INTRODUCTION TO TOPOGRAPHY, BASIC NOTIONS
Hello to all friends in this publication I will start a series related to my professional area, the Topography, which is considered as a set of operations necessary to determine the position in the space of a certain number of points, in order to represent on a plane, portions of the earth's surface and the elements that are placed on it with the details of the smallest details, as well as placing on the ground the points that have been defined in the project, rethinking. It is like geodesy but on a smaller scale, they are usually small extensions, because if they are large you have to rely on geodesy.
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GENERAL IDEAS .GEODESIA.
_ As we have seen in the previous section, both Geodesy and Topography have the same purpose, the flat representation of the earth's surface. The difference is that the Geodesy does it of surfaces of great extension and the Topography of reduced extension. It is difficult to establish a clear differentiation between the two sciences with the evolution of the devices. Anyway, the topography will need to rely on geodesy for a lot of practical applications._
One of the greatest utilities of Geodesy is that through its techniques it is possible to cartographically represent very extensive territories. This is achieves through the establishment of a network of vertices located on the surface of the earth related to each other, called geodesic vertices, whose coordinates are known. _These points are related by a triangulation network of vertices distant from each other from 40 to 100 km or more, called the First Order network; said vertices.
they are linked by a second-order network, whose vertices are separated by an average of 25 km and, finally, a geodesic network of the third order with sides ranging from 3 to 10 km. The set of geodesic vertices form a dense mesh of points, on which the topographic works can be supported. To determine the position and relationships between the geodesic vertices, it is necessary to refer them to a system, a known surface on which to situate ourselves.
In order to study the terrestrial surface and place points in it, it is necessary to establish a reference system to be able to project the points of the terrestrial relief on it and allow the elaboration of maps and plans.
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To establish this reference system we study the shape of the earth and the first figure that appears to us and that most closely resembles the shape of the earth is called Geoid, equipotential surface (we join the points with equal gravity) that results from extending the seas below the continents. But this is irregular and too complex, hence the need to find a known surface, determined and that adapts as much as possible to the shape of the earth, then resorted to the ellipsoid of revolution that adapts as much as possible to the geoid and that is defined by mathematical parameters, being called the reference ellipsoid. The mathematical parameters that define it are major axis, minor axis, and crushing.
Given the difficulty of choosing a common ellipsoid, each country has chosen the one that best suits its specific geographical area. In the following figure we see the comparison between the three surfaces studied, that is, between the real earth surface, the Geoid and the ellipsoid reference.
VERTICAL OF A POINT AND WAVE OF THE GEOIDE.
The vertical of a point is the line perpendicular to the surface of the Earth at that point. If we consider the terrestrial surface as a geoid, it is called astronomical vertical and passes through the center of the Earth. If, on the other hand, we refer to the reference ellipsoid, it is called vertical geodesic and does not have to pass through the center of the ellipsoid. It is called deviation from the vertical at a point P of the terrain to the angle formed by said verticals.
GEOGRAPHICAL COORDINATES.
To proceed to the knowledge and determinations on the terrestrial surface, it is used to project and section on it, with which a series of lines, points and surfaces that are defined below appear:
Terrestrial axis, Poles.
It is the imaginary line, around which the earth turns in its rotation movement. The ends of this axis at its intersection with the surface of the earth are called poles, the name of the North Pole corresponding to the one on the constellation side of the greater bear. The other end of the axis is the South Pole.
Meridian plane and meridians, zero meridian.
Every plane that contains the Earth's axis is called the Meridian Plane. The line intersection of this plane with the earth's surface is called the maximum circle or meridian. Obviously they pass through the poles and one is taken as origin, giving reference to the others, the zero meridian.
Plane Parallel, Parallel, Ecuador.
They are called parallel planes to all those that are perpendicular to the Earth's Axis, being called
Equatorial Plane that passes through the center of the Earth and divides the Earth Globe into two hemispheres, the Northern Hemisphere and the Southern Hemisphere, which correspond to each respective Pole.
Cenit and nadir.
If we place ourselves on a point on the surface of the earth and we draw a vertical we will have on our head the zenith and crossing the earth would go to the other end, Nadir, receiving the name of antipodal both ends of each other.
Horizontal plane at a point Meridiana and Cardinal points.
It is called a horizontal plane at a point to the plane tangent to the spherical surface at that point, perpendicular to the direction of the vertical. The geographical meridian of a point is the straight intersection between a horizontal plane and the meridian plane passing through that point. This one marks us on said horizontal plane the North-South direction of the corresponding pole. A perpendicular to the meridian would define a new direction, with which starting from the intersection point we would have four opposite directions.
The points of projection to the infinity of these directions are called Cardinal Points, those corresponding to each pole receive the same name, north and south, the other two East and West, correspond to the right and left of an observer located at the point and having the North in front, its back to the South. By international agreement are marked with the letters N, S, E and W.
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Geographical coordinates. Geographic azimuth.
The location of a point on the surface of the earth is determined by the intersection of the meridian and the parallel passing through that point, and which are defined as Geographic Coordinates, Longitude and Latitude.
Length of a point is the measure, in sexagesimal degrees, of the parallel arc intersected between the meridian passing through the point and the meridian origin or zero meridian (Greenwich). The lengths are measured from 0º to 180º and can be towards the East (positive) or towards the West (negative) and usually represented by the letter λ.
Latitude of a point is the measure, in sexagesimal degrees, of the meridian arc intercepted between the equator and the parallel of the place; towards the North it is considered positive and towards the South negative and is usually represented by the Greek letter φ.
The geographic north, as we have seen in the previous section, is defined by the geographical meridian. It is called the geographical azimuth of an AB address, the angle that forms that direction with the geographic north and is designated with the letter θ. Likewise, the azimuth is measured from the geographical north and clockwise from 0º to 360º.
DATUM.
It receives the name of Datum the point tangent to the ellipsoid and the geoid, where both are coincident.
Each Datum is composed of:
- A reference Ellipsoid, defined by a, b and crush.
- The point of coincidence of geoid and ellipsoid, called fundamental, defined by its geographical coordinates longitude and latitude. In addition to the azimuth of an address originating in the fundamental point, since the astronomical and geodesic vertical does not coincide, a deviation occurs.
in the vertical and in the meridian. This deviation is called:
Eta> Deviation in the vertical.
Xi> Deviation in the meridian.
The deviation from the vertical has already been studied, we will focus on the Deviation in the meridian. When the geodesic vertical does not pass through the center of the ellipsoid of revolution, a fictitious point originates and it may not belong to the earth axis, when this happens there is a Xi.
These two deviations are only calculated for the fundamental point and not for the others.
EARTH MAGNETIC FIELD.
If a magnetic needle is suspended by its center of gravity so that it can rotate freely around it, it is observed that, after a series of oscillations, it takes a fixed direction that it recovers whenever it is separated from it and in addition it inclines more or less with respect to the horizontal one. This fact indicates the existence of a terrestrial magnetic field, the Earth behaving like a huge magnet with its two magnetic poles. These magnetic poles are close to the geographical ones, but they are not coinciding with them, and they receive the name of north magnetic pole and south magnetic pole.
The angle α that forms the needle with the horizontal is called magnetic tilt angle. As the geographical poles do not coincide with the magnetic poles, it is evident that the respective geographical and magnetic meridians will not coincide either, but that at any point of the earth's surface both will form a angle that is denominated of magnetic declination, δ. Then another north appears, the magnetic north, defined by the magnetic meridian and which the compass materializes.
It is called the direction of an AB direction, the angle that forms that direction with the magnetic north and is designated with the letter R, measured from magnetic north and clockwise from 0º to 360º.
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Bibliography :
- https://onlinelibrary.wiley.com/doi/10.1002/9781444360790.ch1
- Basic topography, elements of basic notions. 2017 WIILLER M. YASLEIS V.
- http://academic.brooklyn.cuny.edu/geology/leveson/core/linksa/top_describe/describe_intro2.html
- https://en.wikipedia.org/wiki/Topography.
- Topography and Geodesy, basic concepts and elements conjugated by definition. 2014 . Martini Lucas, Davilucio S.
- http://www.bbc.co.uk/history/domesday/dblock/GB-432000-552000/page/3