Many of these errors are eliminated by offset.
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